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The Autonomic Nervous
System and Visceral
Sensory Neurons 15
Overview of the Autonomic Nervous System 468
Comparison of the Autonomic and Somatic Motor Systems 468 Divisions of the Autonomic Nervous System 470
The Parasympathetic Division 472
Cranial Outflow 472 Sacral Outflow 473
The Sympathetic Division 473
Basic Organization 473 Sympathetic Pathways 477 The Role of the Adrenal Medulla in the Sympathetic Division 480
Visceral Sensory Neurons 481
Visceral Reflexes 481
Central Control of the Autonomic
Nervous System 483
Control by the Brain Stem and Spinal Cord 483 Control by the Hypothalamus and Amygdaloid Body 483 Control by the Cerebral Cortex 483
Disorders of the Autonomic
Nervous System 483
The Autonomic Nervous System
Throughout Life 484
Neurons of the myenteric plexus in a section of the small intestine ( light micrograph 1200×).
▲
The general visceral sensory system continuously monitors the activities of the visceral organs so that the autonomic motor neurons can make adjustments as necessary to ensure optimal performance of visceral functions. A third component of visceral innervation, the enteric nervous system, also innervates smooth muscle and glands, specifically those within the digestive tract. The enteric nervous system regulates the activity of the digestive tract and functions completely independently of the CNS. Autonomic neurons to the digestive tract can influence enteric neurons by either stimulating or inhibiting their activity. This ANS influence acts as a “volume control” rather than as an “on/off” switch. The details of the enteric nervous system and its interaction with the ANS are described with the innerva- tion of the digestive system (Chapter 23). Before describing the ANS, we need to review two terms. A synapse is a junction between two neurons that com- municates the message from one neuron, called the presyn- aptic neuron, to another neuron, the postsynaptic neuron. A ganglion (plural: ganglia ) is a cluster of neuronal cell bodies in the PNS. (See Chapter 12 for review of these terms.)
Comparison of the Autonomic and Somatic Motor Systems Other discussions of motor innervation focus largely on the somatic motor system, which innervates skeletal muscles. Each somatic motor neuron runs from the central nervous system all the way to the muscle being innervated, and each motor unit consists of a single neuron plus the skeletal muscle cells that it innervates (Figure 15.2). Typical somatic motor axons are thick, heavily myelinated fibers that conduct nerve impulses rapidly. By contrast, the comparable motor unit in the ANS includes a chain of two motor neurons (Figure 15.2). The first of these is called a preganglionic neuron. The cell body of this neuron lies within the CNS. Its axon, the pregangli- onic axon (also called a preganglionic fiber ), synapses with the second motor neuron, the postganglionic neuron, in a peripheral autonomic ganglion. The postganglionic axon
C
onsider the following situations: You wake up at night after having eaten at a restaurant where the food did not taste quite right, and you find yourself waiting helplessly for your stomach to “decide” whether it can hold the food down. A few days later, you are driving to school after drinking too much coffee and wish in vain that your full bladder would stop its uncomfortable contractions. Later that day, your professor asks you a hard question in front of the class, and you try not to let them see you sweat—but the sweat runs down your face anyway. All of these are exam- ples of visceral motor functions that are not easily controlled by the conscious will and that sometimes seem to “have a mind of their own.” These functions are performed by the autonomic nervous system (ANS), a motor system that does indeed operate with a certain amount of independence ( autonomic = self-governing).
OVERVIEW OF THE AUTONOMIC NERVOUS SYSTEM learning outcomes ▶ Define the autonomic nervous system (ANS), and explain its relationship to the peripheral nervous system as a whole. ▶ Compare autonomic neurons to somatic motor neurons. ▶ Describe the basic differences between the parasympathetic and sympathetic divisions of the ANS. The ANS is the system of motor neurons that innervate the smooth muscle, cardiac muscle, and glands of the body. By controlling these effectors, the ANS regulates such visceral functions as heart rate, blood pressure, digestion, and urina- tion, which are essential for maintaining the stability of the body’s internal environment. The ANS is the general visceral motor division of the peripheral nervous system and is dis- tinct from the general somatic motor division, which inner- vates the skeletal muscles (Figure 15.1). Although this chapter focuses on autonomic (general visceral motor) functions, it also considers the general visceral senses.
Figure 15.1 Place of the autonomic nervous system (ANS) and visceral sensory components in the structural organization of the nervous system.
