Autonomic Nervous System Autonomic Nervous System (ANS)(ANS)

n The ANS consists of motor neurons that: The ANS consists of motor neurons that: n Innervate smooth and cardiac muscle and glandsInnervate smooth and cardiac muscle and glandsn Make adjustments to ensure optimal support for Make adjustments to ensure optimal support for

body activitiesbody activitiesn Operate via subconscious controlOperate via subconscious controln Have viscera as most of their effectorsHave viscera as most of their effectors

ANS in the Nervous SystemANS in the Nervous System

ANS Versus Somatic Nervous ANS Versus Somatic Nervous System (SNS)System (SNS)

n The ANS differs from the SNS in the The ANS differs from the SNS in the following three areasfollowing three areasn EffectorsEffectorsn Efferent pathwaysEfferent pathwaysn Target organ responsesTarget organ responses

EffectorsEffectorsn The effectors of the SNS are skeletal musclesThe effectors of the SNS are skeletal musclesn The effectors of the ANS are cardiac muscle, The effectors of the ANS are cardiac muscle,

smooth muscle, and glandssmooth muscle, and glands

Efferent PathwaysEfferent Pathwaysn Heavily myelinated axons of the somatic Heavily myelinated axons of the somatic

motor neurons extend from the CNS to the motor neurons extend from the CNS to the effectoreffector

n Axons of the ANS are a two-neuron chainAxons of the ANS are a two-neuron chainn The preganglionic (first) neuron has a lightly The preganglionic (first) neuron has a lightly

myelinated axonmyelinated axonn The ganglionic (second) neuron extends to an The ganglionic (second) neuron extends to an

effector organeffector organ

Neurotransmitter EffectsNeurotransmitter Effectsn All somatic motor neurons release All somatic motor neurons release

Acetylcholine (ACh), which has an excitatory Acetylcholine (ACh), which has an excitatory effecteffect

n In the ANS:In the ANS:n Preganglionic fibers release AChPreganglionic fibers release AChn Postganglionic fibers release norepinephrine or Postganglionic fibers release norepinephrine or

ACh and the effect is either stimulatory or ACh and the effect is either stimulatory or inhibitoryinhibitory

n ANS effect on the target organ is dependent upon ANS effect on the target organ is dependent upon the neurotransmitter released and the receptor type the neurotransmitter released and the receptor type of the effectorof the effector

Comparison of Somatic and Comparison of Somatic and Autonomic SystemsAutonomic Systems

Divisions of the ANSDivisions of the ANSn ANS divisions: sympathetic and ANS divisions: sympathetic and

parasympatheticparasympatheticn The sympathetic mobilizes the body during The sympathetic mobilizes the body during

extreme situationsextreme situationsn The parasympathetic performs maintenance The parasympathetic performs maintenance

activities and conserves body energyactivities and conserves body energy n The two divisions counterbalance each otherThe two divisions counterbalance each other

Role of the Parasympathetic Role of the Parasympathetic DivisionDivision

n Concerned with keeping body energy use lowConcerned with keeping body energy use lown Involves the Involves the D D activities – digestion, activities – digestion,

defecation, and diuresisdefecation, and diuresisn Its activity is illustrated in a person who Its activity is illustrated in a person who

relaxes after a mealrelaxes after a mealn Blood pressure, heart rate, and respiratory rates are Blood pressure, heart rate, and respiratory rates are

lowlown Gastrointestinal tract activity is highGastrointestinal tract activity is highn The skin is warm and the pupils are constrictedThe skin is warm and the pupils are constricted

Role of the Sympathetic DivisionRole of the Sympathetic Division

n The sympathetic division is the “fight-or-flight” systemThe sympathetic division is the “fight-or-flight” systemn Involves Involves EE activities – exercise, excitement, emergency, activities – exercise, excitement, emergency,

and embarrassmentand embarrassmentn Promotes adjustments during exercise – blood flow to Promotes adjustments during exercise – blood flow to

organs is reduced, flow to muscles is increasedorgans is reduced, flow to muscles is increasedn Its activity is illustrated by a person who is threatenedIts activity is illustrated by a person who is threatened

n Heart rate increases, and breathing is rapid and deepHeart rate increases, and breathing is rapid and deepn The skin is cold and sweaty, and the pupils dilateThe skin is cold and sweaty, and the pupils dilate

