MBBS Year 1 - Autonomic Nervous System PDF

MBBS -YEAR 1 AUTONOMIC NERVOUS SYSTEM Desired Learning Outcomes ◼ On completion of this topic, you should be able to: 1. identify the divisions and subdivisions of the peripheral nervous system 2. list the functions of sympathetic and parasympathetic systems 3. compare and contrast the tw...

MBBS -YEAR 1 AUTONOMIC NERVOUS SYSTEM Desired Learning Outcomes ◼ On completion of this topic, you should be able to: 1. identify the divisions and subdivisions of the peripheral nervous system 2. list the functions of sympathetic and parasympathetic systems 3. compare and contrast the two subdivisions of ANS 4. list the higher control of ANS 5. outline the role of ANS and endocrines in stress responses ◼ Contents 1. Divisions of autonomic nervous system – sympathetic and parasympathetic 2. Functions of autonomic nervous system 3. Importance of ANS in maintenance of homeostasis 4. Control by higher centers  AUTONOMIC NERVOUS SYSTEM ◼ Definition: Part of the nervous system that controls most of the visceral functions: Eg: 1) Arterial blood pressure 2) G.I tract motility 3) G.I secretions 4) Sweating 5) Body Temperature ◼ All these functions are quick  CENTERS FOR ANS ◼ 1)Spinal cord ◼ 2) Brain stem ◼ 3)Hypothalamus ◼ 4) Limbic cortex ◼ ANS also operates by visceral reflexes AUTONOMIC NERVOUS SYSTEM ◼ Supplies & controls cardiac & smooth muscles, glandular cells, & special cells such as J.G. cells ◼ Maintains internal environment of the body, meeting it’s demands on minute- to-minute basis ◼ Involuntary in control ◼ Regulation mainly through reflexes  Autonomic Nervous System ◼ Two divisions - Sympathetic N.S, - Parasympathetic N.S. ◼ Two links- Preganglionic, postganglionic ◼ Sympathetic- short preganglionic fibers & long postganglionic fibers ◼ Parasympathetic- long preganglionic fibers & short postganglionic fibers  Autonomic outflow Sympathetic outflow Preganglionic Postganglionic Ach NE Parasympathetic outflow Postgan. Preganglionic Ach Ach  Autonomic outflow ◼ Parasympathetic-Cranio-sacral outflow III, VII, IX, X cranial nerves S 2,3,4 segments of spinal cord ◼ Sympathetic- Thoraco-lumbar outflow T1 – L2 segments of spinal cord ◼ Sympathetic & Sacral Parasympathetic neuron cell bodies in lateral horn of spinal cord  Sympathetic outflow ◼ Thoracolumbar outflow (T1-L2) ◼ Preganglionic axons short, leave the spinal cord through the ventral roots ◼ Axons pass via the white rami (myelinated) to the paravertebral sympathetic ganglion at the same segmental level ◼ Sympathetic chain extends from cervical to coccygeal level  Sympathetic outflow ◼ Most preganglionic neurons synapse on the cell bodies of postganglionic neurons in the sympathetic ganglia. ◼ Axons of the postganglionic neurons reenter the spinal nerves via the gray rami (nonmyelinated) ◼ Distributed to the autonomic effectors ◼ Each of the 31 pairs of spinal nerves has a gray ramus. ◼ Some preganglionic neurons end on postganglionic neurons of collateral or prevertebral ganglia ◼ Sympathetic chain forms the distributing system LHC White ramus Gray ramus Sympathetic ganglion  Effectors of the sympathetic nervous system ◼ Blood vessels ◼ Abdominal viscera, liver ◼ Piloerector muscles ◼ Pelvic organs ◼ Eccrine sweat glands ◼ Kidneys ◼ Heart ◼ Genitalia ◼ Lungs ◼ Salivary glands ◼ Bronchi ◼ Eye (Dilator pupillae) ◼ Adrenal medulla  Parasympathetic outflow I.Cranial outflow: Edinger Westphal nucleus (of III cranial nerve) Superior salivatory nucleus (of VII cranial nerve) Inferior salivatory nucleus (of IX cranial nerve) Dorsal motor nucleus and nucleus ambiguous (of X cranial nerve) II.Sacral outflow: ◼ Intermediolateral gray columns of S2, S3 and S4 segments  Parasympathetic outflow ◼ The parasympathetic ganglia near or within walls of the visceral organs. Ciliary ganglion of occulomotor (III cranial) nerve, behind the eyeball. The Pterigopalatine and Submandibular ganglion of facial (VII cranial) nerve. The Otic ganglion of glossopharyngeal (IX cranial) nerve The parasympathetic ganglia situated within the walls of thoracic, abdominal and pelvic viscera.  