Chapter14 The Somatic Nervous System
The somatic nervous system is traditionally considered a division within the peripheral nervous system. However, this misses an important point: somatic refers to a functional division, whereas peripheral refers to an anatomic division. The somatic nervous system is responsible for our conscious perception of the environment and for our voluntary responses to that perception by means of skeletal muscles. Peripheral sensory neurons receive input from environmental stimuli, but the neurons that produce motor responses originate in the central nervous system. The distinction between the structures (i.e., anatomy) of the peripheral and central nervous systems and functions (i.e., physiology) of the somatic and autonomic systems can most easily be demonstrated through a simple reflex action.
14.1 Sensory Perception
A major role of sensory receptors is to help us learn about the environment around us, or about the state of our internal environment. Stimuli from varying sources, and of different types, are received and changed into the electrochemical signals of the nervous system. This occurs when a stimulus changes the cell membrane potential of a sensory neuron. The stimulus causes the sensory cell to produce an action potential that is relayed into the central nervous system (CNS), where it is integrated with other sensory information—or sometimes higher cognitive functions—to become a conscious perception of that stimulus. The central integration may then lead to a motor response.
Describing sensory function with the term sensation or perception is a deliberate distinction. Sensation is the activation of sensory receptor cells at the level of the stimulus. Perception is the central processing of sensory stimuli into a meaningful pattern. Perception is dependent on sensation, but not all sensations are perceived. Receptors are the cells or structures that detect sensations. A receptor cell is changed directly by a stimulus. A transmembrane protein receptor is a protein in the cell membrane that mediates a physiological change in a neuron, most often through the opening of ion channels or changes in the cell signalling processes. Transmembrane receptors are activated by chemicals called ligands. For example, a molecule in food can serve as a ligand for taste receptors. Other transmembrane proteins, which are not accurately called receptors, are sensitive to mechanical or thermal changes. Physical changes in these proteins increase ion flow across the membrane and can generate an action potential or a graded potential in the sensory neurons.
Sensory Receptors
Stimuli in the environment activate specialized receptor cells in the peripheral nervous system. Different types of stimuli are sensed by different types of receptor cells. Receptor cells can be classified into types on the basis of three different criteria: cell type, position, and function. Receptors can be classified structurally on the basis of cell type and their position in relation to the stimuli they sense. They can also be classified functionally on the basis of the transduction of stimuli, or how the mechanical stimulus, light, or chemical changed the cell membrane potential.
Structural Receptor Types
The cells that interpret information about the environment can be either (1) a neuron that has a free nerve ending, with dendrites embedded in tissue that would receive a sensation; (2) a neuron that has an encapsulated ending in which the sensory nerve endings are encapsulated in connective tissue that enhances their sensitivity; or (3) a specialized receptor cell, which has distinct structural components that interpret a specific type of stimulus (Figure 14.1). The pain and temperature receptors in the dermis of the skin are examples of neurons that have free nerve endings. Also located in the dermis of the skin are lamellated corpuscles, neurons with encapsulated nerve endings that respond to pressure and touch. The cells in the retina that respond to light stimuli are an example of a specialized receptor, a photoreceptor.
Figure 14.1 Receptor Classification by Cell Type. Receptor cell types can be classified on the basis of their structure. Sensory neurons can have either (a) free nerve endings or (b) encapsulated endings. Photoreceptors in the eyes, such as rod cells, are examples of (c) specialized receptor cells. These cells release neurotransmitters onto a bipolar cell, which then synapses with the optic nerve neurons.
Another way that receptors can be classified is based on their location relative to the stimuli. An exteroceptor is a receptor that is located near a stimulus in the external environment, such as the somatosensory receptors that are located in the skin. An interoceptor is one that interprets stimuli from internal organs and tissues, such as the receptors that sense the increase in blood pressure in the aorta or carotid sinus. Finally, a proprioceptor is a receptor located near a moving part of the body, such as a muscle and tendons, that interprets the positions of the tissues as they move.
Functional Receptor Types
A third classification of receptors is by how the receptor transduces stimuli into membrane potential changes. Stimuli are of three general types. Some stimuli are ions and macromolecules that affect transmembrane receptor proteins when these chemicals diffuse across the cell membrane. Some stimuli are physical variations in the environment that affect receptor cell membrane potentials. Other stimuli include the electromagnetic radiation from visible light. For humans, the only electromagnetic energy that is perceived by our eyes is visible light. Some other organisms have receptors that humans lack, such as the heat sensors of snakes, the ultraviolet light sensors of bees, or magnetic receptors in migratory birds.
Receptor cells can be further categorized on the basis of the type of stimuli they transduce. Chemical stimuli can be interpreted by a chemoreceptor that interprets chemical stimuli, such as an object’s taste or smell. Osmoreceptors respond to solute concentrations of body fluids. Additionally, pain is primarily a chemical sense that interprets the presence of chemicals from tissue damage or similar intense stimuli through a nociceptor. Physical stimuli, such as pressure and vibration, as well as the sensation of sound and body position (balance), are interpreted through a mechanoreceptor. Another physical stimulus that has its own type of receptor is temperature, which is sensed through a thermoreceptor that is sensitive to temperatures either above (heat) or below (cold) normal body temperature. Photoreceptors are located in the eyes, and they detect light.
Sensory Modalities
Ask anyone what the senses are, and they are likely to list the five major senses—taste, smell, touch, hearing, and sight. However, these are not all the senses. The most obvious omission from this list is balance. Also, what is referred to simply as touch can be further subdivided into pressure, vibration, stretch, and hair-follicle position, on the basis of the type of mechanoreceptors that perceive these touch sensations. Other overlooked senses include temperature perception by thermoreceptors and pain perception by nociceptors.
Within the realm of physiology, senses can be classified as either general or specific. A general sense is one that is distributed throughout the body and has receptor cells within the structures of other organs. Mechanoreceptors in the skin, muscles, or the walls of blood vessels or the walls of the urinary bladder are examples of this type. General senses often contribute to the sense of touch, to proprioception (body movement) and kinesthesia (body movement), or to a visceral sense, which is most important to autonomic functions. A special sense is one that has a specific organ devoted to it, namely, the eye, inner ear, tongue, or nose.
Each of the senses is referred to as a sensory modality. Modality refers to the way that information is encoded, which is similar to the idea of transduction. The main sensory modalities can be described on the basis of how each is transduced. The chemical senses are taste and smell. The general sense that is usually referred to as touch includes chemical sensation in the form of nociception, or pain. Pressure, vibration, muscle stretch, and the movement of hair by an external stimulus are all sensed by mechanoreceptors. Hearing and balance are also sensed by mechanoreceptors. Finally, vision involves the activation of photoreceptors.
Listing all the different sensory modalities involves separating the five major senses into more specific categories, or submodalities, of the larger sense. An individual sensory modality represents the sensation of a specific type of stimulus. For example, the general sense of touch, which is known as somatosensation, can be separated into light pressure, deep pressure, vibration, itch, pain, temperature, or hair movement.
Gustation (Taste)
Only a few recognized submodalities exist within the sense of taste, or gustation. Until recently, only four tastes were recognized: sweet, salty, sour, and bitter. Umami is a Japanese word that means “delicious taste,” and is often translated to mean savory. Very recent research has suggested that there may also be a sixth taste for fats, or lipids.
Gustation is the special sense associated with the tongue. The surface of the tongue, along with the rest of the oral cavity, is lined by a stratified squamous epithelium. Raised bumps called papillae (singular = papilla) contain the structures for gustatory transduction. There are four types of papillae, based on their appearance: circumvallate, foliate, filiform, and fungiform (Figure 14.2). Within the structure of the papillae are taste buds that contain specialized gustatory receptor cells for the transduction of taste stimuli. These receptor cells are sensitive to the chemicals contained within foods that are ingested, and they release neurotransmitters based on the amount of the chemical in the food. Neurotransmitters from the gustatory cells can activate sensory neurons in the facial, glossopharyngeal, and vagus cranial nerves.
Figure 14.2 The Tongue. The tongue is covered with small bumps, called papillae, which contain taste buds that are sensitive to chemicals in ingested food or drink. Different types of papillae are found in different regions of the tongue. The taste buds contain specialized gustatory receptor cells that respond to chemical stimuli dissolved in the saliva. These receptor cells activate sensory neurons that are part of the facial and glossopharyngeal nerves. LM × 1600. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)
Salty taste is simply the perception of sodium ions (Na+) in the saliva. When you eat something salty, the salt crystals dissociate into the component ions Na+ and Cl−, which dissolve into the saliva in your mouth. The Na+ concentration becomes high outside the gustatory cells, creating a strong concentration gradient that drives the diffusion of the ion into the cells. The entry of Na+ into these cells results in the depolarization of the cell membrane and the generation of a receptor potential.
Sour taste is the perception of H+ concentration. Just as with sodium ions in salty flavors, these hydrogen ions enter the cell and trigger depolarization. Sour flavors are, essentially, the perception of acids in our food. Increasing hydrogen-ion concentrations in the saliva (lowering saliva pH) triggers progressively stronger graded potentials in the gustatory cells. For example, orange juice—which contains citric acid—will taste sour because it has a pH value of approximately 3. Of course, it is often sweetened so that the sour taste is masked. These two tastes (salty and sour) are triggered by the cations Na+ and H+.
The other tastes result from food molecules binding to a G protein–coupled receptor. A G protein signal transduction system ultimately leads to depolarization of the gustatory cell. The sweet taste is the sensitivity of gustatory cells to the presence of glucose dissolved in the saliva. Other monosaccharides, such as fructose, and artificial sweeteners such as aspartame (NutraSweet™), saccharine, or sucralose (Splenda™) also activate the sweet receptors. The affinity for each of these molecules varies, and some will taste sweeter than glucose because they bind to the G protein–coupled receptor differently.
Bitter taste is similar to sweet in that food molecules bind to G protein–coupled receptors. However, there are a number of different ways in which this can happen because there is a large diversity of bitter-tasting molecules. Some bitter molecules depolarize gustatory cells, whereas others hyperpolarize gustatory cells. Likewise, some bitter molecules increase G protein activation within the gustatory cells, whereas other bitter molecules decrease G protein activation. The specific response depends on which molecule is binding to the receptor.
One major group of bitter-tasting molecules is alkaloids. Alkaloids are nitrogen-containing molecules that are commonly found in bitter-tasting plant products, such as coffee, hops (in beer), tannins (in wine), tea, and aspirin. By containing toxic alkaloids, the plant is less susceptible to microbial infection and less attractive to herbivores.
