Autonomic Nervous System

ANSHonors Anatomy & Physiology

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Comparison Of Autonomic & Somatic Nervous Systems

2 Neuron Chain

Preganglionic Neurons

Sympathetic Parasympatheitc

• 1st neuron in spinal cord: thoracic and lumbar spinal nerves

• 1st neuron in 4 cranial nerves in the brain stem (III, VII, IX, X) or in S2 –S4 of spinal cord

Postganglionic Neurons

Sympathetic Parasympathetic

• 2nd neuron in sympathetic trunk ganglion, a chain of ganglion that is found on either side lateral to spine

• 2nd neuron in individual ganglion closer to organ it serves

• both sympathetic & parasympathetic neurons lie outside CNS

Axons of Autonomic Nerves

• Preganglionic:– in cranial or spinal n. (from CNS

ganglion)– myelinated

• Postganglionic:– from ganglion visceral effector– unmyelinated

ANS Neurotransmitters

• based on the neurotransmitter they produce & release autonomic neurons are classified as either:

1. Cholinergic• release acetylcholine (ACh)1. Adrenergic • release norepinephrine (NE) aka

noradrenalin

Cholinergic Neurons & Receptors

• Cholinergic neurons include:1. all Sympathetic &

Parasympathetic preganglionic neurons

2. Sympathetic postganglionic neurons that innervate most sweat glands

3. all Parasympathetic postganglionic neurons

Receptors• that bind ACh called

cholinergic receptors• 2 types:1. Nicotinic receptors• in plasma membranes & dendrites of

symp. & parasymp postganglionic neurons & in NMJ

1. Muscarinic receptors• in plasma membrane of all effectors

(smooth muscle, cardiac muscle, glands)• Muscarine: mushroom poisonmimics

actions of ACh

ACh

• when activates nicotinic receptors depolarization (excitation)

• when activates muscarinic receptors sometimes depolarization, sometimes hyperpolarization (inhibition) depending on the cell

NE

• most sympathetic postganglionic neurons are adrenergic

• Adrenergic receptors bind both NE & Epinephrine (Epi)

• 2 types receptors:1. Alpha receptors (α)– subtypes: α1, α2

2. Beta receptors (β)– subtypes: β1, β2, β3

αlpha & βeta Receptors

• α1 & β1 produce excitation when activated

• α2 & β2 receptors cause inhibition of effector tissues

• β3 found only on cells of brown adipose where activation causes thermogenesis (heat production)

αlpha & βeta Receptors

• cells of most effectors have either α or β receptors

• some visceral effectors contain both

• NE stimulates α more strongly than β

• Epi is potent stimulator of both

MAO

• MonoAmine Oxidase: inactivates NE in synaptic cleft

• group of pharmaceuticals that are MAO inhibitors so prolong effect of NE

Agonists • substance that binds to & activates

a receptor• in the process mimicking the

effect of a natural neurotransmitter or hormone

• example: phenylephrine is an adrenergic agonist @ α1 receptors; used in cold remedies– constricts blood vessels in nasal

mucosa reduces production of mucus

Antagonists • substance that blocks receptors

so prevents the natural neurotransmitter or hormone from exerting its effect

• example: propanolol a β1 blocker, used to treat HTN decreases heart rate & force of contraction lowers BP– side effects: hypoglycemia, mild

bronchoconstriction, decreases frequency & severity of migraines

Autonomic Tone

• balance between sympathetic & parasympathetic activity

• regulated by hypothalamus– if turns up sympathetic tone, turns

down parasympathetic tone @ same time

Sympathetic Responses

• dominate during physical or emotional stress

• occur during “E situations”– Exercise– Emotions– Emergency– Excitement

Sympathetic Responses• Fight or Flight Response– pupils dilate– HR, force of contraction, & BP increase– airways dilate– vessels to kidneys & GI tract constrict

slowing down digestion & urine production– vessels muscles (skeletal & cardiac),

liver, & adipose tissue dilate– hepatocytes increase glycogenolysis &

adipose increase lipolysis blood glucose increases

– anything nonessential slowed down

Sympathetic Stimulation

• effects longer lasting than parasympathetic responses (NE lasts longer in synaptic cleft than ACh)

• effects are more widespread (more tissues activated)

Parasympathetic Responses• enhance “rest & digest” activities• remeber SLUDD:– Salivation– Lacrimation–Urination–Digestion–Defecation

Disorders of the ANS

• Raynaud’s phenomena:• digits become ischemic after

exposure to cold or w/ emotional stress

• due to excessive sympathetic stimulation of smooth muscle in arterioles in digits & increased response to stimuli that cause vasoconstriction

Raynaud’s Phenomenon

• treatment options:• Ca++ channel blockers to relax

smooth muscle• Prazosin: blocks α receptors which

blocks smooth muscle contractions