Chapter15 The Autonomic Nervous System
The autonomic nervous system is often associated with the “fight-or-flight response,” which refers to the preparation of the body to either run away from a threat or to stand and fight in the face of that threat. The autonomic nervous system is not just about responding to threats. Besides the fight-or-flight response, there are the responses referred to as “rest and digest.” If that lioness is successful in her hunting, then she is going to rest from the exertion. Her heart rate will slow. Breathing will return to normal. The digestive system has a big job to do. Much of the function of the autonomic system is based on the connections within an autonomic, or visceral, reflex.
15.1 Divisions of the Autonomic Nervous System
The nervous system can be divided into two functional parts: the somatic nervous system and the autonomic nervous system. The major differences between the two systems are evident in the responses that each produces. The somatic nervous system causes contraction of skeletal muscles. The autonomic nervous system controls cardiac and smooth muscle, as well as glandular tissue. The somatic nervous system is associated with voluntary responses (though many can happen without conscious awareness, like breathing), and the autonomic nervous system is associated with involuntary responses, such as those related to homeostasis.
The autonomic nervous system regulates many of the internal organs through a balance of two aspects, or divisions. In addition to the endocrine system, the autonomic nervous system is instrumental in homeostatic mechanisms in the body. The two divisions of the autonomic nervous system are the sympathetic division and the parasympathetic division. The sympathetic system is associated with the fight-or-flight response, and parasympathetic activity is referred to by the epithet of rest and digest. Homeostasis is the balance between the two systems. At each target effector, dual innervation determines activity. For example, the heart receives connections from both the sympathetic and parasympathetic divisions. One causes heart rate to increase, whereas the other causes heart rate to decrease.
Sympathetic Division of the Autonomic Nervous System
To respond to a threat—to fight or to run away—the sympathetic system causes divergent effects as many different effector organs are activated together for a common purpose. More oxygen needs to be inhaled and delivered to skeletal muscle. The respiratory, cardiovascular, and musculoskeletal systems are all activated together.
Additionally, sweating keeps the excess heat that comes from muscle contraction from causing the body to overheat. The digestive system slows down so that blood is not absorbing nutrients when it should be delivering oxygen to skeletal muscles. To coordinate all these responses, the connections in the sympathetic system diverge from a limited region of the central nervous system (CNS) to a wide array of ganglia that project to the many effector organs simultaneously. The complex set of structures that compose the output of the sympathetic system makes it possible for these disparate effectors to come together in a coordinated, systemic change.
The sympathetic division of the autonomic nervous system influences the various organ systems of the body through connections emerging from the thoracic and upper lumbar spinal cord. It is referred to as the thoracolumbar system to reflect this anatomical basis. A central neuron in the lateral horn of any of these spinal regions projects to ganglia adjacent to the vertebral column through the ventral spinal roots. The majority of ganglia of the sympathetic system belong to a network of sympathetic chain ganglia that runs alongside the vertebral column.
The ganglia appear as a series of clusters of neurons linked by axonal bridges. There are typically 23 ganglia in the chain on either side of the spinal column. Three correspond to the cervical region, 12 are in the thoracic region, four are in the lumbar region, and four correspond to the sacral region. The cervical and sacral levels are not connected to the spinal cord directly through the spinal roots, but through ascending or descending connections through the bridges within the chain.
A diagram that shows the connections of the sympathetic system is somewhat like a circuit diagram that shows the electrical connections between different receptacles and devices. In Figure 15.1, the “circuits” of the sympathetic system are intentionally simplified.
To continue with the analogy of the circuit diagram, there are three different types of “junctions” that operate within the sympathetic system (Figure 15.2). The first type is most direct: the sympathetic nerve projects to the chain ganglion at the same level as the target effector (the organ, tissue, or gland to be innervated). An example of this type is spinal nerve T1 that synapses with the T1 chain ganglion to innervate the trachea. The fibres of this branch are called white rami communicantes (singular = ramus communicans); they are myelinated and therefore referred to as white (see Figure 15.2a). The axon from the central neuron (the preganglionic fibre shown as a solid line) synapses with the ganglionic neuron (with the postganglionic fibre shown as a dashed line). This neuron then projects to a target effector—in this case, the trachea—via grey rami communicantes, which are unmyelinated axons.
In some cases, the target effectors are located superior or inferior to the spinal segment at which the preganglionic fibre emerges. With respect to the “wiring” involved, the synapse with the ganglionic neuron occurs at chain ganglia superior or inferior to the location of the central neuron. An example of this is spinal nerve T1, which innervates the eye. The spinal nerve tracks up through the chain until it reaches the superior cervical ganglion, where it synapses with the postganglionic neuron (see Figure 15.2b). The cervical ganglia are referred to as paravertebral ganglia, given their location adjacent to prevertebral ganglia in the sympathetic chain.
Not all axons from the central neurons terminate in the chain ganglia. Additional branches from the ventral nerve root continue through the chain and on to one of the collateral ganglia as the greater splanchnic nerve or lesser splanchnic nerve. For example, the greater splanchnic nerve at the level of T5 synapses with a collateral ganglion outside the chain before making the connection to the postganglionic nerves that innervate the stomach (see Figure 15.2c).
Collateral ganglia, also called prevertebral ganglia, are situated anterior to the vertebral column and receive inputs from splanchnic nerves as well as central sympathetic neurons. They are associated with controlling organs in the abdominal cavity and are also considered part of the enteric nervous system. The three collateral ganglia are the celiac ganglion, the superiormesenteric ganglion, and the inferior mesenteric ganglion (see Figure 15.1). The word celiac refers to a body cavity (in this case, the abdominal cavity), and the word mesenteric refers to the digestive system.