Therefore, the function of bitter taste may primarily be related to stimulating the gag reflex to avoid ingesting poisons. Because of this, many bitter foods that are normally ingested are often combined with a sweet component to make them more palatable (cream and sugar in coffee, for example). The highest concentration of bitter receptors appears to be in the posterior tongue, where a gag reflex could still spit out poisonous food.
The taste known as umami is often referred to as the savory taste. Like sweet and bitter, it is based on the activation of G protein–coupled receptors by a specific molecule. The molecule that activates this receptor is the amino acid L-glutamate. Therefore, the umami flavor is often perceived while eating protein-rich foods. Not surprisingly, dishes that contain meat are often described as savory.
Once the gustatory cells are activated by the taste molecules, they release neurotransmitters onto the dendrites of sensory neurons. These neurons are part of the facial and glossopharyngeal cranial nerves, as well as a component within the vagus nerve dedicated to the gag reflex. The facial nerve connects to taste buds in the anterior third of the tongue. The glossopharyngeal nerve connects to taste buds in the posterior two-thirds of the tongue. The vagus nerve connects to taste buds in the extreme posterior of the tongue, verging on the pharynx, which are more sensitive to noxious stimuli such as bitterness.
Olfaction (Smell)
Like taste, the sense of smell, or olfaction, is also responsive to chemical stimuli. The olfactory receptor neurons are located in a small region within the superior nasal cavity (Figure 14.3). This region is referred to as the olfactory epithelium and contains bipolar sensory neurons. Each olfactory sensory neuron has dendrites called olfactory hairs that extend from the apical surface of the epithelium into the mucus lining the cavity. As airborne molecules are inhaled through the nose, they pass over the olfactory epithelial region and dissolve into the mucus. These odorant molecules bind to proteins that keep them dissolved in the mucus and help transport them to the olfactory dendrites. The odorant-protein complex binds to a receptor protein within the cell membrane of an olfactory dendrite. These receptors are G protein–coupled and will produce a graded membrane potential in the olfactory neurons.
Figure 14.3 The Olfactory System. (a) The olfactory system begins in the peripheral structures of the nasal cavity. (b) The olfactory receptor neurons are within the olfactory epithelium. (c) Axons of the olfactory receptor neurons project through the cribriform plate of the ethmoid bone and synapse with the neurons of the olfactory bulb (tissue source: simian). LM × 812. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)
The axon of an olfactory neuron extends from the basal surface of the epithelium, through an olfactory foramen in the cribriform plate of the ethmoid bone, and into the brain. The axons of the group called the olfactory nerves connect to the olfactory bulb on the ventral surface of the frontal lobe. From there, the axons form the olfactory tract and then split to travel to several brain regions. Some travel to the cerebrum, specifically to the primary olfactory cortex that is located in the inferior and medial areas of the temporal lobe. Others project to structures within the limbic system and hypothalamus, where smells become associated with long-term memory and emotional responses. This is how certain smells trigger emotional memories, such as the smell of food associated with one’s birthplace. Smell is the one sensory modality that does not synapse in the thalamus before connecting to the cerebral cortex. This intimate connection between the olfactory system and the cerebral cortex is one reason why smell can be a potent trigger of memories and emotion.
The nasal epithelium, including the olfactory cells, can be harmed by airborne toxic chemicals. Therefore, the olfactory neurons are regularly replaced within the nasal epithelium, after which the axons of the new neurons must find their appropriate connections in the olfactory bulb. These new axons grow along the axons that are already in place in the cranial nerve.
Audition (Hearing)
Hearing, or audition, is the transduction of sound waves into a neural signal that is made possible by the structures of the ear (Figure 14.4). The large, fleshy structure on the lateral aspect of the head is known as the auricle. Some sources will also refer to this structure as the pinna, though that term is more appropriate for a structure that can be moved, such as the external ear of a cat. The C-shaped curves of the auricle direct sound waves toward the auditory canal. The canal enters the skull through the external auditory meatus of the temporal bone. At the end of the auditory canal is the tympanic membrane, or eardrum, which vibrates after it is struck by sound waves. The auricle, ear canal, and tympanic membrane are often referred to as the external ear. The middle ear consists of a space spanned by three small bones called the ossicles. The three ossicles are the malleus, incus, and stapes, which are Latin names that roughly translate to hammer, anvil, and stirrup. The malleus is attached to the tympanic membrane and articulates with the incus. The incus, in turn, articulates with the stapes. The stapes is then attached to the inner ear, where the sound waves will be transduced into a neural signal. The middle ear is connected to the pharynx through the Eustachian tube, which helps equilibrate air pressure across the tympanic membrane. The tube is normally closed but will pop open when the muscles of the pharynx contract during swallowing or yawning.
Figure 14.4 Structures of the Ear. The external ear contains the auricle, ear canal, and tympanic membrane. The middle ear contains the ossicles and is connected to the pharynx by the Eustachian tube. The inner ear contains the cochlea and vestibule, which are responsible for audition and equilibrium, respectively.
The inner ear is often described as a bony labyrinth, as it is composed of a series of canals embedded within the temporal bone. It has two separate regions, the cochlea and the vestibule, which are responsible for hearing and balance, respectively. The neural signals from these two regions are relayed to the brain stem through separate fibre bundles. However, these two distinct bundles travel together from the inner ear to the brain stem as the vestibulocochlear nerve. Sound is transduced into neural signals within the cochlear region of the inner ear, which contains the sensory neurons of the spiral ganglia. These ganglia are located within the spiral-shaped cochlea of the inner ear. The cochlea is attached to the stapes through the oval window.
The oval window is located at the beginning of a fluid-filled tube within the cochlea called the scala vestibuli. The scala vestibuli extends from the oval window, travelling alongside the cochlear duct, which is the central cavity of the cochlea that contains the sound-transducing neurons. At the uppermost tip of the cochlea, the scala vestibuli curves alongside the cochlear duct. The fluid-filled tube, now called the scala tympani, returns to the base of the cochlea, this time travelling under the cochlear duct (Figure 14.5). The scala tympani ends at the round window, which is covered by a membrane that contains the fluid within the scala.
As vibrations of the ossicles travel through the oval window, the fluid of the scala vestibuli and scala tympani moves in a wavelike motion. The frequencies of the fluid waves match the frequencies of the sound waves. The membrane covering the round window will bulge out or pucker in with the movement of the fluid within the scala tympani.
A cross-sectional view of the cochlea shows that the scala vestibuli and scala tympani run along both sides of the cochlear duct (Figure 14.6 and Figure 14.7). The cochlear duct contains the organ of Corti, which transduces the wave motion of the two scalae into neural signals. The organ of Corti lies on top of the basilar membrane, which is the side of the cochlear duct located between the organ of Corti and the scala tympani. As the fluid waves move through the scala vestibuli and scala tympani, the basilar membrane moves at a specific spot, depending on the frequency of the waves. Higher frequency waves move the region of the basilar membrane that is close to the base of the cochlea.
Figure 14.5 Transmission of Sound Waves to Cochlea. A sound wave causes the tympanic membrane to vibrate. This vibration is amplified as it moves across the malleus, incus, and stapes. The amplified vibration is picked up by the oval window, causing pressure waves in the fluid of the scala vestibuli and scala tympani. The complexity of the pressure waves is determined by the changes in amplitude and frequency of the sound waves entering the ear.
Figure 14.6 Cross Section of the Cochlea. The three major spaces within the cochlea are highlighted. The scala tympani and scala vestibuli lie on either side of the cochlear duct. The organ of Corti, containing the mechanoreceptor hair cells, is adjacent to the scala tympani, where it sits atop the basilar membrane.
Figure 14.7 Hair Cell. The hair cell is a mechanoreceptor with an array of stereocilia emerging from its apical surface. The stereocilia are tethered together by proteins that open ion channels when the array is bent toward the tallest member of their array and close when the array is bent toward the shortest member of their array.
Lower frequency waves move the region of the basilar membrane that is near the tip of the cochlea.
The organ of Corti contains hair cells, which are named for the hairlike stereocilia extending from the cell’s apical surfaces (Figure 14.7). The stereocilia are an array of microvilli-like structures arranged from tallest to shortest.
Protein fibres tether adjacent stereocilia together within each array, such that the array will bend in response to movements of the basilar membrane.
The stereocilia extend up from the hair cells to the overlying tectorial membrane, which is attached medially to the organ of Corti. When the pressure waves from the scala move the basilar membrane, the stereocilia are bent against the overhanging tectorial membrane. This bends the stereocilia either toward or away from the tallest member of each array. When the stereocilia bend toward the tallest member of their array, tension in the protein tethers opens ion channels in the hair cell membrane. This will depolarize the hair cell membrane, triggering impulses that travel down the afferent cochlear branch of the vestibulocochlear nerve fibres attached to the hair cells. When the stereocilia bend toward the shortest member of their array, the tension on the tethers slackens and the ion channels close. When no sound is present, and the stereocilia are standing straight, a small amount of tension still exists on the tethers, keeping the membrane potential of the hair cell slightly depolarized (Figure 14.7).
Figure 14.8 Cochlea and Organ of Corti. LM × 412. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)
As discussed, a given region of the basilar membrane will only move if the incoming sound is at a specific frequency. Because the tectorial membrane only moves where the basilar membrane moves, the hair cells in this region will also only respond to sounds of this specific frequency. Therefore, as the frequency of a sound changes, different hair cells are activated all along the basilar membrane. The cochlea encodes auditory stimuli for frequencies between 20 and 20,000 Hz, which is the range of sound that human ears can detect. The unit of Hertz measures the frequency of sound waves in terms of cycles produced per second. Frequencies as low as 20 Hz are detected by hair cells at the apex, or tip, of the cochlea. Frequencies in the higher ranges of 20 KHz are encoded by hair cells at the base of the cochlea, close to the round and oval windows (Figure 14.9). Most auditory stimuli contain a mixture of sounds at a variety of frequencies and intensities (represented by the amplitude of the sound wave). The hair cells along the length of the cochlear duct, which are each sensitive to a particular frequency, allow the cochlea to separate auditory stimuli by frequency, just as a prism separates visible light into its component colours.
Figure 14.9 Frequency Coding in the Cochlea. The standing sound wave generated in the cochlea by the movement of the oval window deflects the basilar membrane on the basis of the frequency of sound. Therefore, hair cells at the base of the cochlea are activated only by high frequencies, whereas those at the apex of the cochlea are activated only by low frequencies.
Equilibrium (Balance)
Along with audition, the inner ear is responsible for encoding information about equilibrium, the sense of balance. A similar mechanoreceptor—a hair cell with stereocilia—senses head position, head movement, and whether our bodies are in motion. These cells are located within the vestibular apparatus of the inner ear. Head position is sensed by the utricle and saccule, whereas rotational movements of the head are sensed by the semicircular canals. The neural signals generated in the vestibular ganglion are transmitted through the vestibular branch of the vestibulocochlear nerve to the brain stem and cerebellum.