Central nervous system (CNS) Peripheral nervous system (PNS)
Sensory (afferent) division Motor (efferent) division
Somatic nervous system
Autonomic nervous system (ANS)
Somatic sensory
Visceral sensory
Sympathetic division
Parasympathetic division
Both divisions have chains of two motor neurons that mostly innervate the same visceral organs, but they cause opposite effects: One division stimulates some smooth muscle to con- tract or a gland to secrete; the other division inhibits that action. The sympathetic division mobilizes the body during extreme situations such as fear, exercise, or rage. The para- sympathetic division enables the body to unwind and relax and works to conserve body energy. In other words, the para- sympathetic division controls routine maintenance functions, and the sympathetic division becomes active when extra metabolic effort is needed. The balance between the two divi- sions keeps body systems running smoothly. The sympathetic division is responsible for the fight- or-flight response. Its activity is evident during vigorous exer- cise, excitement, or emergencies. A pounding heart, dilated (widened) eye pupils, and cold, sweaty skin are signs that the sympathetic division has been mobilized (Figure 15.3, right side). All of them help us respond to dangerous situations: The increased heart rate delivers more blood and oxygen to the skeletal muscles used for fighting or running; widened pupils let in more light for clearer vision; and cold skin indi- cates that blood is being diverted from the skin to more vital organs, such as the brain. Additionally, the small air tubes in the lungs (bronchioles) dilate, increasing the uptake of oxy- gen; oxygen consumption by the body’s cells increases; and the liver releases more sugar into the blood to provide for the increased energy needs of the cells. In this way, the body’s “motors are revved up” for vigorous activity. Temporarily nonessential functions, such as digestion and motility of the urinary tract, are inhibited: When you are running to catch the last bus home, digesting lunch can wait. The sympathetic division also innervates the smooth muscle in the walls of blood vessels. Sympathetic input to the blood ves- sels servicing skeletal muscles rises, causing the smooth muscle of the vessels to relax. These vessels dilate, bringing more blood to the active muscles. At the same time, increased sympathetic input to the smooth muscle in other blood vessels stimulates contraction, producing vasoconstriction. This narrowing of vessel diameter forces the heart to work harder to pump blood around the vascular circuit. As a result, sympathetic activity causes blood pressure to rise during excitement and stress. Unlike the sympathetic division, the parasympathetic division is most active when the body is at rest. This division is concerned with conserving body energy and directing vital “housekeeping” activities such as digestion and the elimina- tion of feces and urine (Figure 15.3, left side). The buzzwords to remember are “rest and digest.” Parasympathetic function is best illustrated by a person who is relaxing after dinner and read- ing the newspaper. Heart rate and respiratory rates are at low- normal levels, and the gastrointestinal tract is digesting food. The pupils are constricted as the eyes focus for close vision. As you explore the sympathetic and parasympathetic divisions in detail, you will find their effects on individual visceral organs are easy to learn if you just remember fight- or-flight (sympathetic) versus rest-and-digest (parasympa- thetic). Furthermore, a dynamic counteraction exists between the two divisions, such that they balance each other during times when a person is neither highly excited nor completely at rest.
(or postganglionic fiber ) then extends to the visceral organs. Functionally, the preganglionic neuron signals the postganglionic neuron; the postganglionic neuron then stimulates muscle con- traction or gland secretion in the effector organ. Axons of pregan- glionic neurons are thin, lightly myelinated fibers, whereas axons of postganglionic neurons are even thinner and are unmyelinated. Consequently, impulses are conducted through the autonomic nervous system more slowly than through the somatic motor system. It is important to emphasize that the autonomic ganglia are motor ganglia containing the cell bodies of motor neurons, in contrast to the dorsal root ganglia, which are sensory ganglia.
Divisions of the Autonomic
Nervous System
The ANS has two divisions, the sympathetic and parasym- pathetic (par″ah-sim″pah-thet′ik) divisions, (Figure 15.2).
Figure 15.3 Overview of the subdivisions of the ANS. The parasympathetic and sympathetic divisions differ anatomically in (1) the sites of origin of their nerves, (2) the relative lengths of their preganglionic and postganglionic fibers, and (3) the location of their ganglia (indicated here by synapse sites). *Although sympathetic innervation to the skin and peripheral structures is mapped to the cervical area in this diagram, all nerves to the periphery carry postganglionic sympathetic fibers.
Salivary glands
Eye (dilates pupil)
Skin*
Heart
Lungs (dilates airways)
Liver Gall- bladder
Genitals (ejaculation)
Pancreas
Eye (constricts pupil)
Lungs (constricts airways)
Bladder
Gall- bladder
Pancreas
Stomach
Cervical
Sympathetic ganglia
Cranial
Lumbar
Thoracic
Genitals (erection)
Heart
Salivary glands
Stomach
Bladder
Adrenal gland
Parasympathetic Sympathetic
Sacral
Brain stem
L (^1)
T 1
−
−
−
−
−
−
inhibitory effect
Figure 15.3). All sympathetic ganglia lie near the spinal cord and vertebral column; postganglionic axons extend from these ganglia and travel to their target organs. Parasympathetic ganglia lie far from the CNS, in or near the organs innervated; therefore, the postganglionic axons are quite short.
3. Sympathetic fibers branch profusely, whereas parasym- pathetic fibers do not. Such extensive branching allows each sympathetic neuron to influence a number of dif- ferent visceral organs, enabling many organs to mobi- lize simultaneously during the fight-or-flight response. Indeed, the literal translation of sympathetic, “experi- enced together,” reflects the bodywide mobilization it produces. Parasympathetic effects, by contrast, are more localized and discrete. 4. The main biochemical difference between the two divi- sions of the ANS involves the neurotransmitter released by the post ganglionic axons (Figure 15.2). In the sympathetic division, most postganglionic axons release norepinephrine (also called noradrenaline ); these fibers are termed adrener- gic. * The postganglionic neurotransmitter in the parasym- pathetic division is acetylcholine (ACh); these fibers are termed cholinergic. The pre ganglionic axon terminals of both divisions are always cholinergic (release ACh).
CLINICAL APPLICATION
Autonomic Neuropathy Damage to the autonomic nerves, which may occur as a complication of diabetes, is called autonomic neuropathy. Damage to these nerves results in the inability to control heart rate, blood pressure, and blood sugar levels. Digestion and respiratory functions, urination, sexual response, and vision are also affected. This insidious condition may go untreated because its symptoms are widespread and commonly associated with other conditions. It can be detected via a noninvasive heart rate variability test (HRV).
In addition to these functional differences, there are ana- tomical and biochemical differences between the sympathetic and parasympathetic divisions (Table 15.1).