Sympathetic OutflowSympathetic Outflown Arises from spinal cord segments TArises from spinal cord segments T11 through L through L22n Sympathetic neurons produce the lateral horns Sympathetic neurons produce the lateral horns

of the spinal cordof the spinal cordn Preganglionic fibers pass through the white rami Preganglionic fibers pass through the white rami

communicantes and synapse in the chain communicantes and synapse in the chain (paravertebral) ganglia(paravertebral) ganglia

n Fibers from TFibers from T55-L-L22 form splanchnic nerves and form splanchnic nerves and synapse with collateral ganglia synapse with collateral ganglia

n Postganglionic fibers innervate the numerous Postganglionic fibers innervate the numerous organs of the bodyorgans of the body

Sympathetic Trunks and Sympathetic Trunks and PathwaysPathways

n The paravertebral ganglia form part of the The paravertebral ganglia form part of the sympathetic trunk or chainsympathetic trunk or chain

n Typically there are 23 ganglia – 3 cervical, 11 Typically there are 23 ganglia – 3 cervical, 11 thoracic, 4 lumbar, 4 sacral, and 1 coccygealthoracic, 4 lumbar, 4 sacral, and 1 coccygeal

Sympathetic Trunks and Sympathetic Trunks and PathwaysPathways

n A preganglionic fiber follows one of three A preganglionic fiber follows one of three pathways upon entering the paravertebral pathways upon entering the paravertebral gangliaganglian Synapse with the ganglionic neuron within the Synapse with the ganglionic neuron within the

same ganglionsame ganglionn Ascend or descend the sympathetic chain to Ascend or descend the sympathetic chain to

synapse in another chain ganglionsynapse in another chain ganglionn Pass through the chain ganglion and emerge Pass through the chain ganglion and emerge

without synapsingwithout synapsing

Pathways with Synapses in Chain Pathways with Synapses in Chain GangliaGanglia

n These fibers innervate sweat glands and These fibers innervate sweat glands and arrector pili musclesarrector pili muscles

n Rami communicantes are associated only with Rami communicantes are associated only with the sympathetic divisionthe sympathetic division

Pathways to the HeadPathways to the Head

n Preganglionic fibers emerge from TPreganglionic fibers emerge from T11-T-T44 and and synapse in the superior cervical ganglionsynapse in the superior cervical ganglion

n These fibers:These fibers:n Serve the skin and blood vessels of the headServe the skin and blood vessels of the headn Stimulate dilator muscles of the irisStimulate dilator muscles of the irisn Inhibit nasal and salivary glandsInhibit nasal and salivary glands

Pathways to the ThoraxPathways to the Thoraxn Preganglionic fibers emerge from TPreganglionic fibers emerge from T11-T-T66 and synapse and synapse

in the cervical chain gangliain the cervical chain ganglian These fibers innervate the heart via the cardiac These fibers innervate the heart via the cardiac

plexus, as well as innervating the thyroid and the skinplexus, as well as innervating the thyroid and the skinn Postganglionic fibers emerge from the middle and Postganglionic fibers emerge from the middle and

inferior cervical ganglia and enter nerves Cinferior cervical ganglia and enter nerves C44-C-C88

n Postganglionic fibers directly serve the heart, aorta, Postganglionic fibers directly serve the heart, aorta, lungs, and esophagus lungs, and esophagus

Pathways with Synapses in Pathways with Synapses in Collateral GangliaCollateral Ganglia

n These fibers (TThese fibers (T55-L-L22) leave the sympathetic ) leave the sympathetic chain without synapsingchain without synapsing

n They form thoracic, lumbar, and sacral They form thoracic, lumbar, and sacral splanchnic nerves splanchnic nerves

n Their ganglia include the celiac, the superior Their ganglia include the celiac, the superior and inferior mesenterics, and the hypogastric and inferior mesenterics, and the hypogastric

Pathways to the AbdomenPathways to the Abdomenn Sympathetic nerves innervating the abdomen Sympathetic nerves innervating the abdomen

have preganglionic fibers from Thave preganglionic fibers from T55-L-L22

n They travel through the thoracic splanchnic They travel through the thoracic splanchnic nerves and synapse at the celiac and superior nerves and synapse at the celiac and superior mesenteric gangliamesenteric ganglia

n Postganglionic fibers serve the stomach, Postganglionic fibers serve the stomach, intestines, liver, spleen, and kidneysintestines, liver, spleen, and kidneys