Figure 15.1 Connections of the Sympathetic Division of the Autonomic Nervous System. Neurons from the lateral horn of the spinal cord (preganglionic nerve fibres—solid lines) project to the chain ganglia on either side of the vertebral column or to collateral (prevertebral) ganglia that are anterior to the vertebral column in the abdominal cavity. Axons from these ganglionic neurons (postganglionic nerve fibres—dotted lines) then project to target effectors throughout the body.
Figure 15.2 Sympathetic Connections and Chain Ganglia. The axon from a central sympathetic neuron in the spinal cord can project to the periphery in a number of different ways. (a) The fibre can project out to the ganglion at the same level and synapse on a ganglionic neuron. (b) A branch can project to a more superior or inferior ganglion in the chain. (c) A branch can project through the white ramus communicans but cannot terminate on a ganglionic neuron in the chain. Instead, it projects through one of the splanchnic nerves to a collateral ganglion or the adrenal medulla (not pictured).
An axon from the central neuron that projects to a sympathetic ganglion is referred to as a preganglionic fibre or neuron and represents the output from the CNS to the ganglion. Because the sympathetic ganglia are adjacent to the vertebral column, preganglionic sympathetic fibres are relatively short, and they are myelinated. A postganglionic fibre—the axon from a ganglionic neuron that projects to the target effector—represents the output of a ganglion that directly influences the organ. Compared with the preganglionic fibres, postganglionic sympathetic fibres are long because of the relatively greater distance from the ganglion to the target effector. These fibres are unmyelinated. (Note that the term postganglionic neuron may be used to describe the projection from a ganglion to the target. The problem with that usage is that the cell body is in the ganglion, and only the fibre is postganglionic. Typically, the term neuron applies to the entire cell.)
One type of preganglionic sympathetic fibre does not terminate in a ganglion. These are the axons from central sympathetic neurons that project to the adrenal medulla, the interior portion of the adrenal gland. These axons are still referred to as preganglionic fibres, but the target is not a ganglion. The adrenal medulla releases signalling molecules into the bloodstream, rather than using axons to communicate with target structures. The cells in the adrenal medulla that are contacted by the preganglionic fibres are called chromaffin cells. These cells are neurosecretory cells that develop from the neural crest along with the sympathetic ganglia, reinforcing the idea that the gland is, functionally, a sympathetic ganglion.
The projections of the sympathetic division of the autonomic nervous system diverge widely, resulting in a broad influence of the system throughout the body. As a response to a threat, the sympathetic system would increase heart rate and breathing rate, cause blood flow to the skeletal muscle to increase, and cause blood flow to the digestive system to decrease. Sweat gland secretion should also increase as part of an integrated response. All those physiological changes are going to be required to occur together to run away from the hunting lioness, or the modern equivalent. This divergence is seen in the branching patterns of preganglionic sympathetic neurons—a single preganglionic sympathetic neuron may have 10–20 targets. An axon that leaves a central neuron of the lateral horn in the thoracolumbar spinal cord will pass through the white ramus communicans and enter the sympathetic chain, where it will branch toward a variety of targets. At the level of the spinal cord at which the preganglionic sympathetic fibre exits the spinal cord, a branch will synapse on a neuron in the adjacent chain ganglion. Some branches will extend up or down to a different level of the chain ganglia. Other branches will pass through the chain ganglia and project through one of the splanchnic nerves to a collateral ganglion. Finally, some branches may project through the splanchnic nerves to the adrenal medulla. All these branches mean that one preganglionic neuron can influence different regions of the sympathetic system very broadly, by acting on widely distributed organs.
Parasympathetic Division of the Autonomic Nervous System
The parasympathetic system can also be referred to as the craniosacral system (or outflow) because the preganglionic neurons are located in nuclei of the brain stem and the lateral horn of the sacral spinal cord.
The connections, or “circuits,” of the parasympathetic division are similar to the general layout of the sympathetic division, with a few specific differences (Figure 15.3). The preganglionic fibres from the cranial region travel in cranial nerves, whereas the preganglionic fibres from the sacral region travel in spinal nerves. The targets of these fibres are terminal ganglia, which are located near—or even within—the target effector. These ganglia are often referred to as intramural ganglia when they are found within the walls of the target organ. The postganglionic fibre projects from the terminal ganglia a short distance to the target effector, or to the specific target tissue within the organ.
Comparing the relative lengths of axons in the parasympathetic system, the preganglionic fibres are long and the postganglionic fibres are short because the ganglia are close to—and sometimes within—the target effectors.
Chemical Signalling in the Autonomic Nervous System
Where an autonomic neuron connects with a target, there is a synapse. The electrical signal of the action potential causes the release of a signalling molecule, which will bind to receptor proteins on the target cell. Synapses of the autonomic system are classified as either cholinergic, meaning that acetylcholine (ACh) is released, or adrenergic, meaning that norepinephrine is released. The terms cholinergic and adrenergic refer not only to the signalling molecule that is released but also to the class of receptors that each binds.
The cholinergic system includes two classes of receptors: the nicotinic receptor and the muscarinic receptor. Both receptor types bind to ACh and cause changes in the target cell. The receptors are named for, and differentiated by, other molecules that bind to them. Whereas nicotine will bind to the nicotinic receptor, and muscarine will bind to the muscarinic receptor, there is no cross-reactivity between the receptors. These molecules are exogenous, meaning that they are made outside of the human body, so a researcher can use them without any confounding endogenous results (results caused by the molecules produced in the body).