1. The two divisions originate from different regions of the CNS (Figure 15.2 and Figure 15.3). The sympathetic division can also be called the thoracolumbar division because its fibers emerge from the thoracic and superior lumbar parts of the spinal cord. The parasympathetic division can also be termed the craniosacral division because its fibers emerge from the brain (cranial part) and the sacral spinal cord (sacral part). 2. The sympathetic pathways have long postganglionic axons, whereas the postganglionic axons for parasympa- thetic pathways are comparatively short (Figure 15.2 and
Characteristic Sympathetic Parasympathetic
Origin Thoracolumbar outflow; lateral horn of gray matter of spinal cord segments T 1 –L 2
Craniosacral outflow: brain stem nuclei of cranial nerves III, VII, IX, and X; spinal cord segments S 2 –S 4 Location of ganglia Ganglia close to CNS: alongside vertebral column (sympathetic trunk ganglia) and anterior to vertebral column (collateral ganglia)
Ganglia in or close to visceral organ served
Relative length of pre- and postganglionic axons
Short preganglionic (splanchic nerves are exceptions); long postganglionic
Long preganglionic; short postganglionic
Rami communicantes (see p. 476)
Gray and white rami communicantes; white contain myelinated preganglionic axons; gray contain unmyelinated postganglionic axons
None
Degree of branching of preganglionic axons
Extensive Minimal
Functional role Prepares body to cope with emergencies and intense muscular activity; fight-or-flight response
Maintenance functions; conserves and stores energy; rest and digest response Neurotransmitters All preganglionic axons release ACh; most postganglionic axons release norepinephrine (adrenergic axons); postganglionic axons to sweat glands and blood vessels of skeletal muscles release ACh; neurotransmitter activity augmented by release of adrenal medullary hormones (epinephrine and norepinephrine)
All axons, preganglionic and postganglionic, release ACH (cholinergic axons)
Table 15.1 Anatomical and Physiological Differences Between the Parasympathetic and Sympathetic Divisions
*Not all postganglionic fibers in the sympathetic division are adrenergic: Those that innervate the sweat glands and blood vessels in skeletal muscle are cholinergic.
(the submandibular and sublingual glands). In the pathway leading to the lacrimal and nasal glands, the preganglionic neurons originate in the lacrimal nucleus in the pons and synapse with postganglionic neurons in the pterygopalatine ganglion, just posterior to the maxilla (Table 14.2, p. 439). In the pathway leading to the submandibular and sublingual glands, the preganglionic neurons originate in the superior salivatory nucleus in the pons and synapse with postgangli- onic neurons in the submandibular ganglion, deep to the mandibular angle.
Glossopharyngeal Nerve (IX)
The parasympathetic fibers (VM) of the glossopharyngeal nerve stimulate secretion of a large salivary gland, the parotid gland, which lies anterior to the ear. The preganglionic neu- rons originate in the inferior salivatory nucleus in the medulla and synapse with postganglionic neurons in the otic ganglion inferior to the foramen ovale of the skull (Table 14.2, p. 441). The three cranial nerves considered so far (III, VII, IX) supply the entire parasympathetic innervation of the head. Note, however, that only the pre ganglionic axons run within these three nerves. These axons synapse in the ganglia described above, which are located along the path of the tri- geminal nerve (V), and then the postganglionic axons travel via the trigeminal to their final destinations. This routing by way of the trigeminal nerve occurs because the trigeminal has the widest distribution within the face.
Vagus Nerve (X)
Parasympathetic fibers (VM) from the vagus innervate the visceral organs of the thorax and most of the abdomen (Figure 15.4). Note that this does not include innervation of the pelvic organs and that the vagal innervation of the diges- tive tube ends halfway along the large intestine. The vagus is an extremely important part of the ANS, containing nearly 90% of the preganglionic parasympathetic fibers in the body. Functionally, the parasympathetic fibers in the vagus nerve bring about typical rest-and-digest activities in visceral mus- cle and glands—stimulation of digestion (secretion of diges- tive glands and increased motility of the smooth muscle of the digestive tract), reduction in the heart rate, and constric- tion of the bronchi in the lungs, for example. The preganglionic cell bodies are mostly in the dor- sal motor nucleus of vagus in the medulla (see Fig. 13.7c, p. 382), and the preganglionic axons run through the entire length of the vagus nerve. Most postganglionic neurons are confined within the walls of the organs being innervated, and their cell bodies form intramural ganglia (in″trah-mu′ral; “within the walls”). The vagus nerve is essential to the functioning of many organs. As the vagus descends through the neck and trunk, it sends branches through many autonomic nerve plexuses to the organs being innervated (Figure 15.5). (Recall from Chapter 14 that a nerve plexus is a network of nerves.) Specifically, the vagus sends branches through the cardiac plexus to the heart, through the pulmonary plexus to the lungs, through the esophageal plexus to the esophagus and into the stomach wall, and through the celiac plexus and the
superior mesenteric plexus to the other abdominal organs (intestines, liver, pancreas, and so on). Fibers from both divi- sions of the ANS, parasympathetic and sympathetic, travel to the thoracic and abdominal organs through these plexuses.
Sacral Outflow The sacral part of the parasympathetic outflow emerges from segments S 2 –S 4 of the spinal cord (bottom part of Figure 15.4). Continuing where the vagus ends, it inner- vates the organs in the pelvis, including the distal half of the large intestine, the bladder, and reproductive organs such as the uterus, and the erectile tissues of the external genitalia. Parasympathetic effects on these organs include stimulation of defecation, voiding of urine, and erection. The preganglionic cell bodies of the sacral parasympa- thetics lie in the visceral motor region of the spinal gray mat- ter (Figure 13.27, p. 412). The axons of these preganglionic neurons run in the ventral roots to the ventral rami, from which they branch to form pelvic splanchnic nerves (see Figure 15.4). These nerves then run through an autonomic plexus in the pelvic floor, the inferior hypogastric plexus (or pelvic plexus; Figure 15.5) to reach the pelvic organs. Some preganglionic axons synapse in ganglia in this plexus, but most synapse in intramural ganglia in the organs. This plexus also contains fibers from both divisions of the ANS. The specific effects of parasympathetic innervation on various organs are presented in comparison with the effects of sympathetic innervation (Table 15.2, p. 475).
check your understanding ◻ 5. Which spinal nerves carry parasympathetic preganglionic fibers? ◻ 6. What is the result of vagal stimulation of (a) the heart, (b) the small intestine, (c) the salivary glands? ◻ 7. Cranial nerves III, VII, and IX carry preganglionic parasympathetic fibers. Which cranial nerve carries the postganglionic fibers to the target organs innervated by these three nerves? (For answers, see Appendix B.)
