Y khoa, y dược - The autonomic nervous system

14 The Autonomic Nervous SystemAutonomic Nervous System (ANS)The ANS consists of motor neurons that: Innervate smooth and cardiac muscle and glandsMake adjustments to ensure optimal support for body activitiesOperate via subconscious controlAutonomic Nervous System (ANS)Other namesInvoluntary nervous system General visceral motor systemCentral nervous system (CNS)Peripheral nervous system (PNS)Motor (efferent) divisionSensory (afferent)divisionSomatic nervoussystemAutonomic nervoussystem (ANS)SympatheticdivisionParasympatheticdivisionFigure 14.1Somatic and Autonomic Nervous Systems The two systems differ inEffectorsEfferent pathways (and their neurotransmitters)Target organ responses to neurotransmittersEffectorsSomatic nervous systemSkeletal musclesANSCardiac muscleSmooth muscleGlandsEfferent PathwaysSomatic nervous systemA, thick, heavily myelinated somatic motor fiber makes up each pathway from the CNS to the muscleANS pathway is a two-neuron chainPreganglionic neuron (in CNS) has a thin, lightly myelinated preganglionic axonGanglionic neuron in autonomic ganglion has an unmyelinated postganglionic axon that extends to the effector organNeurotransmitter EffectsSomatic nervous systemAll somatic motor neurons release acetylcholine (ACh)Effects are always stimulatoryANSPreganglionic fibers release AChPostganglionic fibers release norepinephrine or ACh at effectorsEffect is either stimulatory or inhibitory, depending on type of receptorsSkeletal muscleCell bodies in centralnervous systemPeripheral nervous systemEffect++EffectororgansAChAChSmooth muscle(e.g., in gut),glands, cardiacmuscleGanglionAdrenal medullaBlood vesselAChAChAChNEEpinephrine andnorepinephrineAcetylcholine (ACh)Norepinephrine (NE)Ganglion Heavily myelinated axon Lightly myelinatedpreganglionic axon Lightly myelinatedpreganglionic axons Neuro-transmitterat effectorUnmyelinatedpostganglionicaxonUnmyelinatedpostganglionic axonStimulatoryStimulatoryor inhibitory,dependingon neuro-transmitterandreceptorson effectororgansSingle neuron from CNS to effector organsTwo-neuron chain from CNS to effector organsSOMATICNERVOUSSYSTEMAUTONOMIC NERVOUS SYSTEMPARASYMPATHETICSYMPATHETICFigure 14.2Divisions of the ANSSympathetic divisionParasympathetic divisionDual innervationAlmost all visceral organs are served by both divisions, but they cause opposite effectsRole of the Parasympathetic DivisionPromotes maintenance activities and conserves body energyIts activity is illustrated in a person who relaxes, reading, after a mealBlood pressure, heart rate, and respiratory rates are lowGastrointestinal tract activity is highPupils are constricted and lenses are accommodated for close visionRole of the Sympathetic DivisionMobilizes the body during activity; is the “fight-or-flight” systemPromotes adjustments during exercise, or when threatenedBlood flow is shunted to skeletal muscles and heartBronchioles dilate Liver releases glucoseANS AnatomySalivaryglandsEyeSkin*HeartLungsLiverand gall-bladderGenitalsPancreasEyeLungsBladderLiver andgall-bladderPancreasStomachCervicalSympatheticgangliaCranialLumbarThoracicGenitalsHeartSalivaryglandsStomachBladderAdrenalglandParasympatheticSympatheticSacralBrainstemL1T1Figure 14.3Parasympathetic (Craniosacral) Division OutflowPterygopalatineganglionEyeLacrimalglandNasalmucosaCiliaryganglionPterygopalatineganglionSubmandibularganglionSubmandibularand sublingualglandsCN IIICN VIICN IXCN XOtic ganglionParotid glandHeartLungLiver andgallbladderStomachPancreasUrinarybladderand uretersSmallintestineLargeintestineS2PelvicsplanchnicnervesGenitalia(penis,clitoris, and vagina)RectumCeliacplexusInferiorhypogastricplexusCardiac andpulmonaryplexusesS4PreganglionicPostganglionicCranial nerveFigure 14.4Sympathetic (Thoracolumbar) DivisionPreganglionic neurons are in spinal cord segments T1 – L2Sympathetic neurons produce the lateral horns of the spinal cordPreganglionic fibers pass through the white rami communicantes and enter sympathetic trunk (paravertebral) gangliaFigure 14.6SuperiorcervicalganglionMiddlecervicalganglionInferiorcervicalganglionSympathetic trunk(chain) gangliaPonsL2T1White ramicommunicantesLiver andgallbladder StomachSpleenKidneyAdrenal medullaSmallintestineLargeintestineGenitalia (uterus, vagina, andpenis) and urinary bladderCeliac ganglionInferiormesenteric ganglionLesser splanchnic nerveGreater splanchnic nerveSuperior mesenteric ganglionLumbarsplanchnic nerves EyeLacrimal glandNasal mucosaBlood vessels;skin (arrector pilimuscles andsweat glands)Salivary glandsHeartLungRectumCardiac andpulmonaryplexusesPreganglionicPostganglionicSacralsplanchnicnervesSympathetic Trunks and PathwaysThere are 23 paravertebral ganglia in the sympathetic trunk (chain) 3 cervical 11 thoracic 4 lumbar 4 sacral 1 coccygeal Figure 14.5aSpinal cordDorsal