Figure 15.3 Connections of the Parasympathetic Division of the Autonomic Nervous System. Neurons from brain stem nuclei, or from the lateral horn of the sacral spinal cord, project to terminal ganglia near or within the various organs of the body. Axons from these ganglionic neurons then project short distances to those target effectors.
The adrenergic system also has two types of receptors, named the alpha-adrenergic (α-adrenergic) receptor and beta-adrenergic (β-adrenergic) receptor. Unlike cholinergic receptors, these receptor types are not classified by which drugs can bind to them. All of them are G protein–coupled receptors. There are two types of α-adrenergic receptors, termed α1 and α2, and there are three types of β-adrenergic receptors, termed β1, β2, and β3. An additional aspect of the adrenergic system is that there is a second signalling molecule called epinephrine. The chemical difference between norepinephrine and epinephrine is the addition of a methyl group (CH3) in epinephrine.
The term adrenergic should remind you of the word adrenaline, which is associated with the fight-or-flight response described at the beginning of the chapter. Adrenaline and epinephrine are two names for the same molecule. The adrenal gland secretes adrenaline. Similarly, norepinephrine and noradrenaline are two names for the same molecule.
Having understood the cholinergic and adrenergic systems, their role in the autonomic system is relatively simple to understand. All preganglionic fibres, both sympathetic and parasympathetic, release ACh. All ganglionic neurons—the targets of these preganglionic fibres—have nicotinic receptors in their cell membranes. The nicotinic receptor is a ligand-gated cation channel that results in depolarization of the postsynaptic membrane. The postganglionic parasympathetic fibres also release ACh, but the receptors on their targets are muscarinic receptors, which are G protein–coupled receptors and do not exclusively cause depolarization of the postsynaptic membrane. Postganglionic sympathetic fibres release norepinephrine, except for fibres that project to sweat glands and to blood vessels associated with skeletal muscles, which release ACh (Table 15.1).
Sympathetic | Parasympathetic | |
|---|---|---|
Preganglionic | Acetylcholine → nicotinic receptor | Acetylcholine → nicotinic receptor |
Postganglionic | Norepinephrine → α- or β-adrenergic receptors Acetylcholine → muscarinic receptor (associated with sweat glands and the blood vessels within skeletal muscles) | Acetylcholine → muscarinic receptor |
Signalling molecules can belong to two broad groups. Neurotransmitters are released at synapses, whereas hormones are released into the bloodstream. These are simplistic definitions, but they can help clarify this point. Acetylcholine can be considered a neurotransmitter because it is released by axons at synapses. The adrenergic system, however, presents a challenge. Postganglionic sympathetic fibres release norepinephrine, which can be considered a neurotransmitter. But the adrenal medulla releases epinephrine and norepinephrine into circulation, so they should be considered hormones.
What are referred to here as synapses may not fit the strictest definition of a synapse. Some sources will refer to the connection between a postganglionic fibre and a target effector as neuroeffector junctions; neurotransmitters, as defined above, would be called neuromodulators. The structure of postganglionic connections is not the typical synaptic end bulb that is found at the neuromuscular junction; rather, they are chains of swellings along the length of a postganglionic fibre called a varicosity (Figure 15.4).
Figure 15.4 Autonomic Varicosities. The connection between autonomic fibres and target effectors is not the same as the typical synapse, such as the neuromuscular junction. Instead of a synaptic end bulb, a neurotransmitter is released from swellings along the length of a fibre that makes an extended network of connections in the target effector.
15.2 Autonomic Reflexes and Homeostasis
The autonomic nervous system regulates organ systems through circuits that resemble the reflexes described in the somatic nervous system. The main difference between the somatic and autonomic systems is which target tissues are the effectors. Somatic responses are solely based on skeletal muscle contraction. The autonomic system, however, targets cardiac and smooth muscle, as well as glandular tissue. Whereas the basic circuit is a reflex arc, there are differences in the structure of those reflexes for the somatic and autonomic systems.
The Structure of Reflexes
One difference between a somatic reflex, such as the withdrawal reflex, and a visceral reflex, which is an autonomic reflex, is in the efferent branch. The output of a somatic reflex is the lower motor neuron in the ventral horn of the spinal cord that projects directly to a skeletal muscle to cause its contraction. The output of a visceral reflex is a two-step pathway starting with the preganglionic fibre emerging from a lateral horn neuron in the spinal cord, or a cranial nucleus neuron in the brain stem, to a ganglion—followed by the postganglionic fibre projecting to a target effector. The other part of a reflex, the afferent branch, is often the same between the two systems. Sensory neurons receiving input from the periphery—with cell bodies in the sensory ganglia, of either a cranial nerve or a dorsal root ganglion adjacent to the spinal cord—project into the CNS to initiate the reflex (Figure 15.5).
Afferent Branch
The afferent branch of a reflex arc differs between somatic and visceral reflexes in some instances. Many of the inputs to visceral reflexes are from special or somatic senses, but particular senses are associated with the viscera that are not part of the conscious perception of the environment through the somatic nervous system. For example, there is a specific type of mechanoreceptor, called a baroreceptor, in the walls of the aorta and carotid sinuses that senses the stretch of those organs when blood volume or pressure increases. You do not have a conscious perception of having high blood pressure, but that is an important afferent branch of the cardiovascular and, particularly, vasomotor reflexes. The sensory neuron is essentially the same as any other general sensory neuron. The baroreceptor apparatus is part of the ending of a unipolar neuron that has a cell body in a sensory ganglion. The baroreceptors from the carotid arteries have axons in the glossopharyngeal nerve, and those from the aorta have axons in the vagus nerve.