THE SYMPATHETIC DIVISION learning outcomes ▶ Describe the pathways of sympathetic innervation from the spinal cord to the effector organs in the body periphery, the head, and the visceral organs. ▶ Describe the effect of sympathetic innervation on each effector organ. ▶ Explain the sympathetic function of the adrenal medulla.
Basic Organization The sympathetic division exits from the thoracic and supe- rior lumbar part of the spinal cord, from segments T 1 to L (^2) (Figure 15.3). Its preganglionic cell bodies lie in the visceral motor region of the spinal gray matter, where they form the lateral gray horn (Figure 13.27, p. 412).
Figure 15.5 Autonomic nerves, plexuses, and ganglia. All autonomic plexuses contain both parasympathetic and sympathetic axons. The ganglia are almost exclusively sympathetic.
Left vagus nerve Cardiac branches of the vagus
Trachea
Thoracic spinal nerves (ventral rami)
Cardiac plexus
Pulmonary plexus on the bronchus
Vagus nerve
Esophageal plexus
Diaphragm
Stomach with vagus nerve
Celiac ganglion and plexus
Superior mesenteric ganglion and plexus
Inferior mesenteric ganglion and plexus
Aortic plexus
Inferior hypogastric (pelvic) plexus
Pelvic sympathetic trunk
Superior cervical ganglion
Middle cervical ganglion
Sympathetic cardiac nerves
Stellate ganglion
Aortic arch
Sympathetic trunk ganglia
Esophagus
Thoracic splanchnic nerves
Adrenal (suprarenal) gland
Kidney
Lumbar and sacral splanchnic nerves (^) Superior hypogastric plexus
Aorta
stellate (“star-shaped”) ganglion in the superior thorax (Figure 15.5). Overall, there are 22–24 sympathetic trunk ganglia per side, and a typical person may have 3 cervical, 11 thoracic, 4 lumbar, 4 sacral, and 1 coccygeal ganglia (Figure 15.5). The cervical ganglia lie just anterior to the transverse processes of the cervical vertebrae; the thoracic ganglia lie on the heads of the ribs; the lumbar ganglia lie on the anterolateral sides of the vertebral bodies; the sacral ganglia lie medial to the sacral foramina; and the coccygeal ganglion is anterior to the coccyx. Take care not to confuse the sympathetic trunk ganglia with the dorsal root ganglia. The dorsal root ganglia are sensory and lie along the dorsal roots of spinal nerves in the intervertebral foramina; the sympathetic trunk ganglia are motor and lie anterior to the ventral rami of spinal nerves.
Collateral Ganglia
The collateral ganglia, or prevertebral ganglia, differ from the sympathetic trunk ganglia in at least three ways: (1) They are not paired and are not segmentally arranged; (2) they occur only in the abdomen and pelvis; and (3) they all lie anterior to the vertebral column (hence the name pre- vertebral ), mostly on the large artery called the abdomi- nal aorta. The main collateral ganglia, the celiac, superior mesenteric, inferior mesenteric, and inferior hypogastric ganglia (Figure 15.5), lie within the autonomic nerve plex- uses of the same names.
sweat glands, arrector pili, or (with minor exceptions) blood vessels. Another reason the sympathetic division is more com- plex is that it has more ganglia. The sympathetic ganglia fall into two classes: (1) sympathetic trunk ganglia and (2) col- lateral ganglia.
Sympathetic Trunk Ganglia
The numerous sympathetic trunk ganglia, located along both sides of the vertebral column from the neck to the pelvis, are linked by short nerves into long sympathetic trunks (sympathetic chains) that resemble strings of beads (Figure 15.6). The sympathetic trunk ganglia are also called chain ganglia and paravertebral (“near the vertebrae”) gan- glia. These ganglia are joined to the ventral rami of nearby spinal nerves by white and gray rami communicantes (singular: ramus communicans, “communicating arm”). White rami communicantes lie lateral to gray rami communicantes. There is approximately one sympathetic trunk ganglion for each spinal nerve. However, the number of sympathetic trunk ganglia and spinal nerves is not identical, because some adjacent ganglia fuse during development. Such fusion is most evident in the neck region, where there are eight spinal nerves but only three sympathetic trunk gan- glia: the superior, middle, and inferior cervical ganglia (Figure 15.5). Furthermore, the inferior cervical ganglion usually fuses with the first thoracic ganglion to form the
Figure 15.6 The sympathetic trunk, thoracic region.