rootVentral rootSympathetictrunk ganglionSympathetictrunkRibVentral ramusof spinal nerveGray ramuscommunicansWhite ramuscommunicansThoracicsplanchnic nerves(a) Location of the sympathetic trunkSympathetic Trunks and PathwaysUpon entering a sympathetic trunk ganglion a preganglionic fiber may do one of the following: Synapse with a ganglionic neuron within the same ganglionAscend or descend the sympathetic trunk to synapse in another trunk ganglionPass through the trunk ganglion and emerge without synapsingFigure 14.5b (1 of 3)To effectorBlood vesselsSkin (arrectorpili musclesand sweatglands)Dorsal root ganglionDorsal ramus ofspinal nerveDorsal rootSympathetictrunk ganglionLateral horn (visceralmotor zone)Ventral rootSympathetic trunkGray ramuscommunicansWhite ramuscommunicansVentral ramus ofspinal nerveSynapse at the same level(b) Three pathways of sympathetic innervation1Figure 14.5b (2 of 3)To effectorBlood vesselsSkin (arrectorpili musclesand sweatglands)Synapse at a higher or lower level(b) Three pathways of sympathetic innervation2Figure 14.5b (3 of 3)Splanchnic nerveCollateral ganglion(such as the celiac)Target organin abdomen(e.g., intestine) Synapse in a distant collateral ganglionanterior to the vertebral column (b) Three pathways of sympathetic innervation3Pathways with Synapses in Chain GangliaPostganglionic axons enter the ventral rami via the gray rami communicantesThese fibers innervateSweat glandsArrector pili musclesVascular smooth musclePathways to the HeadFibers emerge from T1 – T4 and synapse in the superior cervical ganglionThese fibersInnervate skin and blood vessels of the headStimulate dilator muscles of the iris Inhibit nasal and salivary glandsPathways to the ThoraxPreganglionic fibers emerge from T1 – T6 and synapse in the cervical trunk gangliaPostganglionic fibers emerge from the middle and inferior cervical ganglia and enter nerves C4 – C8These fibers innervate:Heart via the cardiac plexus Thyroid gland and the skinLungs and esophagusPathways with Synapses in Collateral GangliaMost fibers from T5 – L2 synapse in collateral ganglia They form thoracic, lumbar, and sacral splanchnic nerves Their ganglia include the celiac and the superior and inferior mesentericPathways to the AbdomenPreganglionic fibers from T5 – L2 travel through the thoracic splanchnic nervesSynapses occur in the celiac and superior mesenteric gangliaPostganglionic fibers serve the stomach, intestines, liver, spleen, and kidneysPathways to the PelvisPreganglionic fibers from T10 – L2 travel via the lumbar and sacral splanchnic nervesSynapses occur in the inferior mesenteric and hypogastric gangliaPostganglionic fibers serve the distal half of the large intestine, the urinary bladder, and the reproductive organsPathways with Synapses in the Adrenal MedullaSome preganglionic fibers pass directly to the adrenal medulla without synapsingUpon stimulation, medullary cells secrete norepinephrine and epinephrine into the bloodVisceral ReflexesVisceral reflex arcs have the same components as somatic reflexesMain difference: visceral reflex arc has two neurons in the motor pathwayVisceral pain afferents travel along the same pathways as somatic pain fibers, contributing to the phenomenon of referred painFigure 14.7Spinal cordDorsal root ganglionAutonomic ganglionStimulusResponse Visceral sensoryneuron Integration center• May be preganglionic neuron (as shown)• May be a dorsal horn interneuron• May be within walls of gastrointestinal tract Sensory receptorin viscera2315Visceral effector Efferent pathway(two-neuron chain)• Preganglionic neuron• Ganglionic neuron4Referred PainVisceral pain afferents travel along the same pathway as somatic pain fibersPain stimuli arising in the viscera are perceived as somatic in originFigure 14.8HeartLungs anddiaphragmLiverStomachKidneysOvariesSmall intestineUretersUrinarybladderColonPancreasLiverHeartAppendixGallbladderNeurotransmitters Cholinergic fibers release the neurotransmitter AChAll ANS preganglionic axonsAll parasympathetic postganglionic axonsAdrenergic fibers release the neurotransmitter NEMost sympathetic postganglionic axonsExceptions: sympathetic postganglionic fibers secrete ACh at sweat glands and some blood vessels in skeletal musclesFigure 14.2+AChSmooth muscle(e.g., in gut),glands, cardiacmuscleGanglionAdrenal medullaBlood vesselAChAChAChNEEpinephrine andnorepinephrineAcetylcholine (ACh)Norepinephrine (NE)GanglionLightly myelinatedpreganglionic axon Lightly myelinatedpreganglionic axons UnmyelinatedpostganglionicaxonUnmyelinatedpostganglionic axonStimulatoryor inhibitory,dependingon neuro-transmitterandreceptorson effectororgansTwo-neuron chain from CNS to effector organsAUTONOMIC NERVOUS SYSTEMPARASYMPATHETICSYMPATHETICReceptors for NeurotransmittersCholinergic receptors for AChAdrenergic receptors for NE Cholinergic