Figure 15.5 Comparison of Somatic and Visceral Reflexes. The afferent inputs to somatic and visceral reflexes are essentially the same, whereas the efferent branches are different. Somatic reflexes, for instance, involve a direct connection from the ventral horn of the spinal cord to the skeletal muscle. Visceral reflexes involve a projection from the central neuron to a ganglion, followed by a second projection from the ganglion to the target effector.
Though visceral senses are not primarily a part of conscious perception, those sensations sometimes make it to conscious awareness. If a visceral sense is strong enough, it will be perceived. The sensory homunculus—the representation of the body in the primary somatosensory cortex—only has a small region allotted for the perception of internal stimuli. If you swallow a large bolus of food, for instance, you will probably feel the lump of that food as it pushes through your esophagus, or even if your stomach is distended after a large meal. If you inhale especially cold air, you can feel it as it enters your larynx and trachea. These sensations are not the same as feeling high blood pressure or high blood sugar levels.
When particularly strong visceral sensations rise to the level of conscious perception, the sensations are often felt in unexpected places. For example, strong visceral sensations of the heart will be felt as pain in the left shoulder and left arm. This irregular pattern of projection of conscious perception of visceral sensations is called referred pain.
Depending on the organ system affected, the referred pain will project to different areas of the body (Figure 15.6). The location of referred pain is not random, but a definitive explanation of the mechanism has not been established. The most broadly accepted theory for this phenomenon is that the visceral sensory fibres enter into the same level of the spinal cord as the somatosensory fibres of the referred pain location. By this explanation, the visceral sensory fibres from the mediastinal region, where the heart is located, would enter the spinal cord at the same level as the spinal nerves from the shoulder and arm, so the brain misinterprets the sensations from the mediastinal region as being from the axillary and brachial regions. Projections from the medial and inferior divisions of the cervical ganglia do enter the spinal cord at the middle to lower cervical levels, which is where the somatosensory fibres enter.
Figure 15.6 Referred Pain Chart. Conscious perception of visceral sensations maps to specific regions of the body, as shown in this chart. Some sensations are felt locally, whereas others are perceived as affecting areas that are quite distant from the involved organ.
Efferent Branch
The efferent branch of the visceral reflex arc begins with the projection from the central neuron along the preganglionic fibre. This fibre then makes a synapse on the ganglionic neuron that projects to the target effector.
The effector organs that are the targets of the autonomic system range from the iris and ciliary body of the eye to the urinary bladder and reproductive organs. The thoracolumbar output, through the various sympathetic ganglia, reaches all these organs. The cranial component of the parasympathetic system projects from the eye to part of the intestines. The sacral component reaches the majority of the large intestine and the pelvic organs of the urinary and reproductive systems.
Short and Long Reflexes
Somatic reflexes involve sensory neurons that connect sensory receptors to the CNS and motor neurons that project back out to the skeletal muscles. Visceral reflexes that involve the thoracolumbar or craniosacral systems share similar connections. However, there are reflexes that do not need to involve any CNS components. A long reflex has afferent branches that enter the spinal cord or brain and involve the efferent branches, as previously explained. A short reflexis completely peripheral and only involves the local integration of sensory input with motor output (Figure 15.7).
The difference between short and long reflexes is in the involvement of the CNS. Somatic reflexes always involve the CNS, even in a monosynaptic reflex in which the sensory neuron directly activates the motor neuron. That synapse is in the spinal cord or brain stem, so it has to involve the CNS. However, in the autonomic system, there is the possibility that the CNS is not involved. Because the efferent branch of a visceral reflex involves two neurons—the central neuron and the ganglionic neuron—a “short circuit” can be possible. If a sensory neuron projects directly to the ganglionic neuron and causes it to activate the effector target, then the CNS is not involved.
Figure 15.7 Short and Long Reflexes. Sensory input can stimulate either a short or a long reflex. A sensory neuron can project to the CNS or to an autonomic ganglion. The short reflex involves the direct stimulation of a postganglionic fibre by the sensory neuron, whereas the long reflex involves integration in the spinal cord or brain.
A division of the nervous system that is related to the autonomic nervous system is the enteric nervous system. The word enteric refers to the digestive organs, so this represents the nervous tissue that is part of the digestive system. There are a few myenteric plexuses in which the nervous tissue in the wall of the digestive tract organs can directly influence digestive function. If stretch receptors in the stomach are activated by the filling and distension of the stomach, a short reflex will directly activate the smooth muscle fibres of the stomach wall to increase motility to digest the excessive food in the stomach. No CNS involvement is needed because the stretch receptor is directly activating a neuron in the wall of the stomach that causes the smooth muscle to contract. That neuron, connected to the smooth muscle, is a postganglionic parasympathetic neuron that can be controlled by a fibre found in the vagus nerve.
Balance in Competing Autonomic Reflex Arcs
The autonomic nervous system is important for homeostasis because its two divisions compete at the target effector. The balance of homeostasis is attributable to the competing inputs from the sympathetic and parasympathetic divisions (dual innervation). At the level of the target effector, the signal of which system is sending the message is strictly chemical. A signalling molecule binds to a receptor that causes changes in the target cell, which in turn causes the tissue or organ to respond to the changing conditions of the body.