Spinal cord Dorsal root
Ventral root
Sympathetic trunk ganglion Sympathetic trunk
Rib
Phrenic nerve
Body of thoracic vertebra
Esophageal plexus
Azygos vein
Ventral ramus of spinal nerve
Gray ramus communicans White ramus communicans Thoracic splanchnic nerves (a) Location of the sympathetic trunk (^) (b) Dissection of posterior thoracic wall, right side
Thoracic aorta
Diaphragm
Explore Cadaver
To effector
Blood vessels
Skin (arrector pili muscles and sweat glands)
Dorsal root ganglion Dorsal ramus of spinal nerve
Dorsal root
Sympathetic trunk ganglion
Lateral horn (visceral motor zone)
Ventral root
Sympathetic trunk
Gray ramus communicans White ramus communicans
Splanchnic nerve
Collateral ganglion (such as the celiac)
Target organ in abdomen (e.g., intestine)
Ventral ramus of spinal nerve
(^1) Synapse in trunk ganglion at the same level
(^2) Synapse in trunk ganglion at a higher or lower level
Pass through sympathetic trunk to synapse in a collateral ganglion anterior to the vertebral column
3
Figure 15.7 Three pathways of sympathetic innervation.
Sympathetic Pathways
In the sympathetic pathways to all body regions (Figure 15.7), preganglionic neurons in the thoracolumbar spinal cord send their motor axons through the adjacent ventral root into the spinal nerve, white ramus communicans, and associated sympathetic trunk ganglion. From there, these preganglionic axons follow one of three pathways:
1 The preganglionic axon synapses with a postganglionic neuron in the sympathetic trunk ganglion at the same level and exits via the gray ramus communicans into the spinal nerve at that level. 2 The preganglionic axon ascends or descends in the sym- pathetic trunk to synapse in another trunk ganglion. The postganglionic fiber exits the sympathetic trunk via the gray ramus communicans at the level of the synapse. 3 The preganglionic axon passes through the sympathetic trunk, exits on a splanchnic nerve, and synapses in a col- lateral ganglion. The postsynaptic fiber extends from the collateral ganglion to the visceral organ via an autonomic nerve plexus. Keep this overview in mind as you consider the pathways to the specific body regions in more detail.
Figure 15.8 Sympathetic division of the ANS. Sympathetic innervation to peripheral structures (blood vessels, glands, and arrector pili muscles) occurs in all areas but is shown only in the cervical area.
Superior cervical ganglion
Middle cervical ganglion
Inferior cervical ganglion
Sympathetic trunk (chain) ganglia
Pons
L (^2)
T 1
White rami communicantes
Liver and gallbladder
Stomach
Spleen
Kidney
Adrenal medulla
Small intestine
Large intestine
Genitalia (uterus, vagina, and penis) and urinary bladder
Celiac ganglion
Inferior mesenteric ganglion
Lesser splanchnic nerve
Greater splanchnic nerve
Superior mesenteric ganglion
Lumbar splanchnic nerves
Eye Lacrimal gland
Nasal mucosa
Blood vessels; skin (arrector pili muscles and sweat glands)
Salivary glands
Heart
Lung
Rectum
Cardiac and pulmonary plexuses
Preganglionic Postganglionic
Sacral splanchnic nerves
The Role of the Adrenal Medulla in the Sympathetic Division On the superior aspect of each kidney lies an adrenal (suprarenal) gland (Figure 15.9). The internal portion of this gland—the adrenal medulla —is a major organ of the sympathetic nervous system. The adrenal medulla is a spe- cialized sympathetic ganglion containing a collection of modified postganglionic neurons that completely lack nerve processes. These neuron-derived cells secrete great quanti- ties of two excitatory hormones into the blood of nearby capillaries during the fight-or-flight response. The hormones secreted are norepinephrine (the chemical secreted by other postganglionic sympathetic neurons as a neurotransmitter) and greater amounts of a related excitatory molecule called epinephrine (adrenaline). Once released, these hormones travel throughout the body in the bloodstream, producing the widespread excitatory effects that we have all experienced as a “surge of adrenaline.” The cells of the adrenal medulla are stimulated to secrete by preganglionic sympathetic fibers that arise from cell bod- ies in the T 8 –L^1 region of the spinal cord. From there, they run in the thoracic splanchnic nerves and pass through the celiac plexus before reaching the adrenal medulla (Figure 15.8). Not surprisingly, the adrenal medulla has a more concentrated sympathetic innervation than any other organ in the body. The effects of sympathetic innervation to various organs are presented in comparison with the effects of parasympa- thetic innervation (Table 15.2, p. 475).
fibers inhibit the activity of the muscles and glands in these visceral organs.
Pathways to the Pelvic Organs
In the sympathetic innervation of pelvic organs (Figure 15.8), the preganglionic axons originate in the most inferior part of the thoracolumbar spinal cord (T 10 –L 2 ), then descend in the sympathetic trunk to the lumbar and sacral ganglia of the sympathetic trunk. Some axons synapse there, and the post- ganglionic axons run in lumbar and sacral splanchnic nerves to plexuses on the lower aorta and in the pelvis—namely, the inferior mesenteric plexus, the aortic plexus, and the hypogastric plexuses. Other preganglionic axons, by con- trast, pass directly to these autonomic plexuses and synapse in collateral ganglia there—the inferior mesenteric ganglia and inferior hypogastric ganglia. Postganglionic axons pro- ceed from these plexuses to the pelvic organs, including the bladder, the reproductive organs, and the distal half of the large intestine. These sympathetic fibers inhibit urination and defecation and promote ejaculation.
Figure 15.9 Sympathetic innervation of the adrenal medulla.
