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For both parasympathetic and sympathetic divisions, acetylcholine is the ganglionic neurotransmitter.Postganglionic fibers from parasympathetic ganglia release acetylcholine, which binds to muscarinic receptors at the effector organs. Sympatheticpostganglionic axons release mainly norepinephrine, although acetylcholine is released at sweat glands. The adrenal gland (not illustrated) functionsas part of the SNS. Chromaffin cells of the adrenal medulla act as postganglionic cells of the SNS, releasing epinephrine (and to some extent, norepinephrine) directly into the blood stream. Sympathetic postganglionic nerves, in addition to releasing catecholamines (norepinephrine and epinephrine), release several cotransmitters, including neuropeptide Y, ATP, and substance P.Table 7.1General Characteristics of the Parasympathetic and Sympathetic Nervous SystemsCharacteristicParasympathetic Nervous SystemSympathetic Nervous SystemLocation of preganglionic nervecell bodiesBrainstem (nuclei of cranial nerves II, VII,IX, and X) or sacral spinal cord (S2–S4;sacral parasympathetic nucleus)Intermediolateral and intermediomedial cellcolumns of the thoracolumbar spinal cord (T1–L3)Location of gangliaIn or adjacent to target organsParavertebral and prevertebralNeurotransmitter of preganglionicneuronsAcetylcholine (acts at nicotinic receptors)Acetylcholine (acts at nicotinic receptors)Major neurotransmitter released bypostganglionic neuronAcetylcholine (acts at muscarinicreceptors)Norepinephrine (acts at α and β adrenergicreceptors)The Autonomic Nervous System89Preganglionic sympatheticPostganglionic sympatheticEyeC1Cardiac plexusC2C3C4C5C6C7C8T1HeartLungsT2T3Pulmonary plexusT4T5T6CeliacganglionT7T8T10T12L1Lumbarsplanchnic nervesL2L4Small intestineColonInferiormesentericganglionL3Postganglionic fibersto spinal nerves (toblood vessels, sweatglands, and arrectorpili muscles in skin)PancreasSuperiormesentericganglionT11SpleenStomachThoracicsplanchnic nervesT9Liver and gallbladderL5Adrenal medullaKidneyS1S2S3S4S5CoSympatheticchain gangliaUrinary bladderOvaryPenisUterusScrotumFigure 7.2 Autonomic Nervous System: Sympathetic Division The two divisions of the autonomic nervous system are the sympatheticnervous system and the parasympathetic nervous system.
Preganglionic fibers of the SNS emerge from the spinal cord at levels T1–T2. The SNS isinvolved in “fight-or-flight” responses, as well as responses to exercise, hemorrhage, and other challenges to homeostasis. Both SNS and PNSinnervate smooth and cardiac muscle and glands; in general, they work together in reciprocal fashion to regulate bodily function.Sweat gland secretion is stimulated by activation of thesympathetic nervous system. Most of the postganglionicsympathetic neurons innervating these glands are atypical,releasing the neurotransmitter acetylcholine instead of norepinephrine.
Acetylcholine acts on muscarinic receptors, inducingsweat secretion. However, in some specific areas, such as thepalms of the hands, adrenergic nerves stimulate sweat glandsthrough the release of norepinephrine, which acts at α1 receptors to stimulate secretion.AUTONOMIC RECEPTORSAutonomic receptors are coupled to specific G proteins withinthe cell membrane and produce their effects through varioussignal transduction systems in the effector cells. Acetylcholinereceptors are classified as nicotinic or cholinergic, based ontheir pharmacology. Nicotinic receptors are activated by thedrug nicotine and are blocked by curare, the active agent inSouth American dart poison.
Muscarinic receptors, on theother hand, are activated by the mushroom toxin muscarine90The Nervous System and MuscleCN IIICiliary ganglionCN VIICN IXC1C2C3C4C5C6C7C8EyePterygopalatine ganglionLacrimal glandSubmandibular ganglionNasal mucosaParanasal sinusesCN XOtic ganglionSalivary glandsT1T2T3T4HeartSpinal cordT5T6LungsT7PreganglionicparasympatheticPostganglionicparasympatheticT8T9Liver and gallbladderT10SpleenT11StomachT12PancreasL1L2Small intestineL3ColonL4L5S1Pelvic splanchnic nervesS2KidneyS3S4S5CoUrinary bladderOvaryPenisUterusScrotumFigure 7.3 Autonomic Nervous System: Parasympathetic Division Preganglionic fibers of thePNS are associated with cranial nerves III, VII, IX, and X and also emerge from the sacral spinal cord atlevels S2–S4. The PNS is involved in “vegetative” processes such as digestion as well as homeostatic functions.
