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

Skeletal muscle

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

Mesenteric Nervous System

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

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)

Collateral Ganglia

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

CNS

Somatic nervous system

Effector

/

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

Neural Control Mechanisms CHAPTER EIGHT

Neural Control Mechanisms CHAPTER EIGHT

TABLE 8-13 Some Effects of Autonomic Nervous System Activity

Sympathetic

Receptor

Parasympathetic

Effector Organ

Type*

Effect

Effect

Eyes

Iris muscles

Alpha

Contracts radial muscle

Contracts sphincter

(widens pupil)

muscle (makes pupil

smaller)

Ciliary muscle

Beta

Relaxes (flattens lens

Contracts (allows lens

for far vision)

to become more convex

for near vision)

Heart

SA node

Beta

Increases heart rate

Decreases heart rate

Atria

Beta

Increases contractility

Decreases contractility

AV node

Beta

Increases conduction

Decreases conduction

velocity

velocity

Ventricles

Beta

Increases contractility

Decreases contractility

slightly

Arterioles

Coronary

Alpha

Constricts

Beta

Dilates

Skin

Alpha

Constricts

_t

Skeletal muscle

Alpha

Constricts

Beta

Dilates

Abdominal viscera

Alpha

Constricts

Beta

Dilates

Salivary glands

Alpha

Constricts

Dilates

Veins

Alpha

Constricts

Beta

Dilates

Lungs

Bronchial muscle

Beta

Relaxes

Contracts

Bronchial glands

Alpha

Inhibits secretion

Stimulates secretion

Beta

Stimulates secretion

Salivary glands

Alpha

Stimulates watery

Stimulates watery

secretion

secretion

Beta

Stimulates enzyme

secretion

Stomach

Motility, tone

Alpha and Beta

Decreases

Increases

Sphincters

Alpha

Contracts

Relaxes

Secretion

Inhibits (?)

Stimulates

Intestine

Motility

Alpha and Beta

Decreases

Increases

Sphincters

Alpha

Contracts (usually)

Relaxes (usually)

Secretion

Alpha

Inhibits

Stimulates

Gallbladder

Beta

Relaxes

Contracts

Liver

Alpha and Beta

Glycogenolysis and

gluconeogenesis

Pancreas

Exocrine glands

Alpha

Inhibits secretion

Stimulates secretion

Endocrine glands

Alpha

Inhibits secretion

Beta

Stimulates secretion

PART TWO Biological Control Systems

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

II. Biological Control Systems

8. Neural Control Mechanisms

PART TWO Biological Control Systems

TABLE 8-13 Some Effects of Autonomic Nervous System Activity (cont.)

Effector Organ

Receptor Type*

Sympathetic Effect

Parasympathetic Effectt

Fat cells

Alpha and Beta

Increases fat breakdown

Kidneys

Beta

Increases renin secretion

Bladder wall

Beta

Relaxes

Contracts

Sphincter

Alpha

Contracts

Relaxes

Uterus

Alpha Beta

Contracts in pregnancy Relaxes

Variable

Reproductive tract

(male)

Alpha

Ejaculation

Erection

Skin

Muscles causing hair erection

Alpha

Contracts

Sweat glands

Alpha

Localized secretion

Generalized secretion

Lacrimal glands

Alpha

Secretion

Secretion

Table adapted from "Goodman and Gilman's The Pharmacological Basis of Therapeutics," Joel G. Hardman, Lee E. Limbird, Perry B. Molinoff, Raymond W. Ruddon, and Alfred Goodman Gilman, eds., 9th edn., McGraw-Hill, New York, 1996.

*Note that in many effector organs, there are both alpha-adrenergic and beta-adrenergic receptors. Activation of these receptors may produce either the same or opposing effects. For simplicity, except for the arterioles and a few other cases, only the dominant sympathetic effect is given when the two receptors oppose each other. tThese effects are all mediated by muscarinic receptors.

^A dash means these cells are not innervated by this branch of the autonomic nervous system or that these nerves do not play a significant physiological role.

Table adapted from "Goodman and Gilman's The Pharmacological Basis of Therapeutics," Joel G. Hardman, Lee E. Limbird, Perry B. Molinoff, Raymond W. Ruddon, and Alfred Goodman Gilman, eds., 9th edn., McGraw-Hill, New York, 1996.

*Note that in many effector organs, there are both alpha-adrenergic and beta-adrenergic receptors. Activation of these receptors may produce either the same or opposing effects. For simplicity, except for the arterioles and a few other cases, only the dominant sympathetic effect is given when the two receptors oppose each other. tThese effects are all mediated by muscarinic receptors.

^A dash means these cells are not innervated by this branch of the autonomic nervous system or that these nerves do not play a significant physiological role.

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Essentials of Human Physiology

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Responses

  • mary
    Why does the autonomic nervous system use two neurons in a chain to the tissues?
    7 years ago
  • tara
    Why are sympathetic pregangliomic fibers closer to the spinal cord?
    7 years ago
  • rita
    Where is the mesenteric ganglion?
    7 years ago
  • Haylom
    How does the sympathetic nervous system release at the effector organ?
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  • Emppu Takko
    What nerve goes to superior portion of the liver?
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  • zula
    What body system is relatively independent of nervous system control?
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  • Ruby
    Why is noradrenaline the major neurotransmitter of the sympathetic post ganglionic fibres?
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  • uriele
    What are the effects of the two divisions of the ans worksheet?
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  • chloe
    What is the chemical transmitter between sympathetic fibers and the effector organs?
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  • BISIRAT
    What are the neurotransmitters for the pre and postganglionic fibers of both systems?
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  • stefan
    What are pre and post ganglionic fibres images?
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  • ROSE
    What nerve controls the respiratory system?
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  • filiberta
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  • Scudamor
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  • mia
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  • jessamine
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  • Dylan
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  • tarsha
    Is the inferior ganglion a postganglionic?
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  • rosalia
    What bodilly functions are controlled by the ganglion nerve?
    6 years ago
  • ulrich eichelberger
    What is ganglion in nervous system?
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  • amber
    Which is an autonomic body function?
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  • Melilot
    What are the functions of autonomic nervous system in different organs of the body?
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  • ida
    What side of body represents parasympathetic system?
    6 years ago
  • holfast
    Which autonomic nervous system effector undergoes opposing effects without dual innervation?
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  • Zack Reid
    What is autonomic nervous system function to our body?
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  • ruta
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  • Gabriela
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  • karla
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    6 years ago
  • michael
    How the autonomic nervous system heals body?
    5 years ago
  • ronald
    What percentage of body functions is controlled by sympathetic nervous system?
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  • Jessica
    Does the postganglionic undergo divergence?
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