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Physiology of autonomic nervous system Learning objectives • Divisions of ANS. • Pre-ganglionic and Post ganglionic fibers. • Receptors types in the ANS. • Functions of sympathetic and parasympathetic system. • Role of adrenal glands in ANS. Autonomic nervous system The autonomic nervous system is the subdivision of the peripheral nervous system that regulates body activities that are generally not under conscious control Divisions of the autonomic nervous system • Parasympathetic division • Sympathetic division Serve most of the same organs but cause opposing or antagonistic effects Parasympathetic nervous system “rest & digest” • Also called the craniosacral system because all its preganglionic neurons are in the brain stem or sacral levels of the spinal cord – Cranial nerves III,VII, IX and X – In lateral horn of gray matter from S2-S4 • Only innervate internal organs (not skin) Cranial outflow – III - pupils constrict – VII - tears, nasal mucus, saliva – IX – parotid salivary gland – X (Vagus n) – visceral organs of thorax & abdomen: • Stimulates digestive glands • Increases motility of smooth muscle of digestive tract • Decreases heart rate • Causes bronchial constriction • Sacral outflow (S2-4): form pelvic splanchnic nerves nd – Supply 2 half of large intestine – Supply all the pelvic (genitourinary) organs Sympathetic nervous system “fight, flight or fright” • Also called thoracolumbar system: all its neurons are in lateral horn of gray matter from T1-L2 • Lead to every part of the body (unlike parasymp.) – Easy to remember that when nervous, you sweat; when afraid, hair stands on end; when excited blood pressure rises (vasoconstriction): these sympathetic only – Also causes: dry mouth, pupils to dilate, increased heart & respiratory rates to increase O2 to skeletal muscles, and liver to release glucose Regardless of target, all begin same • Preganglionic axons exit spinal cord through ventral root and enter spinal nerve • Exit spinal nerve via communicating ramus • Enter sympathetic trunk/chain where postganglionic neurons are Organization of ANS • Autonomic nervous system – Chain of two motor neurons • Preganglionic neuron • Postganglionic neuron – Conduction is slower due to thinly or unmyelinated axons • Axon of 1st (preganglionic) neuron leaves CNS to synapse with the 2nd (ganglionic) neuron • Axon of 2nd (ganglionic) neuron extends to the organ it serves. General nerve pathway A. Sympathetic Preganglionic cell body Axons move through ventral root of spinal nerve. Synapses w/postganglionic at sympathetic chain ganglion Axons of post ganglions exit via spinal nerve and sympathetic nerve. Exceptions: some preganglionic fibers do not synapse at symp chain i.e Splanchnic nerve and Adrenal gland Options of preganglionic axons in sympathetic trunk Synapse on postganglionic neuron in chain ganglion then return to spinal nerve and follow its branch to the skin Ascend or descend within sympathetic trunk, synapse with a posganglionic neuron within a chain ganglion, and return to spinal nerve at that level and follow branches to skin Enter sympathetic chain, pass through without synapsing, form a splanchnic nerve that passes toward thoracic or abdominal organs – These synapse in prevertebral ganglion in front of aorta – Postganglionic axons follow arteries to organs b. Parasympathetic nervous Cell bodies of preganglionic – brainstem( nuclei) and sacral region of spinal cord. Axons moves through cranial nerves and spinal nerves Synapses with post ganglion at ganglia or in the target system system Anatomical Differences in Sympathetic and Parasympathetic Divisions • Length of postganglionic fibers – Sympathetic – long postganglionic fibers – Parasympathetic – short postganglionic fibers • Branching of axons – Sympathetic axons – highly branched • Influences many organs – Parasympathetic axons – few branches • Localized effect Anatomical Differences in Sympathetic and Parasympathetic Divisions Figure 15.4a Anatomical Differences in Sympathetic and Parasympathetic Divisions Figure 15.4b Signal transmission A. Parasympathetic Preganglionic fibers secretes acetylcholine Post ganglionic receptor = nicotinic Post ganglionic secretes acetylcholine. Target are smooth muscles ,heart and glands Receptor = muscranic B .Sympathetic Preganglionic fibers secretes acetylcholine Post ganglionic receptor = nicotinic Post ganglionic secretes NE Target are smooth muscles ,heart and glands Receptor = Adrenergic and beta Adrenal gland is exception On top of kidneys Adrenal medulla (inside part) is a major organ of the sympathetic nervous system • Synapse in gland • Can cause body-wide release of epinephrine aka adrenaline and norepinephrine in an extreme emergency (adrenaline “rush” or surge) Receptors types in the ANS Adrenergic receptors a .alpha 1 Receptors - Location : vascular smooth muscle of the skin and splanchnic regions ,GIT and urinary bladder sphincters and the radial muscle of the iris. - Produce :excitation(contraction or constriction) - Equally sensitive to nor-epinephrine and epinephrine. - Mechanism of action : Formation of inositol triphosphate and increase in intracellular ca. b .Alpha 2 Receptors. - Location : in pre-synaptic nerve terminals,platelets ,fat cells and the walls of GIT. - Produces: inhibition.(relaxation or dilatation) - MOA :Inhibition of adenylate cyclase and decreases cAMP C .Beta 1 Receptor - Location: SA Node ,AV node and ventricular muscle wall. - Produces :excitation. - Sensitive to both epinephrine and norephinephrine and are more sensitive than the alpha 1 receptors. - MOA: Excitation of adenylate cyclase and increases cAMP D .Beta 2 Receptor - Location: Vascular smooth muscle,bronchial smooth muscle and wall of GIT bladder. - Produces: relaxation. - More Sensitive to epinephrine than NE - More Sensitive to epinephrine than the alpha 1 receptors. - MOA: Excitation of adenylate cyclase and increases cAMP Cholinergic receptors a .Nicotinic Receptor - Location: Autonomic ganglia of sympathetic and parasympathetic nervous system, at neuromuscular junction and in the adrenal medulla. - Produces :Excitation. - Activated : By Ach Or nicotine. - MOA: Ach binds to nicotinic Ach receptor .The nicotinic Ach Receptors are also ion channels for Na+ and K+ b .Muscarinic Receptor - Location:Heart, smooth muscle and glands. - Produces : inhibitory in the heart excitatory in smooth muscle and glands - Activated : By Ach Or muscarine. - MOA: Heart SA Node : Inhibition of adenylate cyclase . Smooth muscle and glands: Formation of inositol triphosphate and increase in intracellular ca.
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