Autonomic Nervous System
The efferent innervation of all tissues other than skeletal muscle is by way of the autonomic nervous system. A special case occurs in the gastrointestinal tract, where autonomic neurons innervate a nerve network in the wall of the intestinal tract. This network, termed the enteric nervous system, will be described in Chapter 17.
In the autonomic nervous system, parallel chains, each with two neurons, connect the central nervous system and the effector cells (Figure 8-43). (This is in contrast to the single neuron of the somatic system.) The first neuron has its cell body in the central nervous system. The synapse between the two neurons is outside the central nervous system, in a cell cluster called an autonomic ganglion. The nerve fibers passing between the central nervous system and the ganglia are called preganglionic fibers; those passing between the ganglia and the effector cells are post-ganglionic fibers. There is the potential for integration in the autonomic ganglia because of the convergence and divergence of the pathways there.
Anatomical and physiological differences within the autonomic nervous system are the basis for its further subdivision into sympathetic and parasympa-
thetic components (see Table 8-10). The nerve fibers of the sympathetic and parasympathetic components leave the central nervous system at different levels— the sympathetic fibers from the thoracic (chest) and lumbar regions of the spinal cord, and the parasym-pathetic fibers from the brain and the sacral portion of the spinal cord (lower back, Figure 8-44). Therefore, the sympathetic division is also called the thora-columbar division, and the parasympathetic is called the craniosacral division.
The two divisions also differ in the location of ganglia. Most of the sympathetic ganglia lie close to the spinal cord and form the two chains of ganglia—one on each side of the cord—known as the sympathetic trunks (Figure 8-44). Other sympathetic ganglia, called collateral ganglia—the celiac, superior mesen-teric, and inferior mesenteric ganglia—are in the abdominal cavity, closer to the innervated organ (Figure 8-44). In contrast, the parasympathetic ganglia lie within the organs innervated by the postganglionic neurons or very close to the organs.
The anatomy of the sympathetic nervous system can be confusing. Preganglionic sympathetic fibers leave the spinal cord only between the first thoracic and third lumbar segments, whereas sympathetic trunks extend the entire length of the cord, from the cervical levels high in the neck down to the sacral levels. The ganglia in the extra lengths of sympathetic trunks receive preganglionic fibers from the thora-columbar regions because some of the preganglionic fibers, once in the sympathetic trunks, turn to travel upward or downward for several segments before forming synapses with postganglionic neurons (Figure 8-45, numbers 1 and 4). Other possible paths taken by the sympathetic fibers are shown in Figure 8-45, numbers 2, 3, and 5.
The anatomical arrangements in the sympathetic nervous system to some extent tie the entire system together so it can act as a single unit, although small segments of the system can still be regulated independently. The parasympathetic system, in contrast, is
Somatic nervous system
Effector organ
Preganglionic fiber
Autonomic nervous system
Ganglion
Postganglionic fiber
Smooth or cardiac muscles, glands, or GI neurons
FIGURE 8-43
Efferent division of the peripheral nervous system. Overall plan of the somatic and autonomic nervous systems.
PART TWO Biological Control Systems
Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition
PART TWO Biological Control Systems
Parasympathetic preganglionic fibers Parasympathetic postganglionic fibers Sympathetic preganglionic fibers
Brainstem
Parasympathetic preganglionic fibers Parasympathetic postganglionic fibers Sympathetic preganglionic fibers
Brainstem
FIGURE 8-44
The parasympathetic (left) and sympathetic (right) divisions of the autonomic nervous system. The celiac, superior mesenteric, and inferior mesenteric ganglia are collateral ganglia. Only one sympathetic trunk is indicated, although there are two, one on each side of the spinal cord. Not shown are the fibers passing to the liver, blood vessels, genitalia and skin glands.
Inferior mesenteric ganglion
FIGURE 8-44
The parasympathetic (left) and sympathetic (right) divisions of the autonomic nervous system. The celiac, superior mesenteric, and inferior mesenteric ganglia are collateral ganglia. Only one sympathetic trunk is indicated, although there are two, one on each side of the spinal cord. Not shown are the fibers passing to the liver, blood vessels, genitalia and skin glands.
Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition
- Neural Control Mechanisms CHAPTER EIGHT
Sympathetic trunk
(chain of sympathetic ganglia)
Spinal cord (dorsal side)
Sympathetic trunk
(chain of sympathetic ganglia)
Spinal cord (dorsal side)
To 4
collateral ganglion
Preganglionic fiber
Postganglionic fiber
Sympathetic ganglion
FIGURE 8-45
Relationship between a sympathetic trunk and spinal cord (1 through 5) with the various courses that preganglionic sympathetic fibers (solid lines) take through the sympathetic trunk. Dashed lines represent postganglionic fibers. A mirror image of this exists on the opposite side of the spinal cord.
TABLE 8-12 Classes of Receptors for
Acetylcholine, Norepinephrine, and Epinephrine
To 4
collateral ganglion
Preganglionic fiber
Postganglionic fiber
Sympathetic ganglion
FIGURE 8-45
Relationship between a sympathetic trunk and spinal cord (1 through 5) with the various courses that preganglionic sympathetic fibers (solid lines) take through the sympathetic trunk. Dashed lines represent postganglionic fibers. A mirror image of this exists on the opposite side of the spinal cord.
made up of relatively independent components. Thus, overall autonomic responses, made up of many small parts, are quite variable and finely tailored to the specific demands of any given situation.