Pathways to the PelvisPathways to the Pelvis

n Preganglionic fibers originate from TPreganglionic fibers originate from T1010-L-L22

n Most travel via the lumbar and sacral Most travel via the lumbar and sacral splanchnic nerves to the inferior mesenteric splanchnic nerves to the inferior mesenteric and hypogastric gangliaand hypogastric ganglia

n Postganglionic fibers serve the distal half of Postganglionic fibers serve the distal half of the large intestine, the urinary bladder, and the the large intestine, the urinary bladder, and the reproductive organsreproductive organs

Pathways with Synapses in the Pathways with Synapses in the Adrenal MedullaAdrenal Medulla

n Fibers of the thoracic splanchnic nerve pass Fibers of the thoracic splanchnic nerve pass directly to the adrenal medulladirectly to the adrenal medulla

n Upon stimulation, medullary cells secrete Upon stimulation, medullary cells secrete norepinephrine and epinephrine into the bloodnorepinephrine and epinephrine into the blood

Segmental Sympathetic SuppliesSegmental Sympathetic Supplies

Visceral ReflexesVisceral Reflexesn Visceral reflexes have the same elements as Visceral reflexes have the same elements as

somatic reflexessomatic reflexesn They are always polysynaptic pathwaysThey are always polysynaptic pathwaysn Afferent fibers are found in spinal and Afferent fibers are found in spinal and

autonomic nervesautonomic nerves

Visceral ReflexesVisceral Reflexes

Referred PainReferred Painn Pain stimuli arising Pain stimuli arising

from the viscera are from the viscera are perceived as perceived as somatic in originsomatic in origin

n This may be due to This may be due to the fact that visceral the fact that visceral pain afferents travel pain afferents travel along the same along the same pathways as pathways as somatic pain fiberssomatic pain fibers

Neurotransmitters and ReceptorsNeurotransmitters and Receptorsn Acetylcholine (ACh) and norepinephrine (NE) Acetylcholine (ACh) and norepinephrine (NE)

are the two major neurotransmitters of the ANSare the two major neurotransmitters of the ANSn ACh is released by all preganglionic axons and ACh is released by all preganglionic axons and

all parasympathetic postganglionic axonsall parasympathetic postganglionic axonsn Cholinergic fibers – ACh-releasing fibers Cholinergic fibers – ACh-releasing fibers n Adrenergic fibers – sympathetic postganglionic Adrenergic fibers – sympathetic postganglionic

axons that release NE axons that release NE n Neurotransmitter effects can be excitatory or Neurotransmitter effects can be excitatory or

inhibitory depending upon the receptor type inhibitory depending upon the receptor type

Cholinergic ReceptorsCholinergic Receptorsn The two types of receptors that bind ACh are The two types of receptors that bind ACh are

nicotinic and muscarinicnicotinic and muscarinicn These are named after drugs that bind to them These are named after drugs that bind to them

and mimic ACh effectsand mimic ACh effects

Nicotinic ReceptorsNicotinic Receptorsn Nicotinic receptors are found on:Nicotinic receptors are found on:

n Motor end plates (somatic targets)Motor end plates (somatic targets)n All ganglionic neurons of both sympathetic and All ganglionic neurons of both sympathetic and

parasympathetic divisionsparasympathetic divisionsn The hormone-producing cells of the adrenal The hormone-producing cells of the adrenal

medullamedullan The effect of ACh binding to nicotinic The effect of ACh binding to nicotinic

receptors is always stimulatoryreceptors is always stimulatory

Muscarinic ReceptorsMuscarinic Receptorsn Muscarinic receptors occur on all effector cells Muscarinic receptors occur on all effector cells

stimulated by postganglionic cholinergic fibersstimulated by postganglionic cholinergic fibersn The effect of ACh binding: The effect of ACh binding:

n Can be either inhibitory or excitatoryCan be either inhibitory or excitatoryn Depends on the receptor type of the target organDepends on the receptor type of the target organ

Adrenergic ReceptorsAdrenergic Receptorsn The two types of adrenergic receptors are The two types of adrenergic receptors are

alpha and betaalpha and betan Each type has two or three subclasses Each type has two or three subclasses