The utricle and saccule are both largely composed of macula tissue (plural = maculae). The macula is composed of hair cells surrounded by support cells. The stereocilia of the hair cells extend into a viscous gel called the otolithic membrane (Figure 14.10). On top of the otolithic membrane is a layer of calcium carbonate crystals, called otoliths. The otoliths essentially make the otolithic membrane top-heavy. The otolithic membrane moves separately from the macula in response to head movements. Tilting the head causes the otolithic membrane to slide over the macula in the direction of gravity. The moving otolithic membrane, in turn, bends the stereocilia, causing some hair cells to depolarize as others hyperpolarize. The exact position of the head as well as linear acceleration and linear deceleration are interpreted by the brain based on the pattern of hair-cell depolarization.
The semicircular canals are three ringlike extensions of the vestibule. One is oriented in the horizontal plane, whereas the other two are oriented in the vertical plane. The anterior and posterior vertical canals are oriented at approximately 45 degrees relative to the sagittal plane (Figure 14.11). The base of each semicircular canal, where it meets with the vestibule, connects to an enlarged region known as the ampulla. The ampulla contains the hair cells that respond to rotational movement, such as turning the head while saying “no.” The stereocilia of these hair cells extend into the cupula, a membrane that projects toward the top of the ampulla. As the head rotates in a plane parallel to the semicircular canal, the fluid lags, deflecting the cupula in the direction opposite to the head movement. The semicircular canals contain several ampullae, with some oriented horizontally and others oriented vertically. By comparing the relative movements of both the horizontal and vertical ampullae, the vestibular system can detect the direction of most head movements within three-dimensional (3-D) space.
Figure 14.10 Linear Acceleration and Deceleration by Maculae. The maculae are specialized for sensing linear acceleration and deceleration, such as when gravity acts on the tilting head, or if the head starts moving in a straight line. The difference in inertia between the hair cell stereocilia and the otolithic membrane in which they are embedded leads to a shearing force that causes the stereocilia to bend in the direction of that linear acceleration or deceleration.
Figure 14.11 Rotational Coding by Semicircular Canals. Rotational movement of the head is encoded by the hair cells in the base of the semicircular canals. As one of the canals moves in an arc with the head, the internal fluid moves in the opposite direction, causing the cupula and stereocilia to bend. The movement of two canals within a plane results in information about the direction in which the head is moving, and activation of all six canals can give a very precise indication of head movement in three dimensions.
Somatosensation (Touch)
Somatosensation is considered a general sense, as opposed to the special senses discussed in this section. Somatosensation is the group of sensory modalities that are associated with touch, proprioception, and interoception. These modalities include pressure, vibration, light touch, tickle, itch, temperature, pain, proprioception, and kinesthesia. This means that its receptors are not associated with a specialized organ but are instead spread throughout the body in a variety of organs. Many of the somatosensory receptors are located in the skin, but receptors are also found in muscles, tendons, joint capsules, ligaments, and in the walls of visceral organs.
Two types of somatosensory signals that are transduced by free nerve endings are pain and temperature. These two modalities use thermoreceptors and nociceptors to transduce temperature and pain stimuli, respectively.
Temperature receptors are stimulated when local temperatures differ from body temperature. Some thermoreceptors are sensitive to just cold and others to just heat. Nociception is the sensation of potentially damaging stimuli. Mechanical, chemical, or thermal stimuli beyond a set threshold will elicit painful sensations. Stressed or damaged tissues release chemicals that activate receptor proteins in the nociceptors.
For example, the sensation of heat associated with spicy foods involves capsaicin (a molecule that activates nociceptors by interacting with a temperature-sensitive ion channel and is the basis for “hot” sensations in spicy food), the active molecule in hot peppers. Capsaicin molecules bind to a transmembrane ion channel in nociceptors that is sensitive to temperatures above 37°C. The dynamics of capsaicin binding with this transmembrane ion channel are unusual in that the molecule remains bound for a long time. Because of this, it will decrease the ability of other stimuli to elicit pain sensations through the activated nociceptor. For this reason, capsaicin can be used as a topical analgesic, such as in products such as Icy Hot.
If you drag your finger across a textured surface, the skin of your finger will vibrate. Such low-frequency vibrations are sensed by mechanoreceptorscalled Merkel cells, also known as type I cutaneous mechanoreceptors. Merkel cells are located in the stratum basale of the epidermis. Deep pressure and vibration are transduced by lamellated (Pacinian) corpuscles, which are receptors with encapsulated endings found deep in the dermis, or subcutaneous tissue. Light touch is transduced by the encapsulated corpuscle endings known as tactile (Meissner) corpuscles. Follicles are also wrapped in a plexus of nerve endings known as the hair follicle plexus. These nerve endings detect the movement of hair at the surface of the skin, such as when an insect may be walking along the skin. Stretching of the skin is transduced by stretch receptors known as bulbous corpuscles. Bulbous corpuscles are also known as Ruffini corpuscles, or type II cutaneous mechanoreceptors.
Other somatosensory receptors are found in the joints and muscles. Stretch receptors monitor the stretching of tendons, muscles, and the components of joints. For example, have you ever stretched your muscles before or after exercise and noticed that you can only stretch so far before your muscles spasm back to a less stretched state? This spasm is a reflex that is initiated by stretch receptors to avoid muscle tearing. Such stretch receptors can also prevent overcontraction of a muscle. In skeletal muscle tissue, these stretch receptors are called muscle spindles. Golgi tendon organs similarly transduce the stretch levels of tendons called tendon stretch organs. Bulbous corpuscles are also present in joint capsules, where they measure stretch in the components of the skeletal system within the joint. The types of nerve endings, their locations, and the stimuli they transduce are presented in Table 14.1.
Vision
Vision is the special sense of sight that is based on the transduction of light stimuli received through the eyes. The eyes are located within either orbit in the skull. The bony orbits surround the eyeballs, protecting them and anchoring the soft tissues of the eye. Lacrimal glands are lateral to the orbit that produces tears to wash across the surface of the eye (Figure 14.12).
The eye itself is a hollow sphere composed of three layers of tissue. The outermost layer is the fibrous tunic, which includes the white sclera and clear cornea. The sclera accounts for five-sixths of the surface of the eye, most of which is not visible, though humans are unique compared with many other species in having so much of the “white of the eye” visible (Figure 14.13). The transparent cornea covers the anterior area of the eye and allows light to enter the eye. The middle layer of the eye is the vascular tunic, which is mostly composed of the choroid, ciliary body, and iris. The choroid is a layer of highly vascularized connective tissue that provides a blood supply to the eyeball. The choroid is posterior to the ciliary body, a muscular structure that is attached to the lens by suspensory ligaments, or zonule fibres. These two structures adjust the shape of the lens, allowing it to focus light on the back of the eye. Overlaying the ciliary body, and visible in the anterior eye, is the iris—the coloured part of the eye. The iris is a smooth muscle that opens or closes the pupil, which is the hole at the centre of the iris that allows light to enter. The iris constricts the pupil in response to bright light and dilates the pupil in response to dim light. The innermost layer of the eye is the neural tunic, or retina, which contains the nervous tissue responsible for photoreception.
Name | Historical (eponymous) name | Location(s) | Stimuli |
|---|---|---|---|
Free nerve endings | Dermis, cornea, tongue, joint capsules, visceral organs | Pain, temperature, mechanical deformation | |
Mechanoreceptors | Merkel’s discs | Epidermal-dermal junction, mucosal membranes | Low frequency vibration (5–15 Hz) |
Bulbous corpuscle | Ruffini’s corpuscle | Dermis, joint capsules | Stretch |
Encapsulated corpuscle | Meissner’s corpuscle | Papillary dermis, especially in the fingertips and lips | Light touch, vibrations below 50 Hz |
Lamellated corpuscle | Pacinian corpuscle | Deep dermis, subcutaneous tissue | Deep pressure, high-frequency vibration (around 250 Hz) |
Hair follicle plexus | Wrapped around hair follicles in the dermis | Movement of hair | |
Muscle spindle | In line with skeletal muscle fibres | Muscle contraction and stretch | |
Tendon stretch organ | Golgi tendon organ | In line with tendons | Stretch of tendons |
* No corresponding eponymous name.
Figure 14.12 The Eye in the Orbit. The eye is located within the orbit and surrounded by soft tissues that protect and support its function. The orbit is surrounded by the cranial bones of the skull.
Figure 14.13 Structure of the Eye. The sphere of the eye can be divided into anterior and posterior cavities. The wall of the eye is composed of three layers: the fibrous tunic, vascular tunic, and neural tunic. Within the neural tunic is the retina, with three layers of cells and two synaptic layers in between. The centre of the retina has a small indentation known as the fovea.
The eye is also divided into two cavities: the anterior cavity and the posterior cavity. The anterior cavity is the space between the cornea and lens, including the iris and ciliary body. It is filled with a watery fluid called the aqueous humour. The posterior cavity is the space behind the lens that extends to the posterior side of the interior eyeball, where the retina is located. The posterior cavity is filled with a more viscous fluid called the vitreous humour.
The retina is composed of several layers and contains specialized cells for the initial processing of visual stimuli. The photoreceptors (rods and cones) change their membrane potential when stimulated by light energy. The change in membrane potential alters the amount of neurotransmitter that the photoreceptor cells release onto bipolar cells in the outer synaptic layer. It is the bipolar cell in the retina that connects a photoreceptor to a retinal ganglion cell (RGC) in the inner synaptic layer. There, amacrine cells additionally contribute to retinal processing before an action potential is produced by the RGC. The axons of RGCs, which lie at the innermost layer of the retina, collect at the optic disc and leave the eye as the optic nerve (see Figure 14.13). Because these axons pass through the retina, there are no photoreceptors at the very back of the eye, where the optic nerve begins. This creates a “blind spot” in the retina, and a corresponding blind spot in our visual field.
Note that the photoreceptors in the retina, rods and cones, are located behind the axons, RGCs, bipolar cells, and retinal blood vessels. A significant amount of light is absorbed by these structures before the light reaches the photoreceptor cells. However, at the exact centre of the retina is a small area known as the fovea. At the fovea, the retina lacks the supporting cells and blood vessels and only contains photoreceptors. Therefore, visual acuity, or the sharpness of vision, is greatest at the fovea. This is because the fovea is where the least amount of incoming light is absorbed by other retinal structures (see Figure 14.13). As one moves in every direction from this central point of the retina, visual acuity drops significantly. In addition, each photoreceptor cell of the fovea is connected to a single RGC. Therefore, this RGC does not have to integrate inputs from multiple photoreceptors, which reduces the accuracy of visual transduction. Toward the edges of the retina, several photoreceptors converge on RGCs (through the bipolar cells) up to a ratio of 50 to 1. The difference in visual acuity between the fovea and peripheral retina is easily evidenced by looking directly at a word in the middle of this paragraph. The visual stimulus in the middle of the field of view falls on the fovea and is in the sharpest focus. Without moving your eyes off that word, notice that words at the beginning or end of the paragraph are not in focus. The images in your peripheral vision are focused onto the peripheral retina and have vague, blurry edges and words that are not as clearly identified. As a result, a large part of the neural function of the eyes is concerned with moving the eyes and head so that important visual stimuli are centred on the fovea.