ReceptorsTwo types of receptors bind AChNicotinic MuscarinicNamed after drugs that bind to them and mimic ACh effectsNicotinic ReceptorsFound onMotor end plates of skeletal muscle cells (Chapter 9)All ganglionic neurons (sympathetic and parasympathetic) Hormone-producing cells of the adrenal medullaEffect of ACh at nicotinic receptors is always stimulatoryMuscarinic ReceptorsFound onAll effector cells stimulated by postganglionic cholinergic fibersThe effect of ACh at muscarinic receptorsCan be either inhibitory or excitatoryDepends on the receptor type of the target organTable 14.2Adrenergic ReceptorsTwo types Alpha () (subtypes 1, 2)Beta () (subtypes 1, 2 , 3)Effects of NE depend on which subclass of receptor predominates on the target organTable 14.2Effects of DrugsAtropineAnticholinergic; blocks muscarinic receptorsUsed to prevent salivation during surgery, and to dilate the pupils for examination NeostigmineInhibits acetylcholinesteraseUsed to treat myasthenia gravisEffects of DrugsOver-the-counter drugs for colds, allergies, and nasal congestionStimulate -adrenergic receptorsBeta-blockersDrugs that attach to 2 receptors to dilate lung bronchioles in asthmatics; other usesTable 14.3Interactions of the Autonomic DivisionsMost visceral organs have dual innervationDynamic antagonism allows for precise control of visceral activitySympathetic division increases heart and respiratory rates, and inhibits digestion and eliminationParasympathetic division decreases heart and respiratory rates, and allows for digestion and the discarding of wastesSympathetic ToneSympathetic division controls blood pressure, even at restSympathetic tone (vasomotor tone)Keeps the blood vessels in a continual state of partial constrictionSympathetic ToneSympathetic fibers fire more rapidly to constrict blood vessels and cause blood pressure to rise Sympathetic fibers fire less rapidly to prompt vessels to dilate to decrease blood pressure Alpha-blocker drugs interfere with vasomotor fibers and are used to treat hypertensionParasympathetic ToneParasympathetic division normally dominates the heart and smooth muscle of digestive and urinary tract organsSlows the heartDictates normal activity levels of the digestive and urinary tractsThe sympathetic division can override these effects during times of stressDrugs that block parasympathetic responses increase heart rate and block fecal and urinary retentionCooperative EffectsBest seen in control of the external genitaliaParasympathetic fibers cause vasodilation; are responsible for erection of the penis or clitorisSympathetic fibers cause ejaculation of semen in males and reflex contraction of a female’s vaginaUnique Roles of the Sympathetic DivisionThe adrenal medulla, sweat glands, arrector pili muscles, kidneys, and most blood vessels receive only sympathetic fibersThe sympathetic division controlsThermoregulatory responses to heatRelease of renin from the kidneysMetabolic effectsIncreases metabolic rates of cellsRaises blood glucose levelsMobilizes fats for use as fuelsLocalized Versus Diffuse EffectsParasympathetic division: short-lived, highly localized control over effectorsSympathetic division: long-lasting, bodywide effects Effects of Sympathetic ActivationSympathetic activation is long lasting because NEIs inactivated more slowly than AChNE and epinephrine are released into the blood and remain there until destroyed by the liverControl of ANS FunctioningHypothalamus—main integrative center of ANS activitySubconscious cerebral input via limbic lobe connections influences hypothalamic functionOther controls come from the cerebral cortex, the reticular formation, and the spinal cordFigure 14.9Cerebral cortex(frontal lobe)Limbic system(emotional input)Communication atsubconscious levelHypothalamusOverall integrationof ANS, the bossSpinal cordUrination, defecation,erection, and ejaculationreflexesBrain stem(reticular formation, etc.)Regulation of pupil size,respiration, heart, bloodpressure, swallowing, etc.Hypothalamic ControlControl may be direct or indirect (through the reticular system)Centers of the hypothalamus controlHeart activity and blood pressureBody temperature, water balance, and endocrine activityEmotional stages (rage, pleasure) and biological drives (hunger, thirst, sex)Reactions to fear and the “fight-or-flight” systemDevelopmental Aspects of the ANSDuring youth, ANS impairments are usually due to injuryIn old age, ANS efficiency declines, partially due to structural changes at preganglionic axon terminalsDevelopmental Aspects of the ANSEffects of age on ANSConstipationDry eyesFrequent eye infectionsOrthostatic hypotensionLow blood pressure occurs because aging pressure receptors respond less to changes in blood pressure with changes in body position and because of slowed responses by sympathetic vasoconstrictor centers