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Autonomic Nervous System

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Autonomic Nervous System PHYSIOLOGY THE AUTO NERVOUS 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). ERIC J ZELINSKAS, CST, BA ONOMIC S SYSTEM 207 NOVEMBER 2001 CATEGORY 1 eneral organization Functionally, the PNS is subdivided into two The nervous system includes the central nervous specialized systems: the somatic nervous system system (CNS), consisting of the brain and spinal (SNS) and the autonomic nervous system cord, and the peripheral nervous system (PNS), (ANS).1 The SNS primarily innervates skeletal consisting of 12 pairs of cranial nerves (which muscle, producing consciously controlled, vol­ emerge from the base of the skull) and 31 pairs of untary movement (ie walking and talking). The spinal nerves (which emerge from the spinal ANS primarily innervates glands, smooth muscle cord). All of these nerves consist of fibers that and cardiac muscle. It’s responsible for control­ may be sensory or motor or a mixture of both.1 ling visceral functions and involuntary muscles in Nerves composed of both sensory and motor the respiratory, circulatory, digestive and urogen­ fibers are called mixed nerves. For example, the ital systems, and in the skin6 that are essential for facial nerve CN VII consists of motor fibers that the body to maintain homeostasis. The ANS oper­ control facial expressions (eg frowning and smil­ ates without conscious control. The autonomic ing) and sensory fibers, which transmit taste sen­ nervous system is activated mainly by centers sations from the tongue to the brain. located in the spinal cord, brain stem, and hypo- gReview of basic parts of the neuron Dendrites Cell body Nucleus Axon Myelin sheath Nerve impulse Axon Vesicle Synaptic cleft Direction of impulse Neurotransmitters Receptor molecules Dendrite of receiving neuron Axon terminals thalamus.3 Often the autonomic nervous system autonomic (terminal) ganglia that lie near or FIGURE 1 operates by means of autonomic reflexes. Senso­ within the walls of the organs innervated (Figure ry signals from peripheral nerve receptors relay 1). Since the terminal ganglia are close to the The parasympa­ signals into the centers of the cord, brain stem, or innervated organs/structures, the axons of the hypothalamus, and these in turn transmit appro­ postganglionic fibers are short. PaNS pregan­ thetic nervous priate reflex responses back to the peripheral glionic neurons synapse with only a relatively few organs or tissues to control their activities.4 postganglionic neurons. For this reason they are system.3 much more precise and localized in their effects. Divisions of the autonomic nervous system Some effects of PaNS stimulation include: The autonomic nervous system (ANS) is further divided into two major subdivisions: the • constriction of pupils parasympathetic nervous system (PaNS) and • contraction of smooth muscle of alimentary sympathetic nervous system (SyNS). The two canal divisions are physiological antagonists and are in • constriction of bronchioles equilibrium with each other. Both divisions often • slowing of heart rate innervate the same organ (eg iris of the eye and the heart). Structurally, each division differs in the Ciliary ganglion location of their preganglionic neuron cell bodies Ciliary muscles of the eye within the CNS, the location of their autonomic III Pupillary sphincter ganglion, the relative lengths of their preganglion­ ic and postganglionic axons, and the ratio of pre­ VII Sphenopalatine ganglion IX ganglionic and postganglionic neurons. They V Lacrimal glands both integrate and operate continuously with the X Nasal glands rest of the nervous system by responding in vary­ ing degrees to information provided by the senso­ Submaxillary ganglion ry component of the nervous system. Submaxillary gland The SyNS dominates during stressful or physically strenuous situations. It sends impuls­ Otic ganglion es that increase blood pressure, speed up rate and Parotid gland force of the heartbeat, dilate bronchioles, increase blood sugar concentration and reroute Heart blood flow to skeletal muscle (fight or flight). Conversely, the PaNS dominates during times of Stomach emotional calm and/or physical rest. It sends Pylorus impulses that decrease blood pressure, decrease heart rate and stimulate gastrointestinal motility Colon (digestion and rest). Small intestine Parasympathetic nervous system Sacral The PaNS is the craniosacral division of the ANS. Ileocecal valve Preganglionic fibers originate from nuclei in the 1 midbrain, medulla and sacral portion of the 2 spinal cord. Neurons of the PaNS emerge from 3 4 Anal sphincter the brainstem and pass through as part of the III, Bladder Inc.