In both sympathetic and parasympathetic divisions, acetylcholine is the major neurotransmitter released between pre- and postganglionic fibers in au-tonomic ganglia (Figure 8-46). In the parasympathetic division, acetylcholine is also the major neurotrans-mitter between the postganglionic fiber and the effector cell. In the sympathetic division, norepinephrine is usually the major transmitter between the postgan-glionic fiber and the effector cell. We say "major" and "usually" because acetylcholine is also released by
I. Receptors for acetylcholine a. Nicotinic receptors
On postganglionic neurons in the autonomic ganglia At neuromuscular junctions of skeletal muscle On some central nervous system neurons b. Muscarinic receptors
On smooth muscle On cardiac muscle On gland cells
On some central nervous system neurons
On some neurons of autonomic ganglia (although the great majority of receptors at this site are nicotinic)
II. Receptors for norepinephrine and epinephrine
On smooth muscle On cardiac muscle On gland cells
On some central nervous system neurons some sympathetic postganglionic endings. Moreover, one or more cotransmitters are usually stored and released with the autonomic transmitters; these include ATP, dopamine, and several of the neuropeptides. These all, however, play a relatively small role.
In addition to the classical autonomic neurotrans-mitters just described, there is a widespread network of postganglionic fibers recognized as nonadrenergic and noncholinergic. These fibers use nitric oxide and other neurotransmitters to mediate some forms of blood vessel dilation and to regulate various gastrointestinal, respiratory, urinary, and reproductive functions.
Many of the drugs that stimulate or inhibit various components of the autonomic nervous system affect receptors for acetylcholine and norepinephrine. Recall that there are several types of receptors for each neurotransmitter (Table 8-12). The great majority of acetylcholine receptors in the autonomic ganglia are nicotinic receptors. In contrast, the acetylcholine receptors on smooth-muscle, cardiac-muscle, and gland cells are muscarinic receptors. To complete the story of the peripheral cholinergic receptors, it should be emphasized that the cholinergic receptors on skeletal-muscle fibers, innervated by the somatic motor neurons, not autonomic neurons, are nicotinic receptors.
One set of postganglionic neurons in the sympathetic division never develops axons; instead, upon activation by preganglionic axons, the cells of this "ganglion" release their transmitters into the bloodstream (Figure 8-46). This "ganglion," called
PART TWO Biological Control Systems
Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition
PART TWO Biological Control Systems
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Autonomic nervous system: Parasympathetic division ACh- Autonomic nervous system: Parasympathetic division r?i Ganglion Autonomic nervous system: Sympathetic division Ganglion Ganglion 
(via bloodstream) Epi (also NE, DA, peptides) Effector organ Effector organ Adrenal medulla (via bloodstream) Epi (also NE, DA, peptides) Effector organ FIGURE 8-46 Transmitters used in the various components of the peripheral efferent nervous system. In a few cases (to be described later), sympathetic neurons release a transmitter other than norepinephrine. Notice that the first neuron exiting the central nervous system—whether in the somatic or the autonomic nervous system—releases acetylcholine. ACh, acetylcholine; NE, norepinephrine; Epi, epinephrine; DA, dopamine. the adrenal medulla, therefore functions as an endocrine gland whose secretion is controlled by sympathetic preganglionic nerve fibers. It releases a mixture of about 80 percent epinephrine and 20 percent norepinephrine into the blood (plus small amounts of other substances, including dopamine, ATP, and neuropeptides). These catecholamines, properly called hormones rather than neurotransmitters in this circumstance, are transported via the blood to effector cells having receptors sensitive to them. The receptors may be the same adrenergic receptors that are located near the release sites of sympathetic post-ganglionic neurons and normally activated by the norepinephrine released from these neurons, or the receptors may be located at places that are not near the neurons and therefore activated only by the circulating epinephrine or norepinephrine. Table 8-13 is a reference list of the effects of auto-nomic nervous system activity, which will be described in subsequent chapters. Note that the heart and many glands and smooth muscles are innervated by both sympathetic and parasympathetic fibers; that is, they receive dual innervation. Whatever effect one division has on the effector cells, the other division usually has the opposite effect. (Several exceptions to this rule are indicated in Table 8-13.) Moreover, the two divisions are usually activated reciprocally; that is, as the activity of one division is increased, the activity of the other is decreased. Dual innervation by nerve fibers that cause opposite responses provides a very fine degree of control over the effector organ. A useful generalization is that the sympathetic system increases its response under conditions of physical or psychological stress. Indeed, a full-blown sympathetic response is called the fight-or-flight response, describing the situation of an animal forced to challenge an attacker or run from it. All resources are mobilized: heart rate and blood pressure increase; blood flow to the skeletal muscles, heart, and brain increase; the liver releases glucose; and the pupils dilate. Simultaneously, activity of the gastrointestinal tract and blood flow to the skin are decreased by inhibitory sympathetic effects. The two divisions of the autonomic nervous system rarely operate independently, and autonomic responses generally represent the regulated interplay of both divisions. Autonomic responses usually occur without conscious control or awareness, as though they were indeed autonomous (in fact, the autonomic nervous system has been called the "involuntary" nervous system). However, it is wrong to assume that this is always the case, for it has been shown that discrete visceral or glandular responses can be learned and thus, to this extent, voluntarily controlled. Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition
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