Light falling on the retina causes chemical changes to pigment molecules in the photoreceptors, ultimately leading to a change in the activity of the RGCs. Photoreceptor cells have two parts, the inner segment and the outer segment (Figure 14.14). The inner segment contains the nucleus and other common organelles of a cell, whereas the outer segment is a specialized region in which photoreception takes place. There are two types of photoreceptors—rods and cones—which differ in the shape of their outer segment. The rod-shaped outer segments of the rod photoreceptors (rods) contain a stack of membrane-bound discs that contain the photosensitive pigment rhodopsin. The cone-shaped outer segments of the cone photoreceptors (cones) contain their photosensitive pigments in infoldings of the cell membrane. There are three cone photopigments, called opsins, which are each sensitive to a particular wavelength of light. The wavelength of visible light determines its colour. The pigments in human eyes are specialized in perceiving three different primary colours: red, green, and blue.
Figure 14.14 Photoreceptor. (a) All photoreceptors have inner segments containing the nucleus and other important organelles and outer segments with membrane arrays containing the photosensitive opsin molecules. Rod outer segments are long columnar shapes with stacks of membrane-bound discs that contain the rhodopsin pigment. Cone outer segments are short, tapered shapes with folds of membrane in place of the discs in the rods. (b) The tissue of the retina shows a dense layer of nuclei of the rods and cones. LM × 800. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)
At the molecular level, visual stimuli cause changes in the photopigment molecule that lead to changes in the membrane potential of the photoreceptor cell.
Sensory Nerves
Once any sensory cell transduces a stimulus into a nerve impulse, that impulse has to travel along axons to reach the CNS. In many of the special senses, the axons leaving the sensory receptors have a topographical arrangement, meaning that the location of the sensory receptor relates to the location of the axon in the nerve. For example, in the retina, axons from RGCs in the fovea are located at the centre of the optic nerve, where they are surrounded by axons from the more peripheral RGCs.
Spinal Nerves
Generally, spinal nerves contain afferent axons from sensory receptors in the periphery, such as from the skin, mixed with efferent axons travelling to the muscles or other effector organs. As the spinal nerve nears the spinal cord, it splits into dorsal and ventral roots. The dorsal root contains only the axons of sensory neurons, whereas the ventral roots contain only the axons of the motor neurons. Some of the branches will synapse with local neurons in the dorsal root ganglion, posterior (dorsal) horn, or even the anterior (ventral) horn, at the level of the spinal cord where they enter. Other branches will travel a short distance up or down the spine to interact with neurons at other levels of the spinal cord. A branch may also turn into the posterior (dorsal) column of the white matter to connect with the brain. For the sake of convenience, we will use the terms ventral and dorsal in reference to structures within the spinal cord that are part of these pathways. This will help underscore the relationships between the different components. Typically, spinal nerve systems that connect to the brain are contralateral, in that the right side of the body is connected to the left side of the brain and the left side of the body to the right side of the brain.
Cranial Nerves
Cranial nerves convey specific sensory information from the head and neck directly to the brain. Whereas spinal information is contralateral, cranial nerve systems, with some exceptions, are mostly ipsilateral, meaning that a cranial nerve on the right side of the head is connected to the right side of the brain. Some cranial nerves contain only sensory axons, such as the olfactory, optic, and vestibulocochlear nerves. Other cranial nerves contain both sensory and motor axons, including the trigeminal, facial, glossopharyngeal, and vagus nerves (however, the vagus nerve is not associated with the somatic nervous system). The general senses of somatosensation for the face travel through the trigeminal system.
14.2 Central Processing
Sensory Pathways
Specific regions of the CNS coordinate different somatic processes using sensory inputs and motor outputs of peripheral nerves. A simple case is a reflex caused by a synapse between a dorsal sensory neuron axon and a motor neuron in the ventral horn. More complex arrangements are possible to integrate peripheral sensory information with higher processes. The important regions of the CNS that play a role in somatic processes can be separated into the spinal cord, brain stem, diencephalon, cerebral cortex, and subcortical structures.
Spinal Cord and Brain Stem
A sensory pathway that carries peripheral sensations to the brain is referred to as an ascending pathway, or ascending tract. The various sensory modalities each follow specific pathways through the CNS. Tactile and other somatosensory stimuli activate receptors in the skin, muscles, tendons, and joints throughout the entire body.
However, the somatosensory pathways are divided into two separate systems on the basis of the location of the receptor neurons. Somatosensory stimuli from below the neck pass along the sensory pathways of the spinal cord, whereas somatosensory stimuli from the head and neck travel through the cranial nerves—specifically, the trigeminal system.
The dorsal column system (sometimes referred to as the dorsal column–medial lemniscus) and the spinothalamic tract are two major pathways that bring sensory information to the brain (Figure 14.15). The sensory pathways in each of these systems are composed of three successive neurons.
The dorsal column system begins with the axon of a dorsal root ganglion neuron entering the dorsal root and joining the dorsal column white matter in the spinal cord. As axons of this pathway enter the dorsal column, they take on a positional arrangement so that axons from lower levels of the body position themselves medially, whereas axons from upper levels of the body position themselves laterally. The dorsal column is separated into two component tracts that contain axons from the legs and lower body, and axons from the upper body and arms.
The spinothalamic tract also begins with neurons in a dorsal root ganglion. These neurons extend their axons to the dorsal horn, where they synapse with the second neuron in their respective pathway. The name “spinothalamic” comes from this second neuron, which has its cell body in the spinal cord grey matter and connects to the thalamus. Axons from these second neurons then decussate (i.e., cross over to the opposite side) within the spinal cord and ascend to the brain and enter the thalamus, where each synapses with the third neuron in its respective pathway. The neurons in the thalamus then project their axons to the spinothalamic tract, which synapses in the postcentral gyrus of the cerebral cortex.
Figure 14.15 Anterior View of Ascending Sensory Pathways of the Spinal Cord. The dorsal column system and spinothalamic tract are the major ascending pathways that connect the periphery with the brain.
These two systems are similar in that they both begin with dorsal root ganglion cells, as with most general sensory information. The dorsal column system is primarily responsible for touch sensations and proprioception, whereas the spinothalamic tract pathway is primarily responsible for pain and temperature sensations. Another similarity is that the second neurons in both of these pathways are contralateral, because they project across the midline to the other side of the brain or spinal cord. In the dorsal column system, this decussation takes place in the brain stem; in the spinothalamic pathway, it takes place in the spinal cord at the same spinal cord level at which the information entered. The third neurons in the two pathways are essentially the same. In both, the second neuron synapses in the thalamus, and the thalamic neuron projects to the somatosensory cortex.
The trigeminal pathway carries somatosensory information from the face, head, mouth, and nasal cavity. As with the previously discussed tracts, the sensory pathways of the trigeminal pathway each involve three successive neurons. First, axons from the trigeminal ganglion enter the brain stem at the level of the pons. The sensory pathway for gustation travels along the facial and glossopharyngeal cranial nerves, which synapse with neurons of the solitary nucleus in the brain stem.
The sensory pathway for audition travels along the vestibulocochlear nerve, which synapses with neurons in the cochlear nuclei of the superior medulla. Within the brain stem, input from either ear is combined to extract location information from the auditory stimuli. Whereas the initial auditory stimuli received at the cochlea strictly represent the frequency—or pitch—of the stimuli, the locations of sounds can be determined by comparing information arriving at both ears. Sound localization is a feature of central processing in the auditory nuclei of the brain stem.
Balance is coordinated through the vestibular system, the nerves of which are composed of axons from the vestibular ganglion that carries information from the utricle, saccule, and semicircular canals. The system contributes to controlling head and neck movements in response to vestibular signals. An important function of the vestibular system is coordinating eye and head movements to maintain visual attention. Most of the axons terminate in the vestibular nuclei of the medulla. Some axons project from the vestibular ganglion directly to the cerebellum, with no intervening synapse in the vestibular nuclei. The cerebellum is primarily responsible for initiating movements on the basis of equilibrium information (Figure 14.16).
To assist the visual system, fibres of the vestibular nuclei project to the oculomotor, trochlear, and abducens nuclei to influence signals sent along the cranial nerves. These connections constitute the pathway of the vestibulo-ocular reflex (VOR), which compensates for head and body movement by stabilizing images on the retina (Figure 14.16). Finally, the vestibular nuclei project to the thalamus to join the proprioceptive pathway of the dorsal column system, allowing conscious perception of equilibrium.
The connections of the optic nerve are more complicated than those of other cranial nerves. Instead of the connections being between each eye and the brain, visual information is segregated between the left and right sides of the visual field. In addition, some of the information from one side of the visual field projects to the opposite side of the brain. Within each eye, the axons projecting from the medial side of the retina decussate at the optic chiasm. For example, the axons from the medial retina of the left eye cross over to the right side of the brain at the optic chiasm. However, within each eye, the axons projecting from the lateral side of the retina do not decussate. For example, the axons from the lateral retina of the right eye project back to the right side of the brain.
Figure 14.16 Vestibulo-Ocular Reflex. Connections between the vestibular system and the cranial nerves controlling eye movement keep the eyes centred on a visual stimulus, even though the head is moving. During head movement, the eye muscles move the eyes in the opposite direction to the head movement, keeping the visual stimulus centred in the field of view.
Therefore, the left field of view of each eye is processed on the right side of the brain, whereas the right field of view of each eye is processed on the left side of the brain (Figure 14.17).
Unlike the photoreceptors, however, these photosensitive RGCs cannot be used to perceive images. By simply responding to the absence or presence of light, these RGCs can send information about day length. The perceived proportion of sunlight to darkness establishes the circadian rhythm of our bodies, allowing certain physiological events to occur at approximately the same time every day.
Figure 14.17 Segregation of Visual Field Information at the Optic Chiasm. Contralateral visual field information from the lateral retina projects to the ipsilateral brain, whereas ipsilateral visual field information has to decussate at the optic chiasm to reach the opposite side of the brain. (Note that this is an inferior view.)