Spinal cord: T 8 –L (^1)
Adrenal medulla cells
Sympathetic trunk
Ventral root
Thoracic splanchnic nerves
Epinephrine and norepinephrine
Adrenal gland Adrenal medulla
Capillary
Kidney
CLINICAL APPLICATION
Stress-Induced Hypertension Continual stress can promote overactive sympathetic stimulation causing vasoconstriction that results in hypertension, or high blood pressure, a circulatory condition that can have a variety of contributing factors. Hypertension is always serious because (1) it increases the workload on the heart, possibly precipitating heart disease, and (2) it increases the wear and tear on artery walls. Stress-induced hypertension is treated with drugs that prevent smooth muscle cells in the walls of blood vessels from binding norepinephrine and epinephrine.
check your understanding ◻ 8. Why are white rami communicantes located only on sympathetic trunk ganglia between T 1 and L 2 , and gray rami communicantes branch off each sympathetic trunk ganglion? ◻ 9. What is the general effect of sympathetic innervation to the abdominal organs? ◻ 10. Which types of autonomic fibers (preganglionic, postganglionic, sympathetic, parasympathetic) are located in the thoracic and abdominal autonomic plexuses? (For answers, see Appendix B.)
a visceral reflex that regulates blood pressure (Figure 15.12). When blood pressure is elevated, baroreceptors in the carotid sinus, located at the junction of the internal and external carotid arteries, stimulate visceral sensory neurons in the glossopharyn- geal nerve (cranial nerve IX). Integration in the cardiac center in the medulla stimulates the vagus nerve (cranial nerve X). Vagal stimulation of the heart decreases heart rate, and subsequently blood pressure falls. Some visceral reflex arcs do not involve the central ner- vous system at all—they are strictly peripheral reflexes. In some of these peripheral reflexes, branches from visceral sensory fibers synapse with postganglionic motor neurons within sympathetic ganglia. Furthermore, complete three- neuron reflex arcs (with small sensory, motor, and intrinsic neu- rons) exist entirely within the wall of the digestive tube; these neurons are part of the enteric nervous system (see Chapter 23, p. 686). The peripheral reflexes carry out highly localized auto- nomic responses involving either small segments of an organ or a few different visceral organs. These reflexes enable the peripheral part of the visceral nervous system to control some of its own activity, making it partly independent of the brain and spinal cord. This fact further illustrates the general concept that the autonomic nervous system operates partly on its own.
Figure 15.12 Baroreceptor reflex.
Increased blood pressure
Blood pressure decreases
Sensory impulses are carried on visceral sensory fibers in the glosso- pharyngeal nerves (CN IX).
Integration occurs in cardiac center of medulla oblongata.
Baroreceptors in carotid sinus are stimulated.
2
2 3
3
1
1
5
5
Parasympathetic stimulation of heart decreases heart rate.
Efferent pathway via the vagus nerves (CN X)
4
4
CLINICAL APPLICATION
Mass Reflex Reaction The mass reflex reaction affects quadriplegics and paraplegics with spinal cord injuries above the level of T 6. The cord injury is followed by a temporary loss of all reflexes inferior to the level of the injury. When reflex activity returns, it is exaggerated because of the lack of inhibitory input from higher (brain) centers. The ensuing episodes of mass reflex reaction involve surges of both visceral and somatic motor output from large regions of the spinal cord. The usual trigger for such an episode is a strong stimulus to the skin or the overfilling of a visceral organ, such as the bladder. During the mass reflex episode, the body goes into flexor spasms, the limbs move wildly, the colon and bladder empty, and profuse sweating occurs. Most seriously, sympathetic activity raises blood pressure to life-threatening levels.
Spinal cord
Dorsal root ganglion
Autonomic ganglion
Stimulus
Response
Visceral sensory neuron
Integration center t.BZCFQSFHBOHMJPOJD neuron (as shown) t.BZCFBEPSTBMIPSO interneuron t.BZCFXJUIJOXBMMT of gastrointestinal tract
4FOTPSZSFDFQUPS in viscera 2
3
1
(^5) Visceral effector
&GGFSFOUQBUIXBZ (two-neuron chain) t1SFHBOHMJPOJDOFVSPO t1PTUHBOHMJPOJD neuron
4
Figure 15.11 Visceral reflexes. Visceral reflex arcs have the same five elements as somatic reflex arcs.
activation can occur when people decide to recall a frightful experience; in this case the cerebral cortex acts through the amygdaloid body.
check your understanding ◻ 11. Via which pathway do most visceral pain fibers travel to reach the CNS? ◻ 12. What is a peripheral reflex, and how does it differ from a spinal reflex? ◻ 13. Which region of the CNS is the main control center for the ANS? (For answers, see Appendix B.)
DISORDERS OF THE AUTONOMIC NERVOUS SYSTEM learning outcome ▶ Briefly describe some diseases of the autonomic nervous system. Because the ANS is involved in nearly every important process that occurs within the body, it is not surprising that abnormalities of autonomic functioning can have far-reaching effects. Such abnormalities can impair elimination processes and blood delivery, and can even threaten life.
CENTRAL CONTROL OF THE
AUTONOMIC NERVOUS SYSTEM
learning outcome
▶ Explain how various regions of the CNS help to regulate the autonomic nervous system.
Though the ANS is not considered to be under direct volun- tary control, many of its activities are regulated by the central nervous system. Several sources of central control exist—the brain stem and spinal cord, the hypothalamus and amygdala, and the cerebral cortex (Figure 15.13).
Control by the Brain Stem
and Spinal Cord
The reticular formation of the brain stem appears to exert the most direct influence over autonomic functions. Centers in the medulla oblongata (pp. 379–380) regulate heart rate ( cardiac centers; Figure 15.12), the diameter of blood ves- sels (vasomotor center), and many digestive activities. Also, the periaqueductal gray matter of the midbrain controls many autonomic functions, especially the sympathetic fear response during a threatening encounter (pp. 381–382). Control of autonomic functions at the level of the spi- nal cord involves the spinal visceral reflexes (Figure 15.11). Note, however, that even though the defecation and urination reflexes are integrated by the spinal cord, they are subject to conscious inhibition from the brain. This enables conscious control over when and where to eliminate wastes.