Competing Neurotransmitters
The postganglionic fibres of the sympathetic and parasympathetic divisions both release neurotransmitters that bind to receptors on their targets. Postganglionic sympathetic fibres release norepinephrine, with a minor exception, whereas postganglionic parasympathetic fibres release ACh. For any given target, the difference in which division of the autonomic nervous system is exerting control is just in which chemical binds to its receptors. The target cells will have adrenergic and muscarinic receptors. If norepinephrine is released, it will bind to the adrenergic receptors present on the target cell, and if ACh is released, it will bind to the muscarinic receptors on the target cell.
In the sympathetic system, there are exceptions to this pattern of dual innervation. The postganglionic sympathetic fibres that contact the blood vessels within skeletal muscle and that contact sweat glands do not release norepinephrine; they release ACh. This does not create any problem because there is no parasympathetic input to the sweat glands. Sweat glands have muscarinic receptors and produce and secrete sweat in response to the presence of ACh.
At most of the other targets of the autonomic system, the effector response is based on which neurotransmitter is released and what receptor is present. For example, regions of the heart that establish heart rate are contacted by postganglionic fibres from both systems. If norepinephrine is released onto those cells, it binds to an adrenergic receptor that causes the cells to depolarize faster, and the heart rate increases. If ACh is released onto those cells, it binds to a muscarinic receptor that causes the cells to hyperpolarize so that they cannot reach threshold as easily, and the heart rate slows. Without this parasympathetic input, the heart would work at a rate of approximately 100 beats per minute (bpm). The sympathetic system speeds that up, as it would during exercise, to 120–140 bpm, for example. The parasympathetic system slows it down to the resting heart rate of 60–80 bpm.
Another example is in the control of pupillary size (Figure 15.8). The afferent branch responds to light hitting the retina. Photoreceptors are activated, and the signal is transferred to the retinal ganglion cells that send an action potential along the optic nerve into the diencephalon. If light levels are low, the sympathetic system sends a signal out through the upper thoracic spinal cord to the superior cervical ganglion of the sympathetic chain.
The postganglionic fibre then projects to the iris, where it releases norepinephrine onto the radial fibres of the iris (a smooth muscle). When those fibres contract, the pupil dilates—increasing the amount of light hitting the retina. If light levels are too high, the parasympathetic system sends a signal out from the Edinger-Westphal nucleus through the oculomotor nerve. This fibre synapses in the ciliary ganglion in the posterior orbit. The postganglionic fibre then projects to the iris, where it releases ACh onto the circular fibres of the iris—another smooth muscle. When those fibres contract, the pupil constricts to limit the amount of light hitting the retina.
In this example, the autonomic system is controlling how much light hits the retina. It is a homeostatic reflex mechanism that keeps the activation of photoreceptors within certain limits. In the context of avoiding a threat like the lioness on the savannah, the sympathetic response for fight or flight will increase pupillary diameter so that more light hits the retina and more visual information is available for running away. Likewise, the parasympathetic response of rest reduces the amount of light reaching the retina, allowing the photoreceptors to cycle through bleaching and be regenerated for further visual perception; this is what the homeostatic process is attempting to maintain.
Figure 15.8 Autonomic Control of Pupillary Size. Activation of the pupillary reflex comes from the amount of light activating the retinal ganglion cells, as sent along the optic nerve. The output of the sympathetic system projects through the superior cervical ganglion, whereas the parasympathetic system originates in the midbrain and projects through the oculomotor nerve to the ciliary ganglion, which then projects to the iris. The postganglionic fibres of either division release neurotransmitters onto the smooth muscles of the iris to cause changes in the pupillary size. Norepinephrine results in dilation, and ACh results in constriction.
Autonomic Tone
Organ systems are balanced between the input from the sympathetic and parasympathetic divisions. When something upsets that balance, the homeostatic mechanisms strive to return it to its regular state. For each organ system, there may be more of a sympathetic or parasympathetic tendency to the resting state, which is known as the autonomic tone of the system. For example, the heart rate was described in the Competing Neurotransmitters section. Because the resting heart rate is the result of the parasympathetic system slowing the heart down from its intrinsic rate of 100 bpm, the heart can be said to be in parasympathetic tone.
In a similar fashion, another aspect of the cardiovascular system is primarily under sympathetic control. Blood pressure is partially determined by the contraction of smooth muscle in the walls of blood vessels. These tissues have adrenergic receptors that respond to the release of norepinephrine from postganglionic sympathetic fibres by constricting and increasing blood pressure. The hormones released from the adrenal medulla—epinephrine and norepinephrine—will also bind to these receptors. Those hormones travel through the bloodstream, where they can easily interact with the receptors in the vessel walls. The parasympathetic system has no significant input to the systemic blood vessels, so the sympathetic system determines their tone.
There is a limited number of blood vessels that respond to sympathetic input in a different fashion. Blood vessels in skeletal muscle, particularly those in the lower limbs, are more likely to dilate. It does not have an overall effect on blood pressure to alter the tone of the vessels but rather allows for blood flow to increase for those skeletal muscles that will be active in the fight-or-flight response. The blood vessels that have a parasympathetic projection are limited to those in the erectile tissue of the reproductive organs. Acetylcholine released by these postganglionic parasympathetic fibres causes the vessels to dilate, leading to the engorgement of the erectile tissue.
