Autonomic Nervous System (ANS) Lecture Notes PDF

2024-2025 CBAutonomic Nervous System (ANS) Physiology Division  Objectives By the end of this lecture, the student should be able to: Towards ▪ Identify the origin of parasympathetic outflow. sympathetic outflow and...

2024-2025 CBAutonomic Nervous System (ANS) Physiology Division  Objectives By the end of this lecture, the student should be able to: Towards ▪ Identify the origin of parasympathetic outflow. sympathetic outflow and unbounded ▪ Identify the sympathetic chain connection to the spinal thinking. cord. ▪ Differentiate between the pre- and post-ganglionic fibres. ▪ Describe the organization of ANS and differentiate between sympathetic, parasympathetic and somatic systems ▪ Recognize the different neurotransmitters released by preganglionic and postganglionic sympathetic and parasympathetic neurones and the receptors activated. ▪ Identify the adrenal medulla and the receptors activated by its secretion. ▪ Outline the autonomic nervous system’s role in cardiovascular control and the receptors activated. Date : / 4/ 2017 Walter Cannon’s enduring legacy, from 1915… Everyone knows that sympathetic nerves mediate “flight or fight” Everyone thinks parasympathetic nerves participate in “rest and digest” These concepts can be misleading, for several reasons: De-emphasises importance of ANS in all situations (complementary actions and fine control) Scientifically restrictive (and often is wrong) Clinically limiting (and has misdirected doctors) Walter Cannon (1929) “In an open system, such as our bodies represent, complex and subject to numberless disturbances, the very existence of a steady state is in itself evidence that agencies are at hand keeping the balance, or ready to act in such a way as to keep the balance.” Peripheral Nervous System: Sensory System (Afferent) - exteroceptors (periphery) - interoceptors (interior) Somatomotor System (Efferent) - skeletal muscle Autonomic Nervous System (Efferent) - sympathetic division - parasympathetic division - enteric division Some definitions Afferent (Latin): “carry towards (CNS)” Efferent (Latin): “carry away from (CNS)” Bell-Magendie Law “Dorsal roots are sensory, ventral roots are motor”  Structure of spinal nerves: Somatic pathways dorsal root dorsal dorsal root ganglion ramus spinal dorsal nerve somatic horn sensory nerve CNS (GSA) inter- neuron ventral ventral somatic horn ramus motor nerve (release ventral root gray ramus communicans Ach.) white ramus communicans Mixed Spinal sympathetic Nerve ganglion Autonomic Nervous System A system of motor nerves that Function to regulate the activity of: Cardiac muscle Smooth muscle Exocrine glands Q: Mention the divisions of the autonomic nervous system? Divided into 3 divisions 1. Parasympathetic Nervous System 2. Sympathetic Nervous System 3. Enteric Nervous System organised in plexuses within the gut can function without input from the spinal cord. Origin of Autonomic Nervous System  Pathways from Spinal cord The efferent of both sympathetic and parasympathetic systems has two neurons(preganglionic and postganglionic ) Distribution of the autonomic nervous system Remember: Ganglion (a swelling) on all autonomic nerves Ganglion has cell bodies (neuron-to-neuron synapse) preganglionic nerves myelinated ➔white rami postganglionic nerves unmyelinated ➔grey rami N.B: Dorsal root ganglion is the nerve cell of afferent fibres Remember sympathetic ganglia tend to be remote from target parasympathetic ganglia tend to be close to target Adrenal Medulla Remember no ganglion on adrenal nerve pathway but adrenal medulla cells are neuron-like adrenal medulla considered a modified ganglion neuro-secretion into bloodstream (“divergence”)  Principle of divergence Sympathetic divergence Parasympathetic divergence Principle of convergence  There are two synapses on all autonomic nerve pathways: neuron-to-neuron synapse – at the ganglion neuron-to-effector synapse – at the tissue target [Synapse = a point of communication] Communication (transmission) occurs at each synapse by: Presynaptic neuron action potential (electrical stimulation) causes release of chemical transmitter The chemical transmitter binds to receptors on postsynaptic cell leading to electric excitation  Synaptic Transmission The process of synaptic TRANSDUCTION is changing one form of energy into another. 1. neurons release transmitter substances (chemical energy) 2. transmitters activate receptors (mechanical energy) 3. receptors change the membrane potential (electrical energy) Receptors are specialised proteins: 1. embedded in the cell membrane 2. twisted into two general arrangements A) they make ion channels B) they link to intracellular enzymes Synaptic Transmission Efferent nerves leaving the CNS (preganglionic) release acetylcholine (ACh) & ATP Preganglionic nerves release mainly Ach and some ATP to excite ganglia and adrenal medulla Neurotransmitters of Autonomic Postganglionic Fibers: Receptors of the ANS Come as families 1) Nicotinic receptors: In autonomic ganglia (principally ion channels), sometimes called nAChRs multiple structural subtypes 4 pharmacological phenotypes (inc. ganglion phenotype) 2) Muscarinic receptors: In parasympathetic targets (and some sympathetic targets) (principally enzyme-linked proteins) 5 structural subtypes (M1- M5) 5 pharmacological phenotypes (inc. M2 in heart) 3) Adrenoceptors: (principally enzyme-linked proteins) 10 structural subtypes 2 pharmacological phenotypes (inc. alpha 1&2 and beta 1&2) Alpha 1 adrenoceptors in blood vessels Beta 1 adrenoceptors in heart Distribution of Autonomic Neurotransmitters & their receptors The sympathetic postganglionic neurons release noradrenaline to act on adrenergic receptors, with the exception of: ❑ the sweat glands ❑ the blood vessels of skeletal muscles. Where the neurotransmitter is acetylcholine, which acts on muscarinic receptors. At the adrenal medulla, there is no postsynapic neuron. Instead the presynaptic neuron releases acetylcholine to act on nicotinic receptors Stimulation of the adrenal medulla releases mainly adrenaline and some noradrenaline into the bloodstream which will act on adrenoceptors ➔a widespread increase in sympathetic activity The parasympathetic system postganglionic neurons release acetylcholine that stimulates muscarinic receptors. It will be discussed in details in nervous control of the CVS Adrenergic Receptors Main actions 1. Alpha-1 = vasoconstriction of blood vessels 2. Alpha-2 = inhibits release of norepinephrine 3. Beta-1 = ↑heart rate & force on contraction 4.Beta-2 = relaxation of smooth muscle in bronchi, uterus, peripheral blood vessels Cholinergic Receptors Main actions 1. Muscarinic - stimulates smooth muscle & slows HR 2. Nicotinic - affect skeletal muscle and Ganglia Sympathetic Responses Parasympathetic Responses It will be discussed in details in nervous control of the CVS Drugs acting on Sympathetic Nervous System (Adrenergic) - Drugs that mimic = adrenergic drugs, sympathomimetics, or adrenomemetics * Adrenergic agonists - Drugs initiate a response - Drugs that block = adrenergic blockers, sympatholytics or adrenolytics * Adrenergic antagonists - prevent a response  Drugs acting on Parasympathetic Nervous System (Cholinergic) - Drugs that mimic the neurotransmitter Acetyl choline = cholinergic drugs, -Cholinergics stimulate the parasympathetic nervous system -Parasympathomimetics Cholinergic agonists - initiates a response - Drugs that block = anticholinergic, parasympatholytics Cholinergic antagonists - prevents a response  Direct-acting Parasympathomimetic (cholinergic drugs) direct- drug binds directly to receptor mimicking acetylcholine indirect-drug inhibits cholinesterase allowing acetylcholine to accuumulate and bind to the receptor Four drugs used commonly on the ANS: Nicotinic receptors blockers: Hexamethonium Muscarinic receptors blockers: Atropine Adrenoceptors blockers: Phentolamine (alpha) propranolol (beta) Balance between sympathetic and parasympathetic Katona et al. (1982) J Appl Physiol 52: 1652-57. Altering the work done by the heart Remember No ACh effect on ventricles Parasympathetic supply only to atria, SAN & AVN (not ventricles) Little postganglionic divergence This effect is called positive inotropy (blocked by propranolol) It will be discussed in details in cardiodynamics Altering the work done by resistance arterioles  The effect of total autonomic blockade? ‘’Not present nowadays’’  To Summarize: Autonomic Nervous System Differences between Sympathetic & Parasympathetic Neurotransmitters Sympathetic NE (ACh at sweat glands), ACh, + + / -, α & ß receptors All preganglionics release acetylcholine (ACh) & are excitatory (+) Symp. postgangl. — norepinephrine (NE) & are excitatory (+) or inhibitory (-) Parasymp. postgangl. — ACh & are excitatory (+) or inhibitory (-) Excitation or inhibition is a receptor-dependent & receptor-mediated response Parasympathetic ACh, + Potential for pharmacologic ACh, + / - modulation of autonomicresponses muscarinic receptors Divisions of ANS Sympathetic division of ANS Parasympathetic division found in ventral roots of found in ventral roots of neuraxis neuraxis thoraco-lumbar outflow craniosacral outflow Ganglia: Ganglia are: 1.Paravertebral chains (L & R) widespread; ( close to vertebral column ) localised to targets 2. Prevertebral abdominal plexus ( lies close to Aorta ) Flight or fight Rest & Digest mediated by noradrenaline mediated by acetylcholine  Sympathetic division Parasympathetic division thoracic and cranial nerves (III, VII, IX and X. lumbar segments &the sacral part of spinal cord pass Stress Rest Mostly in sympathetic chain of Mostly terminate on neurons in ganglia & in collateral ganglia terminal ganglia  References: Guyton & Hall: Textbook Of Medical Physiology. Suggested Readings: Human Physiology, The Basis of Medicine 2nd edition – Pocock and Richards Chapter 10, The autonomic nervous system Chapter 15, The nervous control of the heart, pp 314 Figure 15.18 Chapter 15, The nervous control of blood vessels, pp 323-4. Human Physiology, The Basis of Medicine 3rd edition – Pocock and Richards Chapter 10, The autonomic nervous system Chapter 15, The nervous control of the heart, pp 282 Figure 15.20 Chapter 15, The nervous control of blood vessels, pp 290-291. Thank you