((1, 1, 2, 2, 1, 1, 2 , 2 , 3)3)n Effects of NE binding to: Effects of NE binding to:

n receptors is generally stimulatoryreceptors is generally stimulatoryn receptors is generally inhibitoryreceptors is generally inhibitory

n A notable exception – NE binding to A notable exception – NE binding to receptors of the heart is stimulatoryreceptors of the heart is stimulatory

Effects of DrugsEffects of Drugsn Atropine – blocks parasympathetic effectsAtropine – blocks parasympathetic effectsn Neostigmine – inhibits acetylcholinesterase Neostigmine – inhibits acetylcholinesterase

and is used to treat myasthenia gravisand is used to treat myasthenia gravisn Tricyclic antidepressants – prolong the activity Tricyclic antidepressants – prolong the activity

of NE on postsynaptic membranesof NE on postsynaptic membranesn Over-the-counter drugs for colds, allergies, and Over-the-counter drugs for colds, allergies, and

nasal congestion – stimulate nasal congestion – stimulate -adrenergic -adrenergic receptorsreceptors

n Beta-blockers – attach mainly to Beta-blockers – attach mainly to 11 receptors receptors and reduce heart rate and prevent arrhythmiasand reduce heart rate and prevent arrhythmias

Drugs that Influence the ANSDrugs that Influence the ANS

Interactions of the Autonomic Interactions of the Autonomic DivisionsDivisions

n Most visceral organs are innervated by both Most visceral organs are innervated by both sympathetic and parasympathetic fiberssympathetic and parasympathetic fibers

n This results in dynamic antagonisms that This results in dynamic antagonisms that precisely control visceral activity precisely control visceral activity

n Sympathetic fibers increase heart and respiratory Sympathetic fibers increase heart and respiratory rates, and inhibit digestion and eliminationrates, and inhibit digestion and elimination

n Parasympathetic fibers decrease heart and Parasympathetic fibers decrease heart and respiratory rates, and allow for digestion and the respiratory rates, and allow for digestion and the discarding of wastesdiscarding of wastes

Sympathetic ToneSympathetic Tonen The sympathetic division controls blood The sympathetic division controls blood

pressure and keeps the blood vessels in a pressure and keeps the blood vessels in a continual state of partial constrictioncontinual state of partial constriction

n This sympathetic tone (vasomotor tone):This sympathetic tone (vasomotor tone):n Constricts blood vessels and causes blood pressure Constricts blood vessels and causes blood pressure

to rise as neededto rise as neededn Prompts vessels to dilate if blood pressure is to be Prompts vessels to dilate if blood pressure is to be

decreaseddecreasedn Alpha-blocker drugs interfere with vasomotor Alpha-blocker drugs interfere with vasomotor

fibers and are used to treat hypertensionfibers and are used to treat hypertension

Parasympathetic ToneParasympathetic Tonen Parasympathetic tone:Parasympathetic tone:

n Slows the heartSlows the heartn Dictates normal activity levels of the digestive and Dictates normal activity levels of the digestive and

urinary systemsurinary systemsn The sympathetic division can override these The sympathetic division can override these

effects during times of stresseffects during times of stressn Drugs that block parasympathetic responses Drugs that block parasympathetic responses

increase heart rate and block fecal and urinary increase heart rate and block fecal and urinary retentionretention

Unique Roles of the Sympathetic Unique Roles of the Sympathetic DivisionDivision

n Regulates many functions not subject to Regulates many functions not subject to parasympathetic influenceparasympathetic influence

n These include the activity of the adrenal These include the activity of the adrenal medulla, sweat glands, arrector pili muscles, medulla, sweat glands, arrector pili muscles, kidneys, and most blood vesselskidneys, and most blood vessels

n The sympathetic division controls:The sympathetic division controls:n Thermoregulatory responses to heatThermoregulatory responses to heatn Release of renin from the kidneysRelease of renin from the kidneysn Metabolic effectsMetabolic effects