Diencephalon
The diencephalon is beneath the cerebrum and includes the thalamus and hypothalamus. In the somatic nervous system, the thalamus is an important relay for communication between the cerebrum and the rest of the nervous system. The hypothalamus has both somatic and autonomic functions. In addition, the hypothalamus communicates with the limbic system, which controls emotions and memory functions.
Sensory input to the thalamus comes from most of the special senses and ascending somatosensory tracts. Each sensory system is relayed through a particular nucleus in the thalamus. The thalamus is a required transfer point for most sensory tracts that reach the cerebral cortex, where conscious sensory perception begins. The one exception to this rule is the olfactory system. The olfactory tract axons from the olfactory bulb project directly to the cerebral cortex, along with the limbic system and hypothalamus.
The thalamus is a collection of several nuclei that can be categorized into three anatomical groups. White matter running through the thalamus defines the three major regions of the thalamus, which are an anterior nucleus, a medial nucleus, and a lateral group of nuclei. The anterior nucleus serves as a relay between the hypothalamus and the emotion and memory-producing limbic system. The medial nuclei serve as a relay for information from the limbic system and basal ganglia to the cerebral cortex. This allows memory creation during learning but also determines alertness. The special and somatic senses connect to the lateral nuclei, where their information is relayed to the appropriate sensory cortex of the cerebrum.
Cortical Processing
As described earlier, many of the sensory axons are positioned in the same way as their corresponding receptor cells in the body. This allows identification of the position of a stimulus on the basis of which receptor cells are sending information. The cerebral cortex also maintains this sensory topography in the particular areas of the cortex that correspond to the position of the receptor cells. The somatosensory cortex provides an example in which, in essence, the locations of the somatosensory receptors in the body are mapped onto the somatosensory cortex, which is located in the postcentral gyrus. This mapping is often depicted using a sensory homunculus (Figure 14.18).
The term homunculus refers to a map of the human body that is laid across a portion of the cerebral cortex. In the somatosensory cortex, the external genitals, feet, and lower legs are represented on the medial face of the gyrus within the longitudinal fissure. As the gyrus curves out of the fissure and along the surface of the parietal lobe, the body map continues through the thighs, hips, trunk, shoulders, arms, and hands. The head and face are just lateral to the fingers as the gyrus approaches the lateral sulcus.
The cortex has been described as having specific regions that are responsible for processing specific information; there is the visual cortex, somatosensory cortex, gustatory cortex, and so on. However, our experience of these senses is not divided. Instead, we experience what can be referred to as a seamless percept. Our perceptions of the various sensory modalities—though distinct in their content—are integrated by the brain so that we experience the world as a continuous whole.
In the cerebral cortex, sensory processing begins at the primary sensory cortex, then proceeds to an association area, and finally moves into a multimodal integration area. For example, the visual pathway projects from the retinae through the thalamus to the primary visual cortex in the occipital lobe. This area is primarily in the medial wall within the longitudinal fissure. Here, visual stimuli begin to be recognized as basic shapes. Edges of objects are recognized and built into more complex shapes. Also, inputs from both eyes are compared to extract depth information.
Figure 14.18 The Sensory Homunculus. A cartoon representation of the sensory homunculus is arranged adjacent to the cortical region in which the processing takes place.
Because of the overlapping field of view between the two eyes, the brain can begin to estimate the distance of stimuli based on binocular vision.
There are two main regions that surround the primary cortex that are usually referred to as areas V2 and V3 (the primary visual cortex is area V1). These surrounding areas are the visual association cortex. The visual association regions develop more complex visual perceptions by adding colour and motion information. The information processed in these areas is then sent to regions of the temporal and parietal lobes. Visual processing has two separate streams of processing: one into the temporal lobe and one into the parietal lobe. These are the ventral and dorsal streams, respectively (Figure 14.19).
Figure 14.19 Ventral and Dorsal Visual Streams. From the primary visual cortex in the occipital lobe, visual processing continues in two streams—one into the temporal lobe and one into the parietal lobe.
The ventral stream identifies visual stimuli and their significance. Because the ventral stream uses temporal lobe structures, it begins to interact with the nonvisual cortex and may be important in visual stimuli becoming part of memories. The dorsal stream locates objects in space and helps in guiding movements of the body in response to visual inputs. The dorsal stream enters the parietal lobe, where it interacts with somatosensory cortical areas that are important for our perception of the body and its movements. The dorsal stream can then influence frontal lobe activity, where motor functions originate.
14.3 Motor Responses
The defining characteristic of the somatic nervous system is that it controls skeletal muscles. Somatic senses inform the nervous system about the external environment, but the response to that is through voluntary muscle movement. The term voluntary suggests that there is a conscious decision to make a movement. However, some aspects of the somatic system use voluntary muscles without conscious control. One example is the ability of our breathing to switch to unconscious control while we are focused on another task. However, the muscles that are responsible for the basic process of breathing are also utilized for speech, which is entirely voluntary.
Cortical Responses
Let’s start with sensory stimuli that have been registered through receptor cells and the information relayed to the CNS along ascending pathways. In the cerebral cortex, the initial processing of sensory perception progresses to associative processing and then integration in multimodal areas of the cortex. These levels of processing can lead to the incorporation of sensory perceptions into memory, but more importantly, they lead to a response. The completion of cortical processing through the primary, associative, and integrative sensory areas initiates a similar progression of motor processing, usually in different cortical areas.
Whereas the sensory cortical areas are located in the occipital, temporal, and parietal lobes, motor functions are largely controlled by the frontal lobe (see Figure 13.2). The most anterior regions of the frontal lobe—the prefrontal areas—are important for executive functions, which are those cognitive functions that lead to goal-directed behaviours. These higher cognitive processes include working memory, which has been called a “mental scratch pad,” that can help organize and represent information that is not in the immediate environment. The prefrontal lobe is responsible for aspects of attention, such as inhibiting distracting thoughts and actions so that a person can focus on a goal and direct behaviour toward achieving that goal.
The functions of the prefrontal cortex are integral to the personality of an individual because it is largely responsible for what a person intends to do and how they accomplish those plans. A famous case of damage to the prefrontal cortex is that of Phineas Gage, dating back to 1848. He was a railroad worker who had a metal spike impale his prefrontal cortex (Figure 14.20). He survived the accident, but according to secondhand accounts, his personality changed drastically. Friends described him as no longer acting like himself. Whereas he was a hardworking, amiable man before the accident, he turned into an irritable, temperamental, and lazy man after the accident. Many of the accounts of his change may have been inflated in the retelling, and some behaviour was likely attributable to alcohol used as a pain medication. However, the accounts suggest that some aspects of his personality did change. Also, there is new evidence that, though his life changed dramatically, he was able to become a functioning stagecoach driver, suggesting that the brain has the ability to recover even from major trauma such as this.
Secondary Motor Cortices
In generating motor responses, the executive functions of the prefrontal cortex will need to initiate actual movements. One way to define the prefrontal area is any region of the frontal lobe that does not elicit movement when electrically stimulated. These are primarily in the anterior part of the frontal lobe. The regions of the frontal lobe that remain are the regions of the cortex that produce movement. The prefrontal areas project into the secondary motor cortices, which include the premotor cortex and the supplemental motor area.
Figure 14.20 Phineas Gage. The victim of an accident while working on a railroad in 1848, Phineas Gage had a large iron rod impaled through the prefrontal cortex of his frontal lobe. After the accident, his personality appeared to change, but he eventually learned to cope with the trauma and lived as a coach driver even after such a traumatic event. (Illustration (b) by John M. Harlow, MD.)
Two important regions that assist in planning and coordinating movements are located adjacent to the primary motor cortex. The premotor cortex is more lateral, whereas the supplemental motor area is more medial and superior. The premotor area aids in controlling movements of the core muscles to maintain posture during movement, whereas the supplemental motor area is hypothesized to be responsible for planning and coordinating movement. The supplemental motor area also manages sequential movements that are based on prior experience (i.e., learned movements). Neurons in these areas are most active leading up to the initiation of movement. For example, these areas might prepare the body for the movements necessary to drive a car in anticipation of a traffic light changing.
Adjacent to these two regions are two specialized motor planning centres. The frontal eye fields are responsible for moving the eyes in response to visual stimuli. There are direct connections between the frontal eye fields and the superior colliculus. Also, anterior to the premotor cortex and primary motor cortex is Broca’s area (which is found only in the left hemisphere in most people). This area is responsible for controlling movement of the structures of speech production. The area is named after a French surgeon and anatomist who studied patients who could not produce speech. They did not have impairments to understanding speech, only to producing speech sounds, suggesting a damaged or underdeveloped Broca’s area.
Primary Motor Cortex
The primary motor cortex is located in the precentral gyrus of the frontal lobe. This cortex receives input from several areas that aid in planning movement, and its principal output stimulates spinal cord neurons to stimulate skeletal muscle contraction.
The primary motor cortex is arranged in a similar fashion to the primary somatosensory cortex, in that it has a topographical map of the body, creating a motor homunculus. The neurons responsible for musculature in the feet and lower legs are in the medial wall of the precentral gyrus, with the thighs, trunk, and shoulder at the crest of the longitudinal fissure. The hand and face are in the lateral face of the gyrus. Also, the relative space allotted for the different regions is exaggerated in muscles that have greater innervation. The greatest amount of cortical space is given to muscles that perform fine, agile movements, such as the muscles of the fingers and the lower face. The “power muscles” that perform coarser movements, such as the buttock and back muscles, occupy much less space on the motor cortex.
Descending Pathways
The motor output from the cortex descends into the brain stem and to the spinal cord to control the musculature through motor neurons. Neurons located in the primary motor cortex, named Betz cells, are large cortical neurons that synapse with lower motor neurons in the brain stem or in the spinal cord. The two descending pathways travelled by the axons of Betz cells are the corticobulbar tract and the corticospinal tract, respectively. Both tracts are named for their origin in the cortex and their targets—either the brain stem (the term bulbar refers to the brain stem as the bulb, or enlargement, at the top of the spinal cord) or the spinal cord.
These two descending pathways are responsible for the conscious or voluntary movements of skeletal muscles. Any motor command from the primary motor cortex is sent down the axons of the Betz cells to activate lower motor neurons in either the cranial motor nuclei or in the ventral horn of the spinal cord. The axons of the corticobulbar tract are ipsilateral, meaning they project from the cortex to the motor nucleus on the same side of the nervous system. Conversely, the axons of the corticospinal tract are largely contralateral, meaning that they cross the midline of the brain stem or spinal cord and synapse on the opposite side of the body. Therefore, the right motor cortex of the cerebrum controls muscles on the left side of the body and vice versa.