15.3 Central Control
The pupillary light reflex (Figure 15.9) begins when light hits the retina and causes a signal to travel along the optic nerve. This is visual sensation, because the afferent branch of this reflex is simply sharing the special sense pathway. Bright light hitting the retina leads to the parasympathetic response, through the oculomotor nerve, followed by the postganglionic fibre from the ciliary ganglion, which stimulates the circular fibres of the iris to contract and constrict the pupil. When light hits the retina in one eye, both pupils constrict. When that light is removed, both pupils dilate again back to the resting position. When the stimulus is unilateral (presented to only one eye), the response is bilateral (both eyes). The same is not true for somatic reflexes. If you touch a hot radiator, you only pull that arm back, not both. Central control of autonomic reflexes is different from that of somatic reflexes. The hypothalamus, along with other CNS locations, controls the autonomic system.
Figure 15.9 Pupillary Reflex Pathways. The pupil is under competing autonomic control in response to light levels hitting the retina. The sympathetic system will dilate the pupil when the retina is not receiving enough light, and the parasympathetic system will constrict the pupil when too much light hits the retina.
Forebrain Structures
Autonomic control is based on the visceral reflexes, composed of the afferent and efferent branches. These homeostatic mechanisms are based on the balance between the two divisions of the autonomic system, which results in tone for various organs that is based on the predominant input from the sympathetic or parasympathetic systems. Coordinating that balance requires integration that begins with forebrain structures like the hypothalamus and continues into the brain stem and spinal cord.
The Hypothalamus
The hypothalamus is the control centre for many homeostatic mechanisms. It regulates both autonomic function and endocrine function. The roles it plays in the pupillary reflexes demonstrate the importance of this control centre. The optic nerve projects primarily to the thalamus, which is the necessary relay to the occipital cortex for conscious visual perception. Another projection of the optic nerve, however, goes to the hypothalamus.
The Amygdala
The amygdala is a group of nuclei in the medial region of the temporal lobe that is part of the limbic lobe. The limbic lobe includes structures that are involved in emotional responses, as well as structures that contribute to memory function. The limbic lobe has strong connections with the hypothalamus and influences the state of its activity on the basis of emotional state. For example, when you are anxious or scared, the amygdala will send signals to the hypothalamus along the medial forebrain bundle that will stimulate the sympathetic fight-or-flight response. The hypothalamus will also stimulate the release of stress hormones through its control of the endocrine system in response to amygdala input.
The Medulla
The medulla contains nuclei referred to as the cardiovascular centre, which controls the smooth and cardiac muscle of the cardiovascular system through autonomic connections. When the homeostasis of the cardiovascular system shifts, such as when blood pressure changes, the coordination of the autonomic system can be accomplished within this region. Furthermore, when descending inputs from the hypothalamus stimulate this area, the sympathetic system can increase activity in the cardiovascular system, such as in response to anxiety or stress. The preganglionic sympathetic fibres that are responsible for increasing heart rate are referred to as the cardiac accelerator nerves, whereas the preganglionic sympathetic fibres responsible for constricting blood vessels compose the vasomotor nerves.
Several brain stem nuclei are important for the visceral control of major organ systems. One brain stem nucleus involved in cardiovascular function is the solitary nucleus. It receives sensory input about blood pressure and cardiac function from the glossopharyngeal and vagus nerves, and its output will activate sympathetic stimulation of the heart or blood vessels through the upper thoracic lateral horn. Another brain stem nucleus important for visceral control is the dorsal motor nucleus of the vagus nerve, which is the motor nucleus for the parasympathetic functions ascribed to the vagus nerve, including decreasing the heart rate, relaxing bronchial tubes in the lungs, and activating digestive function through the enteric nervous system. The nucleus ambiguus, which is named for its ambiguous histology, also contributes to the parasympathetic output of the vagus nerve and targets muscles in the pharynx and larynx for swallowing and speech, as well as contributing to the parasympathetic tone of the heart along with the dorsal motor nucleus of the vagus.
Key Terms
- acetylcholine (ACh):
- Neurotransmitter that binds at a motor end-plate to trigger depolarization.
- adrenal medulla:
- Interior portion of the adrenal (or suprarenal) gland that releases epinephrine and norepinephrine into the bloodstream as hormones.
- adrenergic:
- Synapse where norepinephrine is released, which binds to α- or β-adrenergic receptors.
- afferent branch:
- Component of a reflex arc that represents the input from a sensory neuron, for either a special or general sense.
- alpha (α)-adrenergic receptor:
- One of the receptors to which epinephrine and norepinephrine bind, which comes in two subtypes: α1 and α2.
- autonomic tone:
- Tendency of an organ system to be governed by one division of the autonomic nervous system over the other, such as heart rate being lowered by parasympathetic input at rest.
- baroreceptor:
- Mechanoreceptor that senses the stretch of blood vessels to indicate changes in blood pressure.
- beta (β)-adrenergic receptor:
- One of the receptors to which epinephrine and norepinephrine bind, which comes in three subtypes: β1, β2, and β3.
- cardiac accelerator nerves:
- Preganglionic sympathetic fibres that cause the heart rate to increase when the cardiovascular centre in the medulla initiates a signal.
- cardiovascular centre:
- Region in the medulla that controls the cardiovascular system through cardiac accelerator nerves and vasomotor nerves, which are components of the sympathetic division of the autonomic nervous system.
- celiac ganglion:
- One of the collateral ganglia of the sympathetic system that projects to the digestive system.
- central neuron:
- The cell body of a neuron in the autonomic system that is located in the central nervous system, specifically the lateral horn of the spinal cord or a brain stem nucleus.
- cholinergic:
- Synapse at which acetylcholine is released and binds to the nicotinic or muscarinic receptor.