Control by the Hypothalamus
and Amygdaloid Body
The main integration center of the autonomic nervous system is the hypothalamus (Figure 15.13). In general, the medial and anterior parts of the hypothalamus direct parasympathetic functions, whereas the lateral and posterior parts direct sym- pathetic functions. These hypothalamic centers influence the preganglionic autonomic neurons in the spinal cord and brain, both through direct connections and through relays in the retic- ular formation and periaqueductal gray matter. It is through the ANS that the hypothalamus controls heart activity, blood pressure, body temperature, and digestive functions. Recall that the amygdaloid body is the main limbic region for emotions, including fear (see p. 400). Through commu- nications with the hypothalamus and periaqueductal gray matter, the amygdaloid body stimulates sympathetic activity, especially previously learned fear-related behavior.
Control by the Cerebral Cortex
Even though it was once thought that the autonomic nervous system was not subject to voluntary control by the cer- ebral cortex, people can exert some conscious control over some autonomic functions by developing control over their thoughts and emotions. For example, feelings of extreme calm achieved during meditation are associated with cerebral cortex influence on the parasympathetic centers in the hypo- thalamus via various limbic structures. Voluntary sympathetic
Cerebral cortex (frontal lobe)
Limbic system (emotional input)
Communication at subconscious level
Hypothalamus The “boss”: Overall integration of ANS
Spinal cord Reflexes for urination, defecation, erection, and ejaculation
Brain stem (reticular formation, etc.) Regulates pupil size, heart, blood pressure, airflow, salivation, etc.
Figure 15.13 Levels of ANS control. The hypothalamus is the main integration center of the ANS. Inputs from the cerebral cortex via the limbic lobe can influence hypothalamic functioning.
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standing up slowly gives the sympathetic nervous system time to adjust blood pressure, eye drops are available for dry eyes, and drinking ample fluid helps alleviate constipation.
check your understanding ◻ 14. Which embryonic tissue forms the postganglionic neurons of the ANS? ◻ 15. Which division of the ANS is deficient in achalasia of the cardia? Which division malfunctions in Raynaud’s disease? (For answers, see Appendix B.)
begins to decline. Elderly people are commonly constipated because the autonomically controlled motility of their gastro- intestinal tract is reduced. Frequent eye infections can result from diminished formation of tears, which contain bactericidal enzymes, and the pupils cannot dilate as widely or as quickly. Whenever a young and healthy person rises to a standing posi- tion, the sympathetic division induces bodywide vasoconstric- tion, raising blood pressure so that blood can be pumped to the head and brain. The response becomes sluggish with age, so elderly people may faint if they stand up too quickly. Although these age-related problems are distressing, they usually are not life-threatening and can be easily alleviated. For example,
RELATED CLINICAL TERMS
Atonic bladder (a-tahn’ik; “without tone”) A condition in which the bladder becomes flaccid and overfills, allowing urine to dribble out. Atonic bladder results from the temporary loss of the micturition reflex following injury to the spinal cord.
Vagotomy (va-got’o-me) Cutting or severing of a vagus nerve, often to decrease the secretion of stomach acid and other caustic digestive juices that aggravate ulcers.
CHAPTER SUMMARY
1. The ANS is the system of motor neurons that innervates smooth muscle, cardiac muscle, and glands. It is the general visceral motor division of the PNS. The ANS largely operates below the level of consciousness.
OVERVIEW OF THE AUTONOMIC NERVOUS SYSTEM (pp. 468–472)
Comparison of the Autonomic and Somatic Motor Systems (pp. 468–470)
2. In the somatic motor division of the nervous system, a single motor neuron forms the pathway from the CNS to the skeletal muscle cells. Autonomic motor pathways, by contrast, consist of chains of two neurons: the preganglionic neuron, whose cell body is in the CNS, and the postganglionic neuron, whose cell body is in an autonomic ganglion. The postganglionic axon innervates the visceral organs.
Divisions of the Autonomic Nervous System (pp. 470–472)
3. The ANS has parasympathetic and sympathetic divisions. Both innervate many of the same organs but produce opposite effects (Table 15.2, p. 475). The sympathetic division prepares the body for “fight or flight,” whereas the parasympathetic division is active during “rest and digest.” 4. The parasympathetic division exits the CNS on cranial and sacral nerves and has comparatively long preganglionic axons. The sym- pathetic division exits on thoracic and lumbar nerves and has com- paratively long postganglionic axons. 5. The two divisions differ in the neurotransmitters they release at the effector organ. Acetylcholine is released by parasympathetic postganglionic fibers, whereas norepinephrine is released by most sympathetic postganglionic fibers.
THE PARASYMPATHETIC DIVISION (pp. 472–473) Cranial Outflow (pp. 472–473)
6. Cranial parasympathetic fibers arise in the brain stem nuclei of cra- nial nerves III, VII, IX, and X and synapse in ganglia in the head, thorax, and abdomen. Axons in cranial nerve (CN) III serve smooth musculature in the eye via a synapse in the ciliary ganglion. Axons in CN VII serve the submandibular, sublingual, lacrimal, and nasal glands and synapse in the submandibular and pterygopalatine gan- glia. Axons in CN IX serve the parotid gland via a synapse in the otic ganglion. 7. Parasympathetic fibers in the vagus nerve (CN X) innervate organs in the thorax and most of the abdomen, including the heart, lungs, esophagus, stomach, liver, and most of the intestines. The axons in the vagus are preganglionic. Almost all postganglionic neurons are located in intramural ganglia within the organ walls. Sacral Outflow (p. 473) 8. Sacral parasympathetic pathways innervate the pelvic viscera. The preganglionic axons exit from the visceral motor region of the gray matter of the spinal cord (S 2 –S 4 ) and form the pelvic splanchnic nerves. Most of these axons synapse in intramural ganglia in the pelvic organs.