The corticospinal tract descends from the cortex through the deep white matter of the cerebrum. It then passes between the caudate nucleus and putamen of the basal nuclei as a bundle called the internal capsule. The tract then passes through the midbrain as the cerebral peduncles, after which it burrows through the pons. Upon entering the medulla, the tracts make up the large white matter tract referred to as the pyramids (Figure 14.21). The defining landmark of the medullary-spinal border is the pyramidal decussation, which is where most of the fibres in the corticospinal tract cross over to the opposite side of the brain. At this point, the tract separates into two parts, which have control over different domains of the musculature.
Appendicular Control
The lateral corticospinal tract is composed of the fibres that cross the midline at the pyramidal decussation (see Figure 14.21). The axons cross over from the anterior position of the pyramids in the medulla to the lateral column of the spinal cord. These axons are responsible for controlling appendicular muscles.
Figure 14.21 Corticospinal Tract. The major descending tract that controls skeletal muscle movements is the corticospinal tract. It is composed of two neurons, the upper motor neuron and the lower motor neuron. The upper motor neuron has its cell body in the primary motor cortex of the frontal lobe and synapses on the lower motor neuron, which is in the ventral horn of the spinal cord and projects to the skeletal muscle in the periphery.
Axial Control
The anterior corticospinal tract is responsible for controlling the muscles of the trunk (see Figure 14.21). These axons do not decussate in the medulla. Instead, they remain in an anterior position as they descend the brain stem and enter the spinal cord. These axons then travel to the spinal cord level, where they synapse with a lower motor neuron. Upon reaching the appropriate level, the axons decussate, entering the ventral horn on the opposite side of the spinal cord from which they entered. In the ventral horn, these axons synapse with their corresponding lower motor neurons. The lower motor neurons are located in the medial regions of the ventral horn, as they control the axial muscles of the trunk.
Because movements of the trunk involve both sides of the body, the anterior corticospinal tract is not entirely contralateral. Some collateral branches of the tract will project into the ipsilateral ventral horn to control synergistic muscles on that side of the body or to inhibit antagonistic muscles through interneurons within the ventral horn.
Through the influence of both sides of the body, the anterior corticospinal tract can coordinate postural muscles in broad movements of the body. These coordinating axons in the anterior corticospinal tract are often considered bilateral, as they are both ipsilateral and contralateral.
Extrapyramidal Controls
Other descending connections between the brain and the spinal cord are called the extrapyramidal system. The name comes from the fact that this system is outside the corticospinal pathway, which includes the pyramids in the medulla. A few pathways originating from the brain stem contribute to this system.
The tectospinal tract projects from the midbrain to the spinal cord and is important for postural movements that are driven by the superior colliculus. The name of the tract comes from an alternate name for the superior colliculus, which is the tectum. The reticulospinal tract connects the reticular system, a diffuse region of grey matter in the brain stem, with the spinal cord. This tract influences trunk and proximal limb muscles related to posture and locomotion. The reticulospinal tract also contributes to muscle tone and influences autonomic functions. The vestibulospinal tract connects the brain stem nuclei of the vestibular system with the spinal cord. This allows posture, movement, and balance to be modulated on the basis of equilibrium information provided by the vestibular system.
The pathways of the extrapyramidal system are influenced by subcortical structures. For example, connections between the secondary motor cortices and the extrapyramidal system modulate spine and cranium movements. The basal nuclei, which are important for regulating movement initiated by the CNS, influence the extrapyramidal system as well as its thalamic feedback to the motor cortex.
The conscious movement of our muscles is more complicated than simply sending a single command from the precentral gyrus down to the proper motor neurons. During the movement of any body part, our muscles relay information back to the brain, and the brain is constantly sending “revised” instructions back to the muscles. The cerebellum is important in contributing to the motor system because it compares cerebral motor commands with proprioceptive feedback. The corticospinal fibres that project to the ventral horn of the spinal cord have branches that also synapse in the pons, which project to the cerebellum. Also, the proprioceptive sensations of the dorsal column system have a collateral projection to the medulla that projects to the cerebellum. These two streams of information are compared in the cerebellar cortex. Conflicts between the motor commands sent by the cerebrum and body position information provided by the proprioceptors cause the cerebellum to stimulate the red nucleus of the midbrain. The red nucleus then sends corrective commands to the spinal cord along the rubrospinal tract. The name of this tract comes from the word for “red” that is seen in the English word “ruby.”
A good example of how the cerebellum corrects cerebral motor commands can be illustrated by walking in water. An original motor command from the cerebrum to walk will result in a highly coordinated set of learned movements.
However, in water, the body cannot actually perform a typical walking movement as instructed. The cerebellum can alter the motor command, stimulating the leg muscles to take larger steps to overcome the water resistance.
The cerebellum can make the necessary changes through the rubrospinal tract. Modulating the basic command to walk also relies on spinal reflexes, but the cerebellum is responsible for calculating the appropriate response. When the cerebellum does not work properly, coordination and balance are severely affected. The most dramatic example of this is during the overconsumption of alcohol. Alcohol inhibits the ability of the cerebellum to interpret proprioceptive feedback, making it more difficult to coordinate body movements, such as walking a straight line or guiding the hand to touch the tip of the nose.
Ventral Horn Output
The somatic nervous system provides output strictly to skeletal muscles. The lower motor neurons, which are responsible for the contraction of these muscles (lack of this is called paresis), are found in the ventral horn of the spinal cord. These large, multipolar neurons have a corona of dendrites surrounding the cell body and an axon that extends out of the ventral horn. This axon travels through the ventral nerve root to join the emerging spinal nerve. The axon is relatively long because it needs to reach muscles in the periphery of the body. The diameters of cell bodies may be on the order of hundreds of micrometres to support the long axon; some axons are a metre in length, such as the lumbar motor neurons that innervate muscles in the first digits of the feet.
The axons will also branch to innervate multiple muscle fibres. Together, the motor neuron and all the muscle fibres that it controls make up a motor unit. Motor units vary in size. Some may contain up to 1,000 muscle fibres, such as in the quadriceps, or they may only have 10 fibres, such as in an extraocular muscle. The number of muscle fibres that are part of a motor unit corresponds to the precision of control of that muscle. Also, muscles that have finer motor control have more motor units connecting to them, and this requires a larger topographical field in the primary motor cortex.
Motor neuron axons connect to muscle fibres at a neuromuscular junction. This is a specialized synaptic structure at which multiple axon terminals synapse with the muscle fibre sarcolemma. The synaptic end bulbs of the motor neurons secrete acetylcholine, which binds to receptors on the sarcolemma. The binding of acetylcholine opens ligand-gated ion channels, increasing the movement of cations across the sarcolemma. This depolarizes the sarcolemma, initiating muscle contraction. Whereas other synapses result in graded potentials that must reach a threshold in the postsynaptic target, activity at the neuromuscular junction reliably leads to muscle fibre contraction with every nerve impulse received from a motor neuron. However, the strength of contraction and the number of fibres that contract can be affected by the frequency of the motor neuron impulses.
Reflexes
This chapter began by introducing reflexes as an example of the basic elements of the somatic nervous system. Simple somatic reflexes do not include the higher centres discussed for conscious or voluntary aspects of movement. Reflexes can be spinal or cranial, depending on the nerves and central components that are involved. The example described at the beginning of the chapter involved heat and pain sensations from a hot stove, causing withdrawal of the arm through a connection in the spinal cord that leads to contraction of the biceps brachii. The description of this withdrawal reflex was simplified, for the sake of the introduction, to emphasize the parts of the somatic nervous system. But to consider reflexes fully, more attention needs to be given to this example.
As you withdraw your hand from the stove, you do not want to slow that reflex down. As the biceps brachii contracts, the antagonistic triceps brachii needs to relax. Because the neuromuscular junction is strictly excitatory, the biceps will contract when the motor nerve is active. Skeletal muscles do not actively relax. Instead, the motor neuron needs to “quiet down,” or be inhibited. In the hot-stove-withdrawal reflex, this occurs through an interneuron in the spinal cord. The interneuron’s cell body is located in the dorsal horn of the spinal cord. The interneuron receives a synapse from the axon of the sensory neuron that detects that the hand is being burned. In response to this stimulation from the sensory neuron, the interneuron then inhibits the motor neuron that controls the triceps brachii. This is done by releasing a neurotransmitter or other signal that hyperpolarizes the motor neuron connected to the triceps brachii, making it less likely to initiate an action potential. With this motor neuron being inhibited, the triceps brachii relaxes. Without the antagonistic contraction, withdrawal from the hot stove is faster and prevents further tissue damage from occurring.
Another example of a withdrawal reflex occurs when you step on a painful stimulus, like a tack or a sharp rock. The nociceptors that are activated by the painful stimulus activate the motor neurons responsible for contraction of the tibialis anterior muscle. This causes dorsiflexion of the foot. An inhibitory interneuron, activated by a collateral branch of the nociceptor fibre, will inhibit the motor neurons of the gastrocnemius and soleus muscles to cancel plantar flexion. An important difference in this reflex is that plantar flexion is most likely in progress as the foot is pressing down onto the tack. Contraction of the tibialis anterior is not the most important aspect of the reflex, as continuation of plantar flexion will result in further damage from stepping onto the tack.
Another type of reflex is a stretch reflex. In this reflex, when a skeletal muscle is stretched, a muscle spindle receptor is activated. The axon from this receptor structure will cause direct contraction of the muscle. A collateral of the muscle spindle fibre will also inhibit the motor neuron of the antagonist muscles. The reflex helps maintain muscles at a constant length. A common example of this reflex is the knee jerk that is elicited by a rubber hammer struck against the patellar ligament in a physical exam.
A specialized reflex to protect the surface of the eye is the corneal reflex, or the eye blink reflex. When the cornea is stimulated by a tactile stimulus, or even by bright light in a related reflex, blinking is initiated. The sensory component travels through the trigeminal nerve, which carries somatosensory information from the face, or through the optic nerve, if the stimulus is bright light. The motor response travels through the facial nerve and innervates the orbicularis oculi on the same side. This reflex is commonly tested during a physical exam using an air puff or a gentle touch of a cotton-tipped applicator.
Key Terms
- alkaloid:
- Substance, usually from a plant source, that is chemically basic with respect to pH and will stimulate bitter receptors.
- amacrine cell:
- Type of cell in the retina that connects to the bipolar cells near the outer synaptic layer and provides the basis for early image processing within the retina.
- ampulla:
- In the ear, the structure at the base of a semicircular canal that contains the hair cells and cupula for transduction of rotational movement of the head.
- anterior corticospinal tract:
- Division of the corticospinal pathway that travels through the ventral (anterior) column of the spinal cord and controls axial musculature through the medial motor neurons in the ventral (anterior) horn.
- aqueous humour:
- Watery fluid that fills the anterior chamber containing the cornea, iris, ciliary body, and lens of the eye.
