The Autonomic Nervous System: A Comprehensive Overview, Exams of Physiology

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THE AUTO

NERVOUS

PHYSIOLOGY

NOMIC

SYSTEM

ERIC J ZELINSKAS, CST, BA

The nervous system is one of the most compli- cated systems in the human body. Along with the endocrine system, it controls many bodily activ- ities. The nervous system senses changes both in the internal and external environments, inter- prets these changes, and then coordinates appro- priate responses in order to maintain homeosta- sis. 1 In response to changing conditions, the autonomic nervous system (ANS) shunts blood to more needy areas, speeds or slows heart and respiratory rates, adjusts blood pressure and body temperature, and increases or decreases stomach secretions. 2 Most of this fine-tuning occurs without our conscious awareness or attention, implying a certain amount of func- tional independence. Hence the term autonomic (auto=self; nom=govern).

thalamus.^3 Often the autonomic nervous system operates by means of autonomic reflexes. Senso- ry signals from peripheral nerve receptors relay signals into the centers of the cord, brain stem, or hypothalamus, and these in turn transmit appro- priate reflex responses back to the peripheral organs or tissues to control their activities.^4

Divisions of the autonomic nervous system

The autonomic nervous system (ANS) is further divided into two major subdivisions: the parasympathetic nervous system (PaNS) and sympathetic nervous system (SyNS). The two divisions are physiological antagonists and are in equilibrium with each other. Both divisions often innervate the same organ (eg iris of the eye and the heart). Structurally, each division differs in the location of their preganglionic neuron cell bodies within the CNS, the location of their autonomic ganglion, the relative lengths of their preganglion- ic and postganglionic axons, and the ratio of pre- ganglionic and postganglionic neurons. They both integrate and operate continuously with the rest of the nervous system by responding in vary- ing degrees to information provided by the senso- ry component of the nervous system. The SyNS dominates during stressful or physically strenuous situations. It sends impuls- es that increase blood pressure, speed up rate and force of the heartbeat, dilate bronchioles, increase blood sugar concentration and reroute blood flow to skeletal muscle (fight or flight). Conversely, the PaNS dominates during times of emotional calm and/or physical rest. It sends impulses that decrease blood pressure, decrease heart rate and stimulate gastrointestinal motility (digestion and rest).

Parasympathetic nervous system

The PaNS is the craniosacral division of the ANS. Preganglionic fibers originate from nuclei in the midbrain, medulla and sacral portion of the spinal cord. Neurons of the PaNS emerge from the brainstem and pass through as part of the III, VII, IX, and X cranial nerves, and 2nd, 3rd, and 4th sacral nerves from the sacral region. 4 They synapse with postganglionic neurons located in

autonomic (terminal) ganglia that lie near or within the walls of the organs innervated (Figure 1). Since the terminal ganglia are close to the innervated organs/structures, the axons of the postganglionic fibers are short. PaNS pregan- glionic neurons synapse with only a relatively few postganglionic neurons. For this reason they are much more precise and localized in their effects. Some effects of PaNS stimulation include:

• constriction of pupils
• contraction of smooth muscle of alimentary canal
• constriction of bronchioles
• slowing of heart rate

FIGURE 1

The parasympa-

thetic nervous

system. 3

Ciliary ganglion

1 2

III

VII

X

IX (^) V

3 4

Ciliary muscles of the eye Pupillary sphincter

Sphenopalatine ganglion Lacrimal glands Nasal glands

Submaxillary ganglion Submaxillary gland

Otic ganglion

Heart

Stomach Pylorus

Colon

Small intestine

Ileocecal valve

Anal sphincter Bladder Detrusor Trigone

Sacral

Parotid gland

ILLUSTRATION REPRINTED BY PERMISSION OF

Pearson Education Inc.© 2001

Transmission of an impulse between pregan- glionic and postganglionic fibers takes place at an electrochemical junction called a synapse. Both pre- and postganglionic neurons of the PaNS are cholinergic and utilize the neurotrans- mitter acetylcholine (ACh). 4 When a nerve impulse reaches the terminus of a preganglionic fiber, it causes the release of ACh, which migrates across the synapse. The ACh combines with receptors on the synaptic membrane of the post- ganglionic fiber, causing depolarization and continuing the impulse down the postganglion- ic fiber. Once the impulse reaches the postgan- glionic terminus and depolarizes it, ACh migrates across the synapse and binds to specific