Thermoregulatory Responses to Thermoregulatory Responses to HeatHeat

n Applying heat to the skin causes reflex dilation Applying heat to the skin causes reflex dilation of blood vesselsof blood vessels

n Systemic body temperature elevation results in Systemic body temperature elevation results in widespread dilation of blood vesselswidespread dilation of blood vessels

n This dilation brings warm blood to the surface This dilation brings warm blood to the surface and activates sweat glands to cool the bodyand activates sweat glands to cool the body

n When temperature falls, blood vessels constrict When temperature falls, blood vessels constrict and blood is retained in deeper vital organsand blood is retained in deeper vital organs

Release of Renin from the Release of Renin from the KidneysKidneys

n Sympathetic impulses activate the kidneys to Sympathetic impulses activate the kidneys to release reninrelease renin

n Renin is an enzyme that promotes increased Renin is an enzyme that promotes increased blood pressureblood pressure

Metabolic EffectsMetabolic Effectsn The sympathetic division promotes metabolic The sympathetic division promotes metabolic

effects that are not reversed by the effects that are not reversed by the parasympathetic divisionparasympathetic divisionn Increases the metabolic rate of body cellsIncreases the metabolic rate of body cellsn Raises blood glucose levelsRaises blood glucose levelsn Mobilizes fat as a food sourceMobilizes fat as a food sourcen Stimulates the reticular activating system (RAS) of Stimulates the reticular activating system (RAS) of

the brain, increasing mental alertnessthe brain, increasing mental alertness

Localized Versus Diffuse EffectsLocalized Versus Diffuse Effectsn The parasympathetic division exerts short-The parasympathetic division exerts short-

lived, highly localized controllived, highly localized control n The sympathetic division exerts long-lasting, The sympathetic division exerts long-lasting,

diffuse effects diffuse effects

Effects of Sympathetic ActivationEffects of Sympathetic Activation

n Sympathetic activation is long-lasting because Sympathetic activation is long-lasting because NE:NE:n Is inactivated more slowly than AChIs inactivated more slowly than AChn Is an indirectly acting neurotransmitter, using a Is an indirectly acting neurotransmitter, using a

second-messenger systemsecond-messenger systemn And epinephrine are released into the blood and And epinephrine are released into the blood and

remain there until destroyed by the liverremain there until destroyed by the liver

Levels of ANS ControlLevels of ANS Controln The hypothalamus is the main integration The hypothalamus is the main integration

center of ANS activitycenter of ANS activityn Subconscious cerebral input via limbic lobe Subconscious cerebral input via limbic lobe

connections influences hypothalamic functionconnections influences hypothalamic functionn Other controls come from the cerebral cortex, Other controls come from the cerebral cortex,

the reticular formation, and the spinal cordthe reticular formation, and the spinal cord

Levels of ANS ControlLevels of ANS Control

Hypothalamic ControlHypothalamic Control

n Centers of the hypothalamus control:Centers of the hypothalamus control:n Heart activity and blood pressureHeart activity and blood pressuren Body temperature, water balance, and endocrine Body temperature, water balance, and endocrine

activityactivityn Emotional stages (rage, pleasure) and biological Emotional stages (rage, pleasure) and biological

drives (hunger, thirst, sex)drives (hunger, thirst, sex)n Reactions to fear and the “fight-or-flight” systemReactions to fear and the “fight-or-flight” system

Embryonic Development of the Embryonic Development of the ANSANS

n Preganglionic neurons are derived from the Preganglionic neurons are derived from the embryonic neural tubeembryonic neural tube

n ANS structures in the PNS – ganglionic ANS structures in the PNS – ganglionic neurons, the adrenal medulla, and all autonomic neurons, the adrenal medulla, and all autonomic ganglia – derive from the neural crestganglia – derive from the neural crest

n Nerve growth factor (NGF) is a protein secreted Nerve growth factor (NGF) is a protein secreted by target cells that aids in the development of by target cells that aids in the development of ANS pathwaysANS pathways

Developmental Aspects of the Developmental Aspects of the ANSANS

n During youth, ANS impairments are usually During youth, ANS impairments are usually due to injurydue to injury

n In old age, ANS efficiency decreases, resulting In old age, ANS efficiency decreases, resulting in constipation, dry eyes, and orthostatic in constipation, dry eyes, and orthostatic hypotensionhypotensionn Orthostatic hypotension is a form of low blood Orthostatic hypotension is a form of low blood

pressure that occurs when sympathetic pressure that occurs when sympathetic vasoconstriction centers respond slowly to vasoconstriction centers respond slowly to positional changespositional changes