- ascending pathway:
- Fibre structure that relays sensory information from the periphery through the spinal cord and brain stem to other structures of the brain.
- association area:
- Region of cortex connected to a primary sensory cortical area that further processes the information to generate more complex sensory perceptions.
- audition:
- Sense of hearing.
- auricle:
- Fleshy external structure of the ear.
- basilar membrane:
- In the ear, the floor of the cochlear duct on which the organ of Corti sits.
- Betz cells:
- Output cells of the primary motor cortex that cause musculature to move through synapses on cranial and spinal motor neurons.
- binocular vision:
- Indications of the distance of visual stimuli on the basis of slight differences in the images projected onto either retina.
- bipolar cell:
- Cell type in the retina that connects the photoreceptors to the RGCs.
- Broca’s area:
- Region of the frontal lobe associated with the motor commands necessary for speech production.
- bulbous (Ruffini) corpuscles:
- Responsible for the detection of stretching of the skin, known as type II cutaneous mechanoreceptors.
- cerebral peduncles:
- Segments of the descending motor pathway that make up the white matter of the ventral midbrain.
- chemoreceptor:
- Sensory receptor cell that is sensitive to chemical stimuli, such as in taste, smell, or pain.
- choroid:
- Highly vascular tissue in the wall of the eye that supplies the outer retina with blood.
- ciliary body:
- Smooth muscle structure on the interior surface of the iris that controls the shape of the lens through the zonule fibres.
- circadian rhythm:
- Internal perception of the daily cycle of light and dark based on retinal activity related to sunlight.
- cochlea:
- Auditory portion of the inner ear containing structures to transduce sound stimuli.
- cochlear duct:
- Space within the auditory portion of the inner ear that contains the organ of Corti and is adjacent to the scala tympani and scala vestibuli on either side.
- cone photoreceptor:
- One of the two types of retinal receptor cell that is specialized for colour vision through the use of three photopigments distributed through three separate populations of cells.
- contralateral:
- Word meaning “on the opposite side,” as in axons that cross the midline in a fibre tract.
- cornea:
- Fibrous covering of the anterior region of the eye that is transparent so that light can pass through it.
- corneal reflex:
- Protective response to stimulation of the cornea that causes contraction of the orbicularis oculi muscle, resulting in blinking of the eye.
- corticobulbar tract:
- Connection between the cortex and the brain stem responsible for generating movement.
- corticospinal tract:
- Connection between the cortex and the spinal cord responsible for generating movement.
- cupula:
- Specialized structure within the base of a semicircular canal that bends the stereocilia of hair cells when the head rotates by way of the relative movement of the enclosed fluid.
- dorsal column system:
- Ascending tract of the spinal cord associated with fine touch and proprioceptive sensations.
- dorsal stream:
- Connections between cortical areas from the occipital to parietal lobes that are responsible for the perception of visual motion and guiding movement of the body in relation to that motion.
- encapsulated corpuscle ending:
- Configuration of a sensory receptor neuron with dendrites surrounded by specialized structures to aid in transduction of a particular type of sensation, such as the lamellated corpuscles in the deep dermis and subcutaneous tissue.
- equilibrium:
- Sense of balance that includes sensations of position and movement of the head.
- Eustachian tube:
- Tube connecting the pharynx with the middle ear, which helps equilibrate air pressure across the tympanic membrane; it is normally closed but will pop open when the muscles of the pharynx contract during swallowing or yawning.
- executive functions:
- Cognitive processes of the prefrontal cortex that lead to directing goal-directed behaviour, which is a precursor to executing motor commands.
- external ear:
- Structures on the lateral surface of the head, including the auricle and the ear canal up to the tympanic membrane.
- exteroceptor:
- Sensory receptor that is positioned to interpret stimuli from the external environment, such as photoreceptors in the eye or somatosensory receptors in the skin.
- extrapyramidal system:
- Pathways between the brain and spinal cord that are separate from the corticospinal tract and are responsible for modulating the movements generated through that primary pathway.
- fibrous tunic:
- Outer layer of the eye primarily composed of connective tissue known as the sclera and cornea.
- fovea:
- Exact centre of the retina at which visual stimuli are focused for maximal acuity, where the retina is thinnest and there is nothing but photoreceptors.
- free nerve ending:
- Configuration of a sensory receptor neuron with dendrites in the connective tissue of the organ, such as in the dermis of the skin, that are most often sensitive to chemical, thermal, and mechanical stimuli.
- frontal eye fields:
- Area of the prefrontal cortex responsible for moving the eyes to attend to visual stimuli.
- general sense:
- Any sensory system that is distributed throughout the body and incorporated into organs of multiple other systems, such as the walls of the digestive organs or the skin.
- gustation:
- Sense of taste.
- gustatory receptor cells:
- Sensory cells in the taste bud that transduce the chemical stimuli of gustation.
- hair cells:
- Mechanoreceptor cells found in the inner ear that transduce stimuli for the senses of hearing and balance.
- hair follicle plexus:
- Follicles are also wrapped in a plexus of nerve endings—detect the movement of hair at the surface of the skin, such as when an insect may be walking along the skin.
- incus (also anvil):
- Ossicle of the middle ear that connects the malleus to the stapes.
- inner ear:
- Structure within the temporal bone that contains the sensory apparati of hearing and balance.
- inner segment:
- In the eye, the section of a photoreceptor that contains the nucleus and other major organelles for normal cellular functions.
- internal capsule:
- Segment of the descending motor pathway that passes between the caudate nucleus and the putamen.
- interoceptor:
- Sensory receptor that is positioned to interpret stimuli from internal organs, such as stretch receptors in the wall of blood vessels.
- ipsilateral:
- Word meaning on the same side, as in axons that do not cross the midline in a fibre tract.
- iris:
- Coloured portion of the anterior eye that surrounds the pupil.
- kinesthesia:
- Sense of body movement based on sensations in skeletal muscles, tendons, joints, and the skin.
- lacrimal glands:
- Are lateral to the orbit that produces tears to wash across the surface of the eye.
- lamellated (Pacinian) corpuscles:
- Responsible for the detection of deep pressure and vibration.
- lateral corticospinal tract:
- Division of the corticospinal pathway that travels through the lateral column of the spinal cord and controls appendicular musculature through the lateral motor neurons in the ventral (anterior) horn.
- lens:
- Component of the eye that focuses light on the retina.
- lower motor neurons:
- Found in the ventral horn of the spinal cord and are responsible for the contraction of these muscles.
- macula:
- Enlargement at the base of a semicircular canal at which transduction of equilibrium stimuli takes place within the ampulla.
- malleus (also hammer):
- Ossicle that is directly attached to the tympanic membrane.
- mechanoreceptor:
- Receptor cell that transduces mechanical stimuli into an electrochemical signal.
- middle ear:
- Space within the temporal bone between the ear canal and bony labyrinth where the ossicles amplify sound waves from the tympanic membrane to the oval window.
- multimodal integration area:
- Region of the cerebral cortex in which information from more than one sensory modality is processed to arrive at higher-level cortical functions such as memory, learning, or cognition.
- muscle spindles:
- Stretch receptors in the skeletal muscle tissue.
- neural tunic:
- Layer of the eye that contains nervous tissue, namely, the retina.
- nociceptor:
- Receptor cell that senses pain stimuli.
- odorant molecules:
- Volatile chemicals that bind to receptor proteins in olfactory neurons to stimulate the sense of smell.
- olfaction:
- Sense of smell.
- olfactory bulb:
- Central target of the first cranial nerve; located on the ventral surface of the frontal lobe in the cerebrum.
- olfactory cortex:
- That is located in the inferior and medial areas of the temporal lobe.
- olfactory epithelium:
- Region of the nasal epithelium where olfactory neurons are located.
- olfactory hairs:
- Dendrites of the olfactory sensory neuron located in the apical surface of the epithelium into the mucus lining the cavity with role in detection of odorant molecules dissolved in the mucus.
- olfactory nerves:
- Group of axons that connect to the olfactory bulb.
- olfactory sensory neuron:
- Receptor cell of the olfactory system, sensitive to the chemical stimuli of smell, the axons of which compose the first cranial nerve.
- olfactory tract:
- Narrow band of nerve fibres in the brain that connects the olfactory bulb to the primary olfactory cortex.
- opsin:
- Protein that contains the photosensitive cofactor retinal for phototransduction.
- optic chiasm:
- Decussation point in the visual system at which medial retina fibres cross to the other side of the brain.
- optic disc:
- Spot on the retina at which RGC axons leave the eye and blood vessels of the inner retina pass.
- optic nerve:
- Second cranial nerve, which is responsible for visual sensation.
- organ of Corti:
- Structure in the cochlea in which hair cells transduce movements from sound waves into electrochemical signals.
- osmoreceptor:
- Receptor cell that senses differences in the concentrations of body fluids on the basis of osmotic pressure.
- ossicles:
- Three small bones in the middle ear.
- otolith:
- Layer of calcium carbonate crystals located on top of the otolithic membrane.
- otolithic membrane:
- Gelatinous substance in the utricle and saccule of the inner ear that contains calcium carbonate crystals and into which the stereocilia of hair cells are embedded.
- outer segment:
- In the eye, the section of a photoreceptor that contains opsin molecules that transduce light stimuli.
- oval window:
- Membrane at the base of the cochlea where the stapes attaches, marking the beginning of the scala vestibuli.
- papilla:
- For gustation, a bumplike projection on the surface of the tongue that contains taste buds.
- photoreceptor:
- Receptor cell specialized to respond to light stimuli.
- premotor cortex:
- Cortical area anterior to the primary motor cortex that is responsible for planning movements.
- primary sensory cortex:
- Region of the cerebral cortex that initially receives sensory input from an ascending pathway from the thalamus and begins the processing that will result in conscious perception of that modality.
- proprioception:
- Sense of position and movement of the body.
- proprioceptor:
- Receptor cell that senses changes in the position and kinesthetic aspects of the body.
- pupil:
- Open hole at the centre of the iris that light passes through into the eye.
- pyramidal decussation:
- Location at which corticospinal tract fibres cross the midline and segregate into the anterior and lateral divisions of the pathway.
- pyramids:
- Segment of the descending motor pathway that travels in the anterior position of the medulla.
- red nucleus:
- Midbrain nucleus that sends corrective commands to the spinal cord along the rubrospinal tract, based on the disparity between an original command and the sensory feedback from movement.
- reticulospinal tract:
- Extrapyramidal connections between the brain stem and spinal cord that modulate movement, contribute to posture, and regulate muscle tone.
- retina:
- Nervous tissue of the eye at which phototransduction takes place.
- retinal ganglion cell (RGC):
- Neuron of the retina that projects along the second cranial nerve.