- chromaffin cells:
- Neuroendocrine cells of the adrenal medulla that release epinephrine and norepinephrine into the bloodstream as part of sympathetic system activity.
- collateral ganglia:
- Ganglia outside of the sympathetic chain that are targets of sympathetic preganglionic fibres, which are the celiac, inferior mesenteric, and superior mesenteric ganglia.
- craniosacral system:
- Alternate name for the parasympathetic division of the autonomic nervous system that is based on the anatomical location of central neurons in brain stem nuclei and the lateral horn of the sacral spinal cord; also referred to as craniosacral outflow.
- efferent branch:
- Component of a reflex arc that represents the output, with the target being an effector, such as muscle or glandular tissue.
- endogenous:
- Describes a substance made in the human body.
- epinephrine:
- Signalling molecule released from the adrenal medulla into the bloodstream as part of the sympathetic response.
- exogenous:
- Describes a substance made outside of the human body.
- fight-or-flight response:
- Set of responses induced by sympathetic activity that lead to either fleeing a threat or standing up to it, which in the modern world is often associated with anxious feelings.
- ganglionic neuron:
- Specifically refers to the cell body of a neuron in the autonomic system that is located in a ganglion.
- greater splanchnic nerve:
- Nerve that contains fibres of the central sympathetic neurons that do not synapse in the chain ganglia but project onto the celiac ganglion.
- grey rami communicantes (singular = ramus communicans):
- Unmyelinated structures that provide a short connection from a sympathetic chain ganglion to the spinal nerve that contains the postganglionic sympathetic fibre.
- inferior mesenteric ganglion:
- One of the collateral ganglia of the sympathetic system that projects to the digestive system.
- intramural ganglia:
- Terminal ganglia of the parasympathetic system that are found within the walls of the target effector.
- lesser splanchnic nerve:
- Nerve that contains fibres of the central sympathetic neurons that do not synapse in the chain ganglia but project onto the inferior mesenteric ganglion.
- limbic lobe:
- Structures arranged around the edges of the cerebrum that are involved in memory and emotion.
- long reflex:
- Reflex arc that includes the central nervous system.
- muscarinic receptor:
- Type of acetylcholine receptor protein that is characterized by also binding to muscarine and is a metabotropic receptor.
- myenteric plexuses:
- Nervous tissue in the wall of the digestive tract organs that can directly influence digestive function.
- nicotinic receptor:
- Type of acetylcholine receptor protein that is characterized by also binding to nicotine and is an ionotropic receptor.
- norepinephrine:
- Signalling molecule released as a neurotransmitter by most postganglionic sympathetic fibres as part of the sympathetic response, or as a hormone into the bloodstream from the adrenal medulla.
- parasympathetic division:
- Division of the autonomic nervous system responsible for restful and digestive functions.
- paravertebral ganglia:
- Autonomic ganglia superior to the sympathetic chain ganglia.
- postganglionic fibre:
- Axon from a ganglionic neuron in the autonomic nervous system that projects to and synapses with the target effector; sometimes referred to as a postganglionic neuron.
- preganglionic fibre:
- Axon from a central neuron in the autonomic nervous system that projects to and synapses with a ganglionic neuron; sometimes referred to as a preganglionic neuron.
- prevertebral ganglia:
- Autonomic ganglia that are anterior to the vertebral column and functionally related to the sympathetic chain ganglia.
- referred pain:
- The conscious perception of visceral sensation projected to a different region of the body, such as the left shoulder and arm pain as a sign of a heart attack.
- reflex arc:
- Circuit of a reflex that involves a sensory input and motor output, or an afferent branch and an efferent branch, and an integrating centre to connect the two branches.
- rest and digest:
- Set of functions associated with the parasympathetic system that lead to restful actions and digestion.
- short reflex:
- Reflex arc that does not include any components of the central nervous system.
- somatic reflex:
- Reflex involving skeletal muscle as the effector, under the control of the somatic nervous system.
- superior cervical ganglion:
- One of the paravertebral ganglia of the sympathetic system that projects to the head.
- superior mesenteric ganglion:
- One of the collateral ganglia of the sympathetic system that projects to the digestive system.
- sympathetic chain ganglia:
- Series of ganglia adjacent to the vertebral column that receive input from central sympathetic neurons.
- sympathetic division:
- Division of the autonomic nervous system associated with the fight-or-flight response.
- target effector:
- Organ, tissue, or gland that will respond to the control of an autonomic or somatic or endocrine signal.
- terminal ganglia:
- Ganglia of the parasympathetic division of the autonomic system, which are located near or within the target effector, the latter also known as intramural ganglia.
- thoracolumbar system:
- Alternate name for the sympathetic division of the autonomic nervous system that is based on the anatomical location of central neurons in the lateral horn of the thoracic.
- varicosity:
- Structure of some autonomic connections that is not a typical synaptic end bulb, but a string of swellings along the length of a fibre that makes a network of connections with the target effector.
- vasomotor nerves:
- Preganglionic sympathetic fibres that cause the constriction of blood vessels in response to signals from the cardiovascular centre.
- visceral reflex:
- Reflex involving an internal organ as the effector, under the control of the autonomic nervous system.
- white rami communicantes (singular = ramus communicans):
- Myelinated structures that provide a short connection from a sympathetic chain ganglion to the spinal nerve that contains the preganglionic sympathetic fibre.