© Education Pearson 2001 VII, IX, and X cranial nerves, and 2nd, 3rd, and Detrusor 4th sacral nerves from the sacral region.4 They Trigone synapse with postganglionic neurons located in ILLUSTRATION REPRINTED BY PERMISSION OF receptor sites in the effector gland, organ or muscle causing the desired effect (eg release of Eye hormones, muscular contraction, etc). (Table 1) The action of acetylcholine is relatively brief and usually lasts for only a fraction of a second. It Pilo-erector muscle B is rapidly broken down by the enzyme Heart cholinesterase, which is present both in the ter­ T-1 minal nerve ending and on the surface of the Sweat gland 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 I2 Bronchi fibers and postganglionic fibers in both the SyNS Blood vessel and the PaNS divisions of the ANS. Muscarinic Celiac ganglion receptor sites for ACh occur at the junction between the postganglionic fibers and effector Pylorus sites in the PaNS division of the ANS. L-1 Adrenal Sympathetic nervous system 5 medulla The SyNS is the thoracolumbar division of the ANS (Figure 2). Preganglionic fibers originate Kidney from cell bodies in the lateral gray horn of all Ureter thoracic and the first two or three lumbar seg­ 5 ments of the spinal cord and leave the cord by way of the of the anterior (ventral) spinal nerve Intestine roots (Figure 3). They pass through the inter­ Ileocecal valve vertebral foramina, enter a white rami commu­ Pearson Education Inc. © 2001 Inc. Education Pearson nicans and connect with the ganglia of the par­ avertebral sympathetic chain, which are situated Anal sphincter Hypogastric plexus anterolaterally to the spinal cord. Each of these Detrusor Trigone ILLUSTRATION REPRINTED BY PERMISSION OF paired chains is a series of 22 ganglia spanning the length of the vertebral column.4 All pregan­ Transmission of an impulse between pregan­ glionic neurons in the SyNS are myelinated, glionic and postganglionic fibers takes place at which gives them a white appearance. Most pre­ an electrochemical junction called a synapse. ganglionic neurons end within the paraverte­ FIGURE 2 Both pre- and postganglionic neurons of the bral sympathetic ganglia and synapse with post­ PaNS are cholinergic and utilize the neurotrans- ganglionic efferent neurons. Some of the The sympathetic mitter acetylcholine (ACh).4 When a nerve postganglionic neurons re-enter the spinal impulse reaches the terminus of a preganglionic nerves via the grey rami communicans. They nervous system.3 fiber, it causes the release of ACh, which migrates appear grey because postganglionic neurons are across the synapse. The ACh combines with nonmyelinated. These neurons extend with Dashed lines represent post- receptors on the synaptic membrane of the post­ other neurons in the spinal nerves, eventually ganglionic fibers in the gray ganglionic fiber, causing depolarization and branch off and form visceral nerves that inner­ rami leading into the spinal continuing the impulse down the postganglion­ vate smooth muscle and sweat glands. Other nerves for distribution to ic fiber. Once the impulse reaches the postgan­ preganglionic neurons pass through the paraver­ blood vessels, sweat glands, glionic terminus and depolarizes it, ACh tebral sympathetic ganglia to a second set of gan­ and pilo-erector muscles. migrates across the synapse and binds to specific glia called collateral ganglia located mainly in the Table 1 ANS parasympathetic preganglionic postganglionic effector organs, fibers fibers glands or muscles Table 2 ANS sympathetic sweat glands, preganglionic postganglionic blood vessels (some), fibers fibers external genitalia heart, preganglionic postganglionic blood vessels (most), fibers fibers smooth muscle in GI tract preganglionic adrenal blood fibers medulla stream Table 3 Cholinergic and adrenergic receptors2 Neurotransmitter Receptor Type Major locations Effect of binding Acetylcholine Cholinergic Nicotinic All postganglionic neurons; adrenal Excitation medullary cells (also neuromuscular junc­ tions of skeletal muscle) Muscarinic All parasympathetic target organs Excitation in most cases;
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