Autonomic Nervous System Lecture Notes PDF

Curriculum: Phase 1/ Semester3/ CVS/ Session 4 Lecturer: teacher Dr. Shahlaa khazaal chabuk Degrees: MSc/Ph.D. Phsiology The Autonomic Nervous System CVS  The Autonomic Nervous System (ANS) Important for regulating many physiological functions Largely outside vo...

Curriculum: Phase 1/ Semester3/ CVS/ Session 4 Lecturer: teacher Dr. Shahlaa khazaal chabuk Degrees: MSc/Ph.D. Phsiology The Autonomic Nervous System CVS  The Autonomic Nervous System (ANS) Important for regulating many physiological functions Largely outside voluntary control Exerts control over – smooth muscle (vascular and visceral) – exocrine secretion  Two divisions of the ANS Parasympathetic nervous system Sympathetic nervous system Some text books include a third division the enteric nervous system – Network of neurones surrounding GI tract – Is normally controlled via sympathetic and parasympathetic fibres  Organization of sympathetic and parasympathetic systems wo neurones arranged in series Central Nervous system Peripheral Nervous Target cell system Preganglionic Postganglionic neuron neuron The autonomic nervous system is activated mainly by centers located in the spinal cord, brain stem, and hypothalamus (i.e. Cell body of preganglionic neurone is in the CNS)  Organization of the sympathetic division Thoracolumbar origin Preganglionic neurones arise from segments T1 to L2 (or L3) Most synapse with postganglionic neurones in the paravertebral chain of ganglia (sympathetic chain) Some synapse in a number of prevertebral rganization of the parasympathetic divis Craniosacral origin Preganglionic fibres travel in cranial nerves (III, VII, IX & X) or sacral outflow from S2-S4 Synapse with neurones in ganglia close to the target tissue  Arrangement of pre- and postganglionic neurones Sympathetic Preganglionic Postganglionic Target tissue Parasympathetic Postganglionic Preganglionic Target tissue  Preganglionic neurones of both divisions release acetylcholine Preganglionic Postganglionic Acetylcholine (ACh) +K +K ACh acts on nicotinic Ach +Na receptors on the postganglionic cell. Nicotinic ACh receptors have  Neurotransmitters at the postganglionic cell to effector cell synapse Postganglionic sympathetic neurones are usually noradrenergic (use noradrenaline (NA) as a transmitter) Postganglionic parasympathetic neurones are usually cholinergic (have ACh as transmitter) The exceptions of sympathetic innervation include The postganglionic sympathetic nerve fibers to : the sweat glands to the piloerector muscles of the hairs  Neurotransmitters in the sympatheticnervous system Adrenergic nicotinic ACh receptors receptors Preganglionic Postganglionic noradrenaline (NA) (norepinephrine) acetylcholine (ACh) Chromaffin cells of the adrenal medulla are like specialized postganglionic sympathetic neurons Preganglionic chromaffin adrenaline cell (epinephrine) acetylcholine (ACh) adrenal chromaffin cells release adrenaline which circulates in the blood Receptors to noradrenaline and adrenaline adrenoreceptors. G protein-coupled receptors – no integral ion channel Types and subtypes of adrenoreceptors – α-adrenoreceptors α1-adrenoreceptors α2-adrenoreceptors – β-adrenoreceptors β1-adrenoreceptors β2-adrenoreceptors Neurotransmitters in the parasympathetic nervous system muscarinic ACh receptors nicotinic ACh receptors Preganglionic Postganglionic effector cell acetylcholine (ACh) Neurotransmitters in the parasympathetic nervous system parasympathetic postganglionic neurones use ACh as a neurotransmitter ACh acts at muscarinic receptors on the effector cells G protein-coupled receptors (M1, M2 & M3) no What does the autonomic nervous system do? regulates physiological functions where parasympathetic and sympathetic divisions both innervate a tissue they often have opposite effects sympathetic activity is increased under stress parasympathetic system is more dominant under basal conditions note: Sympathetic drive to different tissues is independently regulated eg sympathetic activity to the heart can be increased without increasing activity to GI tract on some occasions (fight or flight) there can be a more co-ordinated sympathetic response Control of the cardiovascular system The ANS controls – heart rate – force of contraction of heart – peripheral resistance of blood vessels Parasympathetic input to the heart preganglionic fibres - 10th (X) cranial nerve VAGUS synapse with postganglionic cells onepicardial surface or within walls of heart at SA and AV node postganglionic cells release ACh acts on M2-receptors – decrease heart rate (-ve chronotropic effect) – decrease AV node conduction velocity Sympathetic input to the heart Postganglionic fibres from the sympathetic trunk Innervate SA node AV node and myocardium Release noradrenaline Acts on β1 adrenoreceptors – increases heart rate (+ve chrontropic effect) – increases force of contraction (+ve inotropic The pacemaker of the heart Cells in the sinoatrial node (SA node) steadily depolarise toward threshold – slow depolarising pacemaker potential – turning on a slow Na+ conductance – opening of Ca2+ channels AP firing in the SA node sets the rhythm of the heart Effect of ANS on pacemaker potentials sympathetic activity increases slope Parasympathetic activity decreases slope of the Pacemake potential sympathetic effect parasympathetic mediated by β1 effect mediated receptors G- by M2 receptors protein coupled G-protein coupled receptorsIncrease receptors cAMP speads up Increase K+ pacemaker conductanceand potential decrease cAMP How does noradrenaline increase the force of contraction? NA acting on β1 receptors in myocardium causes and increase in cAMP phosphorylation of Ca2+ channels increased Ca2+ entry during AP increased force of contraction also increased uptake of Ca2+ in sarcoplasmic reticulum ANS effects on vasculature most vessels receive sympathetic innervation – exceptions some specialised tissue eg erectile tissue have parasympathetic innervation most arteries and veins have α1-adrenoreceptors – coronary and skeletal muscle vasculature also have β2- receptors Effects of β2 adrenoreceptors and α1 adrenoreceptors on vascular smooth muscle Activating β2 adrenoreceptors causes vasodilation – increases cAMP opens a type of potassium channel relaxation of smooth muscle Activating α1 adrenoreceptors causes vasoconstriction Role of local metabolites Active tissue produces more metabolites – e.g. adenosine, K+, H+, increase PCO2 Local increases in metabolites have a strong vasodilator effect More important for ensuring adequate perfusion of skeletal and coronary muscle than activation of β2-receptors