- rhodopsin:
- Photopigment molecule found in the rod photoreceptors.
- rod photoreceptor:
- One of the two types of retinal receptor cell that is specialized for low-light vision.
- round window:
- Membrane that marks the end of the scala tympani.
- rubrospinal tract:
- Descending motor control pathway, originating in the red nucleus, that mediates control of the limbs on the basis of cerebellar processing.
- saccule:
- Structure of the inner ear responsible for transducing linear acceleration in the vertical plane.
- scala tympani:
- Portion of the cochlea that extends from the apex to the round window.
- scala vestibuli:
- Portion of the cochlea that extends from the oval window to the apex.
- sclera:
- White of the eye.
- semicircular canals:
- Structures within the inner ear responsible for transducing rotational movement information.
- sensory homunculus:
- Topographic representation of the body within the somatosensory cortex demonstrating the correspondence between neurons processing stimuli and sensitivity.
- sensory modality:
- A particular system for interpreting and perceiving environmental stimuli by the nervous system.
- solitary nucleus:
- Medullar nucleus that receives taste information from the facial and glossopharyngeal nerves.
- somatosensation:
- General sense associated with modalities lumped together as touch.
- specialized receptor cell:
- Cell that transduces environmental stimuli into neural signals.
- special sense:
- Any sensory system associated with a specific organ structure, namely, smell, taste, sight, hearing, and balance.
- spinothalamic tract:
- Ascending tract of the spinal cord associated with pain and temperature sensations.
- spiral ganglion:
- Location of neuronal cell bodies that transmit auditory information along the eighth cranial nerve.
- stapes (also stirrup):
- Ossicle of the middle ear that is attached to the inner ear.
- stereocilia:
- Array of apical membrane extensions in a hair cell that transduce movements when they are bent.
- stretch receptors:
- Monitor the stretching of tendons, muscles, and the components of joints.
- stretch reflex:
- Response to activation of the muscle spindle stretch receptor that causes contraction of the muscle to maintain a constant length.
- submodality:
- Specific sense within a broader major sense, such as sweet as a part of the sense of taste or colour as a part of vision.
- superior colliculus:
- Structure in the midbrain that combines visual, auditory, and somatosensory input to coordinate spatial and topographic representations of the three sensory systems.
- supplemental motor area:
- Cortical area anterior to the primary motor cortex that is responsible for planning movements.
- taste buds:
- Structures within a papilla on the tongue that contain gustatory receptor cells.
- tectorial membrane:
- Component of the organ of Corti that lies over the hair cells, into which the stereocilia are embedded.
- tectospinal tract:
- Extrapyramidal connections between the superior colliculus and spinal cord.
- tendon (Golgi) stretch organs:
- Transduce the stretch levels of tendons.
- thermoreceptor:
- Sensory receptor specialized for temperature stimuli.
- topographical:
- Relating to positional information.
- transduction:
- Process of changing an environmental stimulus into the electrochemical signals of the nervous system.
- tympanic membrane:
- Ear drum.
- utricle:
- Structure of the inner ear responsible for transducing linear acceleration in the horizontal plane.
- vascular tunic:
- Middle layer of the eye primarily composed of connective tissue with a rich blood supply.
- ventral stream:
- Connections between cortical areas from the occipital lobe to the temporal lobe that are responsible for the identification of visual stimuli.
- vestibular apparatus:
- Component of the vestibular system that contains the hair cell with stereocilia responsible for senses, head position, head movement, and whether our bodies are in motion.
- vestibular ganglion:
- Location of neuronal cell bodies that transmit equilibrium information along the eighth cranial nerve.
- vestibular nuclei:
- Targets of the vestibular component of the eighth cranial nerve.
- vestibule:
- In the ear, the portion of the inner ear responsible for the sense of equilibrium.
- vestibulo-ocular reflex (VOR):
- Reflex based on connections between the vestibular system and the cranial nerves of eye movements that ensures images are stabilized on the retina as the head and body move.
- vestibulospinal tract:
- Extrapyramidal connections between the vestibular nuclei in the brain stem and spinal cord that modulate movement and contribute to balance on the basis of the sense of equilibrium.
- visceral sense:
- Sense associated with the internal organs.
- vision:
- Special sense of sight based on the transduction of light stimuli.
- visual acuity:
- Property of vision related to the sharpness of focus, which varies in relation to retinal position.
- vitreous humour:
- Viscous fluid that fills the posterior chamber of the eye.
- withdrawal reflex:
- From a painful stimulus only requires the sensory fibre that enters the spinal cord and the motor neuron that projects to a muscle.
- working memory:
- Function of the prefrontal cortex to maintain a representation of information that is not in the immediate environment.
- zonule fibres:
- Fibrous connections between the ciliary body and the lens.
Chapter Review
14.1 Sensory Perception
The senses are olfaction (smell), gustation (taste), somatosensation (sensations associated with the skin and body), audition (hearing), equilibrium (balance), and vision. With the exception of somatosensation, this list represents the special senses, or those systems of the body that are associated with specific organs such as the tongue or eye. Somatosensation belongs to the general senses, which are those sensory structures that are distributed throughout the body and in the walls of various organs. The special senses are all primarily part of the somatic nervous system in that they are consciously perceived through cerebral processes, though some special senses contribute to autonomic function. The general senses can be divided into somatosensation, which is commonly considered touch, but includes tactile, pressure, vibration, temperature, and pain perception. The general senses also include the visceral senses, which are separate from the somatic nervous system function in that they do not normally rise to the level of conscious perception.
The cells that transduce sensory stimuli into the electrochemical signals of the nervous system are classified on the basis of structural or functional aspects of the cells. The structural classifications are based on either the anatomy of the cell that is interacting with the stimulus (free nerve endings, encapsulated endings, or specialized receptor cells) or where the cell is located relative to the stimulus (interoceptor, exteroceptor, or proprioceptor). Third, the functional classification is based on how the cell transduces the stimulus into a neural signal.
Chemoreceptors respond to chemical stimuli and are the basis for olfaction and gustation. Related to chemoreceptors are osmoreceptors and nociceptors for fluid balance and pain reception, respectively. Mechanoreceptors respond to mechanical stimuli and are the basis for most aspects of somatosensation, as well as being the basis of audition and equilibrium in the inner ear. Thermoreceptors are sensitive to temperature changes, and photoreceptors are sensitive to light energy.
The nerves that convey sensory information from the periphery to the CNS are either spinal nerves, connected to the spinal cord, or cranial nerves, connected to the brain. Spinal nerves have mixed populations of fibres; some are motor fibres and some are sensory. The sensory fibres connect to the spinal cord through the dorsal root, which is attached to the dorsal root ganglion. Sensory information from the body that is conveyed through spinal nerves will project to the opposite side of the brain to be processed by the cerebral cortex. The cranial nerves can be strictly sensory fibres, such as the olfactory, optic, and vestibulocochlear nerves, or mixed sensory and motor nerves, such as the trigeminal, facial, glossopharyngeal, and vagus nerves. The cranial nerves are connected to the same side of the brain from which the sensory information originates.
14.2 Central Processing
Sensory input to the brain enters through pathways that travel through either the spinal cord (for somatosensory input from the body) or the brain stem (for everything else, except the visual and olfactory systems) to reach the diencephalon. In the diencephalon, sensory pathways reach the thalamus. This is necessary for all sensory systems to reach the cerebral cortex, except for the olfactory system, which is directly connected to the frontal and temporal lobes.
The two major tracts in the spinal cord, originating from sensory neurons in the dorsal root ganglia, are the dorsal column system and the spinothalamic tract. The major differences between the two are in the type of information that is relayed to the brain and where the tracts decussate. The dorsal column system primarily carries information about touch and proprioception and crosses the midline in the medulla. The spinothalamic tract is primarily responsible for pain and temperature sensation and crosses the midline in the spinal cord at the level at which it enters. The trigeminal nerve adds similar sensation information from the head to these pathways.
The auditory pathway passes through multiple nuclei in the brain stem, in which additional information is extracted from the basic frequency stimuli processed by the cochlea. Sound localization is made possible through the activity of these brain stem structures. The vestibular system enters the brain stem and influences activity in the cerebellum, spinal cord, and cerebral cortex.
The visual pathway segregates information from the two eyes so that one-half of the visual field projects to the other side of the brain. Within visual cortical areas, the perception of the stimuli and their location is passed along two streams, one ventral and one dorsal. The ventral visual stream connects to structures in the temporal lobe that are important for long-term memory formation. The dorsal visual stream interacts with the somatosensory cortex in the parietal lobe, and together they can influence the activity in the frontal lobe to generate movements of the body in relation to visual information.
14.3 Motor Responses
The motor components of the somatic nervous system begin with the frontal lobe of the brain, where the prefrontal cortex is responsible for higher functions such as working memory. The integrative and associate functions of the prefrontal lobe feed into the secondary motor areas, which help plan movements. The premotor cortex and supplemental motor area then feed into the primary motor cortex, which initiates movements. Large Betz cells project through the corticobulbar and corticospinal tracts to synapse on lower motor neurons in the brain stem and ventral horn of the spinal cord, respectively. These connections are responsible for generating movements of skeletal muscles.
The extrapyramidal system includes projections from the brain stem and higher centres that influence movement, mostly to maintain balance and posture, as well as to maintain muscle tone. The superior colliculus and red nucleus in the midbrain, the vestibular nuclei in the medulla, and the reticular formation throughout the brain stem each have tracts projecting to the spinal cord in this system. Descending inputs from the secondary motor cortices, basal nuclei, and cerebellum connect to the origins of these tracts in the brain stem.
All these motor pathways project to the spinal cord to synapse with motor neurons in the ventral horn of the spinal cord. These lower motor neurons are the cells that connect to skeletal muscle and cause contractions. These neurons project through the spinal nerves to connect to the muscles at neuromuscular junctions. One motor neuron connects to multiple muscle fibres within a target muscle. The number of fibres that are innervated by a single motor neuron varies on the basis of the precision necessary for that muscle and the amount of force necessary for that motor unit. The quadriceps, for example, have many fibres controlled by single motor neurons for powerful contractions that do not need to be precise.
Reflexes are the simplest circuits within the somatic nervous system. A withdrawal reflex from a painful stimulus only requires the sensory fibre that enters the spinal cord and the motor neuron that projects to a muscle. Antagonist and postural muscles can be coordinated with the withdrawal, making the connections more complex. The simple, single neuronal connection is the basis of somatic reflexes. The corneal reflex is a contraction of the orbicularis oculi muscle to blink the eyelid when something touches the surface of the eye. Stretch reflexes maintain a constant length of muscles by causing a contraction of a muscle to compensate for a stretch that can be sensed by a specialized receptor called a muscle spindle.