The Role of the Adrenal Medulla in the Sympathetic Division (p. 480)
18. The adrenal glands, one superior to each kidney, contain a medulla of modified postganglionic sympathetic neurons that secrete the hor- mones epinephrine (adrenaline) and norepinephrine into the blood. The result is the “surge of adrenaline” felt during excitement. 19. The cells of the adrenal medulla are innervated by preganglionic sympathetic neurons, which signal them to secrete the hormones.
VISCERAL SENSORY NEURONS (p. 481)
20. General visceral sensory neurons monitor temperature, pain, irrita- tion, chemical changes, and stretch in the visceral organs. 21. Visceral sensory fibers run within the autonomic nerves, especially within the vagus and the sympathetic nerves. Most pain fibers from the visceral organs of the body follow the sympathetic pathway to the CNS and are carried in the spinothalamic tract to the thalamus and then to the visceral sensory cortex. 22. Visceral pain is induced by stretching, infection, and cramping of internal organs but seldom by cutting or scraping these organs. Visceral pain is referred to somatic regions of the body that receive innervation from the same spinal cord segments.
VISCERAL REFLEXES (pp. 481–482)
23. Many visceral reflexes are spinal reflexes, such as the defecation reflex. Others, such as the baroreceptor reflex, involve integration centers in the brain stem. Some visceral reflexes, however, involve only peripheral neurons.
CENTRAL CONTROL OF THE AUTONOMIC NERVOUS SYSTEM (p. 483)
24. Visceral motor functions are influenced by the medulla oblongata, the periaqueductal gray matter, spinal visceral reflexes, the hypo- thalamus, the amygdaloid body, and the cerebral cortex. Some people can voluntarily regulate some autonomic activities by gain- ing extraordinary control over their emotions.
DISORDERS OF THE AUTONOMIC NERVOUS SYSTEM (pp. 483–484)
25. Most autonomic disorders reflect problems with the control of smooth muscle. Raynaud’s disease and hypertension result from abnormalities in vascular control.
THE AUTONOMIC NERVOUS SYSTEM THROUGHOUT LIFE (pp. 484–485)
26. Preganglionic neurons develop from the neural tube. Postganglionic neurons and visceral sensory neurons develop from the neural crest. 27. The efficiency of the ANS declines in old age: Gastrointestinal motility and production of tears decrease, and the vasomotor response to standing slows.
THE SYMPATHETIC DIVISION (pp. 473–480)
Basic Organization (pp. 473–476)
9. The preganglionic sympathetic cell bodies are in the lateral horn of the spinal gray matter from the level of T 1 to L 2. 10. The sympathetic division supplies some peripheral structures that the parasympathetic division does not: arrector pili, sweat glands, and the smooth muscle of blood vessels. 11. Sympathetic ganglia include 22–24 pairs of sympathetic trunk ganglia (also called chain ganglia and paravertebral ganglia), which are linked together to form sympathetic trunks on both sides of the vertebral column, and unpaired collateral ganglia (preverte- bral ganglia), most of which lie on the aorta in the abdomen. Human Cadaver/Nervous System/Autonomic Nervous System
Sympathetic Pathways (pp. 477–480)
12. Every preganglionic sympathetic axon leaves the lateral gray horn of the thoracolumbar spinal cord through a ventral root and spinal nerve. From there, it runs in a white ramus communicans to a sympathetic trunk ganglion or collateral ganglion, where it synapses with the postganglionic neuron that extends to the vis- ceral effector. Many preganglionic axons ascend or descend in the sympathetic trunk to synapse in a ganglion at another body level. 13. In the sympathetic pathway to the body periphery (to innervate arrector pili, sweat glands, and peripheral blood vessels), the pre- ganglionic axons synapse in the sympathetic trunk ganglia, and the postganglionic axons run in gray rami communicantes to the dor- sal and ventral rami of the spinal nerves for peripheral distribution. 14. In the sympathetic pathway to the head, the preganglionic axons ascend in the sympathetic trunk and synapse in the superior cer- vical ganglion. From there, most postganglionic axons associate with a large artery that distributes them to the glands and smooth musculature of the head. 15. In the sympathetic pathway to thoracic organs, most preganglionic axons synapse in the nearest sympathetic trunk ganglion, and the postganglionic axons run directly to the organs (lungs, esophagus). Many postganglionic axons to the heart, however, descend from the cervical ganglia in the neck. 16. In the sympathetic pathway to abdominal organs, the pregangli- onic axons run in splanchnic nerves to synapse in collateral ganglia on the aorta. From these ganglia, the postganglionic axons follow large arteries to the abdominal viscera (stomach, liver, kidney, and most of the large intestine). 17. In the sympathetic pathway to pelvic organs, the preganglionic axons synapse in sympathetic trunk ganglia or in collateral gan- glia on the aorta, sacrum, and pelvic floor. Postganglionic axons travel through the most inferior autonomic plexuses to the pelvic organs.
Multiple Choice/Matching Questions
(For answers, see Appendix B.)
1. Which of the following does not characterize the ANS? (a) two- neuron motor chains, (b) preganglionic cell bodies in the CNS, (c) presence of postganglionic cell bodies in ganglia, (d) innerva- tion of skeletal muscle. 2. For each of the following terms or phrases, write either S (for sym- pathetic ) or P (for parasympathetic ) division of the autonomic ner- vous system. (1) short preganglionic axons, long postganglionic axons (2) intramural ganglia (3) craniosacral outflow
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