In general, the SNS and PNS work together in reciprocal fashion to regulate bodily function.The Autonomic Nervous SystemTable 7.291Actions of the Autonomic Nervous SystemPARASYMPATHETIC NERVOUS SYSTEMSYMPATHETIC NERVOUS SYSTEMSite of ActionActionReceptor typeActionReceptor typeCardiac pacemakerDecreases heart rateMuscarinicIncreases heart rateβ1Cardiac muscleDecreases contractility ofatria; limited effects onventriclesMuscarinicIncreased contractilityβ1Cardiac AV nodeDecreases conductionvelocityMuscarinicIncreased conduction velocityβ1Vascular smooth muscleIndirect vasodilation (genitalorgans and lower GItract only) by nitric oxidereleased fromendotheliumMuscarinicConstriction (predominanteffect in most vascularbeds)Vasodilationα1Increases motilityMuscarinicReduces motilityα2, β2Relaxes sphinctersMuscarinicConstricts sphinctersα1Gastric parietal cellsAcid secretionMuscarinicPancreasExocrine secretionMuscarinicLung, bronchial smooth muscleConstrictsMuscarinicDilatesβ2Secretion (generalized)Secretion (specific areassuch as palms)Muscarinic*α1Gastrointestinal smooth muscleSweat glandsβ2Male reproductive systemErectionMuscarinicEmission during orgasmαFemale reproductive systemVasocongestion, vaginallubricationMuscarinicOrgasmic smooth muscleconstrictionαPupilMiosis (constriction)MuscarinicMydriasis (dilation)α1AV, atrioventricular.*Atypical sympathetic cholinergic nerve fibers secrete acetylcholine.and are blocked by the drug atropine, the deadly nightshadetoxin.
Nicotinic receptors are membrane channels thatconduct Na+ and K+ when acetylcholine is bound. There areseveral subtypes of muscarinic receptors; the most commonare coupled to G proteins that activate phospholipase C andproduce elevation of IP3 and free intracellular Ca2+.Receptors for catecholamines are classified as α-adrenergic orβ-adrenergic on the basis of their responses to specific pharmacologic agents (drugs). β-receptors are blocked by the drugpropranolol, and α-receptors are blocked by phentolamine.Adrenergic receptors may be further subclassified as α1, α2,β1, β2, and β3 subtypes on the basis of effects of more specificdrugs.
At α1-receptors, binding of an agonist produces elevation of free intracellular Ca2+; binding of an agonist at α2receptors produces inhibition of adenyl cyclase and reductionof the levels of the second messenger cAMP. Activation of βreceptors results in activation of adenyl cyclase and elevationof cAMP. How an organ or tissue responds to sympatheticstimulation is largely a function of the type of adrenergicreceptors present in the tissue (see Table 7.2).92The Nervous System and Musclethetic activity.
Catecholamines in plasma and 24-hour urinecollections are elevated, and unlike catecholamines releasedby normal adrenal medulla, are not suppressed by clonidineadministration (clonidine acts centrally to suppress sympatheticactivity). Treatment is by surgical resection of the tumor. Pheochromocytoma is a rare disorder.CLINICAL CORRELATEPheochromocytomaTumors of the adrenal medulla that secrete epinephrine and norepinephrine are known as pheochromocytomas; catecholaminesecreting tumors in extraadrenal tissue may also occur. In eithercase, signs and symptoms are consistent with increased sympa-Adrenal pheochromocytomaPotential sites of pheochromocytomaSympathetic trunkAortic archDiaphragmSpleenIncreased dopaminesecretion suggestsmalignant tumor.Tumor secretes increasedamounts of catecholamines,usually epinephrine, andnorepinephrine.Adrenal medullaAbdominal aortaHypertensionmay be episodicor sustained.KidneyZuckerkandlbodyOvaryBladder wallTestesVasoconstriction increases peripheralresistance and blood pressure.Pheochromocytoma is a chromaffin cell tumor secretingexcessive catecholamines resulting in increased peripheralvascular resistance and hypertension.Most pheochromocytomas are adrenal in origin, but can occur invarious sites and may be associated with multiple endocrine neoplasia(MEN) syndromes.
Most are sporadic, but some are hereditary.Clinical features of pheochromocytomaHeadacheRandomurine sampleSweating andflushingAnxiety24-hoururine sampleRandom urine assay for creatine and metanephrineor 24-hour urine assay of metanephrine and freecatecholamines used in diagnosis.NauseaPalpitations/chest painsWeaknessBlood pressureEpigastric painTremorSymptoms are secondary to excessive catecholamine secretion and areusually paroxysmal.
More than 90% of patients with pheochromocytomahave headaches, palpitations, and sweating alone or in combination.CT scan or MRI may revealpresence of tumor.Catecholamines secreted by tumors of the adrenal medulla or other sites produce excess sympathetic-likeactivity.Review Questions93Review QuestionsCHAPTER 3: NERVE AND MUSCLE PHYSIOLOGY1.
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