Chapter Review
15.1 Divisions of the Autonomic Nervous System
The primary responsibilities of the autonomic nervous system are to regulate homeostatic mechanisms in the body, which is also part of what the endocrine system does. The key to understanding the autonomic system is to explore the response pathways—the output of the nervous system. The way we respond to the world around us, to manage the internal environment on the basis of the external environment, is divided between two parts of the autonomic nervous system. The sympathetic division responds to threats and produces a readiness to confront the threat or to run away: the fight-or-flight response. The parasympathetic division plays the opposite role. When the external environment does not present any immediate danger, a restful mode descends on the body, and the digestive system is more active.
The sympathetic output of the nervous system originates from the lateral horn of the thoracolumbar spinal cord. An axon from one of these central neurons projects by way of the ventral spinal nerve root and spinal nerve to a sympathetic ganglion, either in the sympathetic chain ganglia or one of the collateral locations, where it synapses on a ganglionic neuron.
These preganglionic fibres release ACh, which excites the ganglionic neuron through the nicotinic receptor. The axon from the ganglionic neuron—the postganglionic fibre—then projects to a target effector where it will release norepinephrine to bind to an adrenergic receptor, causing a change in the physiology of that organ in keeping with the broad, divergent sympathetic response. The postganglionic connections to sweat glands in the skin and blood vessels supplying and upper lumbar spinal cord skeletal muscle are, however, exceptions; those fibres release ACh onto muscarinic receptors. The sympathetic system has a specialized preganglionic connection to the adrenal medulla that causes epinephrine and norepinephrine to be released into the bloodstream rather than exciting a neuron that contacts an organ directly. This hormonal component means that the sympathetic chemical signal can spread throughout the body very quickly and affect many organ systems at once.
The parasympathetic output is based in the brain stem and sacral spinal cord. Neurons from particular nuclei in the brain stem or from the lateral horn of the sacral spinal cord (preganglionic neurons) project to terminal (intramural) ganglia located close to or within the wall of target effectors. These preganglionic fibres also release ACh onto nicotinic receptors to excite the ganglionic neurons. The postganglionic fibres then contact the target tissues within the organ to release ACh, which binds to muscarinic receptors to induce rest-and-digest responses.
Signalling molecules utilized by the autonomic nervous system are released from axons and can be considered as either neurotransmitters (when they directly interact with the effector) or as hormones (when they are released into the bloodstream). The same molecule, such as norepinephrine, could be considered either a neurotransmitter or a hormone on the basis of whether it is released from a postganglionic sympathetic axon or from the adrenal gland. The synapses in the autonomic system are not always the typical type of connection first described in the neuromuscular junction. Instead of having synaptic end bulbs at the very end of an axonal fibre, they may have swellings—called varicosities—along the length of a fibre so that it makes a network of connections within the target tissue.
15.2 Autonomic Reflexes and Homeostasis
Autonomic nervous system function is based on the visceral reflex. This reflex is similar to the somatic reflex, but the efferent branch is composed of two neurons. The central neuron projects from the spinal cord or brain stem to synapse on the ganglionic neuron that projects to the effector. The afferent branch of the somatic and visceral reflexes is very similar, as many somatic and special senses activate autonomic responses. However, there are visceral senses that do not form part of conscious perception. If a visceral sensation, such as cardiac pain, is strong enough, it will rise to the level of consciousness. However, the sensory homunculus does not provide a representation of the internal structures to the same degree as the surface of the body, so visceral sensations are often experienced as referred pain, such as feelings of pain in the left shoulder and arm in connection with a heart attack.
The role of visceral reflexes is to maintain a balance of function in the organ systems of the body. The two divisions of the autonomic system each play a role in effecting change, usually in competing directions. The sympathetic system increases heart rate, whereas the parasympathetic system decreases heart rate. The sympathetic system dilates the pupil of the eye, whereas the parasympathetic system constricts the pupil. The competing inputs can contribute to the resting tone of the organ system. Heart rate is normally under parasympathetic tone, whereas blood pressure is normally under sympathetic tone. The heart rate is slowed by the autonomic system at rest, whereas blood vessels retain a slight constriction at rest.
In a few systems of the body, the competing input from the two divisions is not the norm. The sympathetic tone of blood vessels is caused by the lack of parasympathetic input to the systemic circulatory system. Only certain regions receive parasympathetic input that relaxes the smooth muscle wall of the blood vessels. Sweat glands are another example, which only receive input from the sympathetic system.
15.3 Central Control
The autonomic system integrates sensory information and higher cognitive processes to generate output, which balances homeostatic mechanisms. The central autonomic structure is the hypothalamus, which coordinates sympathetic and parasympathetic efferent pathways to regulate activities of the organ systems of the body. The majority of hypothalamic output travels through the medial forebrain bundle and the dorsal longitudinal fasciculus to influence the brain stem and spinal components of the autonomic nervous system. The medial forebrain bundle also connects the hypothalamus with higher centres of the limbic system, where emotion can influence visceral responses. The amygdala is a structure within the limbic system that influences the hypothalamus in the regulation of the autonomic system as well as the endocrine system.
These higher centres have descending control of the autonomic system through brain stem centres, primarily in the medulla, such as the cardiovascular centre. This collection of medullary nuclei regulates cardiac function, as well as blood pressure. Sensory inputs from the heart, aorta, and carotid sinuses project to these regions of the medulla. The solitary nucleus increases sympathetic tone of the cardiovascular system through the cardiac accelerator and vasomotor nerves. The nucleus ambiguus and the dorsal motor nucleus both contribute fibres to the vagus nerve, which exerts parasympathetic control of the heart by decreasing heart rate.