receptor sites in the effector gland, organ or muscle causing the desired effect (eg release of hormones, muscular contraction, etc). (Table 1) The action of acetylcholine is relatively brief and usually lasts for only a fraction of a second. It is rapidly broken down by the enzyme cholinesterase, which is present both in the ter- minal nerve ending and on the surface of the receptor organ. Acetylcholine (cholinergic) receptor sites are classified as either nicotinic or muscarinic. Nicotinic receptor sites for ACh occur at the junction between the preganglionic fibers and postganglionic fibers in both the SyNS and the PaNS divisions of the ANS. Muscarinic receptor sites for ACh occur at the junction between the postganglionic fibers and effector sites in the PaNS division of the ANS.

Sympathetic nervous system

The SyNS is the thoracolumbar division of the ANS (Figure 2). Preganglionic fibers originate from cell bodies in the lateral gray horn of all thoracic and the first two or three lumbar seg- ments of the spinal cord and leave the cord by way of the of the anterior (ventral) spinal nerve roots (Figure 3). They pass through the inter- vertebral foramina, enter a white rami commu- nicans and connect with the ganglia of the par- avertebral sympathetic chain, which are situated anterolaterally to the spinal cord. Each of these paired chains is a series of 22 ganglia spanning the length of the vertebral column. 4 All pregan- glionic neurons in the SyNS are myelinated, which gives them a white appearance. Most pre- ganglionic neurons end within the paraverte- bral sympathetic ganglia and synapse with post- ganglionic efferent neurons. Some of the postganglionic neurons re-enter the spinal nerves via the grey rami communicans. They appear grey because postganglionic neurons are nonmyelinated. These neurons extend with other neurons in the spinal nerves, eventually branch off and form visceral nerves that inner- vate smooth muscle and sweat glands. Other preganglionic neurons pass through the paraver- tebral sympathetic ganglia to a second set of gan- glia called collateral ganglia located mainly in the

FIGURE 2

The sympathetic

nervous system.^3

Dashed lines represent post- ganglionic fibers in the gray rami leading into the spinal nerves for distribution to blood vessels,sweat glands, and pilo-erector muscles.

Pilo-erector muscle

Sweat gland

Blood vessel

Eye

Heart

Bronchi

Pylorus

Adrenal medulla

Kidney Ureter

Detrusor Trigone

Anal sphincter

Intestine

Ileocecal valve

Celiac ganglion

T-

B

I

5

L-

5

Hypogastric plexus ILLUSTRATION REPRINTED BY PERMISSION OF

Pearson Education Inc.© 2001

abdomen close to the aorta and its major branch- es (eg celiac, superior mesenteric and inferior mesenteric arteries). These bundles of collateral ganglia are often called plexus. 4 The pregan- glionic neurons synapse with nonmyelinated neurons in the collateral ganglion. The postgan- glionic neurons branch off and innervate the smooth muscles of the abdominal and pelvic vis- cera and the endocrine glands in that area. The effects of the SyNS are extremely widespread rather than specific to one organ or muscle. Preganglionic neurons of the SyNS are cholinergic and utilize the neurotransmitter acetylcholine (ACh). A few of the postganglionic neurons of the SyNS are cholinergic and secrete

acetylcholine (ACh). They innervate the sweat glands of the skin, some blood vessels within the skeletal muscles and the external genitalia. But by far, the majority of the sympathetic postgan- glionic nerves are adrenergic and utilize the neu- rotransmitter norepinephrine (NE). 3 The affect of NE released at the effector site produces diff- erent results (excitation or inhibition) depend- ing on the receptor(s) to which it binds. (Table 2) There are two major classes of adrenergic (NE-binding) receptors: alpha (α) and beta (β). Organs that respond to NE (or epinephrine, EPI) display one or both types of receptors. In general, NE or epinephrine binding to alpha receptors is stimulatory, while their binding to

Paravertebral (sympathetic chain) ganglion Dorsal root and dorsal root ganglion

Lateral horn of gray matter (visceral motor zone) Sympathetic trunk Ventral ramus of spinal nerve Gray ramus communicans White ramus communicans

Prevertebral (collateral) ganglion such as the celiac

Target organ (in abdomen)

Splanchnic nerve

Ventral root

Dorsal ramus of spinal nerve (^) Blood vessels

{

To effector

Skin (arrector pili mus- cles and sweat glands)

FIGURE 3

Sympathetic

pathways. 2

synapse in a paraver- tebral (chain) gan- glion at the same level synapse in a paraver- tebral ganglion at a different level synapse in a preverte- bral (collateral) gan- glion anterior to the vertebral column

ILLUSTRATION REPRINTED BY PERMISSION OF

WB Saunders Publishing Co.© 2001

beta receptors is inhibitory. However, there are notable exceptions. For example, binding of NE to the beta receptors of cardiac muscle prods the heart into more vigorous activity. These differ- ences reflect that both alpha and beta receptors have two receptor subclasses (alpha 1 and alpha 2, beta 1 and beta 2). Each receptor type tends to predominate in certain target organs (Table 3).

Adrenal medulla

Some preganglionic sympathetic (thoracic splanchnic) nerve fibers pass through the celiac ganglion without synapsing and terminate by synapsing with hormone-producing medullary cells (chromaffin cells) of the adrenal gland. When stimulated by the preganglionic fibers, the chromaffin cells release large quantities of epi- nephrine and norepinephrine directly into the blood stream. These hormones are then carried to tissues throughout the body where they rein- force the effects of the SyNS. 4 The epinephrine and norepinephrine released by the combined efforts of the SyNS and the adrenal glands is eventually dissipated either by being taken back into the synaptic nerve endings or by action of the enzyme monoamine oxidase.^4

Sympathetic and parasympathetic tone

The autonomic system generally maintains a ‘tone,’ a basal level of activity, which then may be either increased or decreased by central control. 5 The sympathetic and parasympathetic systems are continually active and the basal rates of stim- ulation are known, respectively, as sympathetic tone and parasympathetic tone. 5 The value of tone is that it allows a single nervous system to increase or decrease the activity of an organ. For example, sympathetic tone normally keeps almost all the blood vessels of the body con- stricted to approximately half their maximum diameter. By increasing the degree of sympathet- ic stimulation, the vessels can be constricted even more; but, on the other hand, by decreasing the level of sympathetic stimulation, the vessels can be dilated. 5 Another example of tone is that of the parasympathetics in the gastrointestinal tract.

Surgical removal of the parasympathetic supply to the gut by cutting the vagi can cause serious and prolonged gastric and intestinal atony, thus illustrating that in normal function the parasym- pathetic tone to the gut is strong. This tone can be decreased by the brain, thereby inhibiting gas- trointestinal motility, or it can be increased, thereby promoting increased gastrointestinal activity. 5 The presence of dual innervation and the possibility of either increasing or decreasing the tone permit a wide range of control.^5

Conclusion

The art and science of medicine has changed very rapidly over the last 10 years. The high cost of hospital care has created an impetus for surgical technologists to master the knowledge and advanced procedural skills necessary to meet the growing demands of an ever more complex sur- gical environment. It is my hope this article has provided a useful framework upon which surgi- cal technologists can advance their knowledge and understanding of human physiology as it relates to patient care, thus being better prepared to move into the realm of advanced practice.

References

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