Effectors of the parasympathetic nervous system ❑ Sphincter pupillae and ciliary ❑ Liver,Pancreas and muscles of eyeball (III n) Gastrointestinal tract from ❑ Lacrimal gland (VII n). the esophagus to the splenic ❑ Salivary glands (VII & IX) flexure of the colon (Vagus) ❑ Remainder of the colon, ❑ Heart (X n) rectum, urinary bladder and ❑ Lungs (X n) reproductive organs ❑ Bronchi (X n) ( S 2, 3, 4) Constricts pupil  Parasympathetic Sympathetic Response Organ Response "Fight or Flight" "Rest and Digest" Heart Increased rate and strength of Decreased heart rate (baroreceptor contraction Cardiac output decreases reflex) Cardiac output increases Lung Bronchioles Constriction Dilation Glycogen breakdown Liver Glycogen No effect Blood glucose increases Breakdown of fat Adipose tissue No effect Blood fatty acids increase Basal No effect Increases Metabolism Increased secretion of HCl & Decreased secretion Stomach digestive enzymes Decreased motility Increased motility  Increased secretion of HCl & Decreased secretion Intestine digestive enzymes Decreased motility Increased motility Relaxes internal sphincter Constricts internal sphincter Urinary bladder Detrusor muscle contracts Relaxes detrusor Urination promoted Urination inhibited Relaxes sphincter Constricts sphincter Rectum Contracts wall muscles Relaxes wall muscles Defecation promoted Defecation inhibited Pupil constricts Pupil dilates Eye Adjusts for near vision Adjusts for far vision Male Sex Organs Promotes erection Promotes ejaculation  NEUROTRANSMITTERS IN ANS Sympathetic nervous system Parasympathetic n. system ◼ At the autonomic ganglia ◼ At the autonomic ganglia Acetylcholine Acetylcholine (Nicotinic receptors) (Nicotinic receptors) ◼ At the effector structures ◼ At the effector structures Norepinephrine (Adrenergic Acetylcholine receptors;  & ) (Muscarinic receptors)  Blockers of ANS neurotransmiiters Drugs that block -adrenergic receptors ◼Phentolamine, phenoxybenzamine, prazocin (1), yohimbine (2) Drugs that block -adrenergic receptors ◼Propranolol, atenolol , metaprolol, labetalol Drugs that block muscarinic cholinergic receptors ◼ Atropine, scopalamine Drugs that block autonomic ganglia (nicotinic receptors) ◼ Hexamethonium  AUTONOMIC CONTROL OF ORGAN SYSTEMS ◼ Most organs supplied by both the divisions of autonomic nervous system, ◼ This dual innervation results in reciprocal actions that precisely control visceral activity ◼ E g., when there is a need for decreased heart rate (in sleep), parasympathetic fibers to heart are activated to inhibit SA node & decrease heart rate. Sympathetic NS is inhibited However, some organs only receive sympathetic innervation: Spleen Sweat glands Piloerector muscles Most blood vessels AUTONOMIC CONTROL OF ORGAN SYSTEMS ◼ Most autonomic actions mediated through reflexes ◼ E.g., pupillary reflexes, GI activity, micturition, baroreceptor reflex, etc ◼ Sympathetic nervous system has diffuse actions (‘fight or flight response’ ) – ‘catabolic’ ◼ Parasympathetic nervous system regulates vegetative functions of the body (“Rest and Digest”)- ‘anabolic’  Higher control of ANS Hypothalamus is the main integration center of ANS activity. Posterior nucleus- sympathetic center Anterior nucleus –parasympathetic center Subconscious cerebral input via limbic lobe connections influences hypothalamic function Other controls come from the prefrontal lobe of cerebral cortex, & reticular formation.  A.1. The autonomic nervous system: is regulated by the Hypothalamus It regulates activities of : (1) Visceral muscles : which are involuntary , and include (a) cardiac muscle , and (b) smooth moscle in arterioles and the gastro-intesinal tract (GIT). (2) Glands ( e.g. , sweat glands and GIT glands ) It consists of 2 subdivisions: 11/14/2024 Figure 11-2 25  A.2 The autonomic nervous system pathways: The autonomic division consists of two efferent neurons in series Fig. 11-4 Tasrget tissue : effector Effector 11/14/2024 26  Higher control of autonomic function Sympathetic functions regulated by posterior hypothalamus Parasympathetic functions regulated by anterior hypothalamus 11/14/2024 27  The ANS has 2 subdivisions : sympathetic and parasympathetic 11/14/2024 28 Effect of sympathetic stimulation Sympathetic part of ANS is active during fear, anxiety, sever pain preparing the body for fight or flight Promoting mechanisms which increase energy & metabolism 11/14/2024 29  Effect of parasympathetic stimulation Parasympathetic system activities is related to relaxed state Vegetative function: – Feeding – Resting – Part of the sexual function 11/14/2024 30  Sympathetic (Thoracolumbar) System 1.The preganglionic fibers are short ( because most ganglia are paravertebral ) and postgang fibers are long. 2. Pregang fibers are myelinated and postgang fibers unmyelinated. 3.Pregang fibers secrete ACh in ganglia 4. The cholinergic receptor in ganglia is nicotinic 5.The neurotransmitter secreted by postgang fibers is Norepinephrine , except in sweat glands and blood vessels in skeletal muscle (in the latter cases it is ACh). 6.There is muchdivergence. The ratio (pre/post) being = 1/10 or more. This divergence implies simultaneous diffuse ( generalized ) actions. 11/14/2024 31  Parasympathetic (Craniosacral ) system : 1.The preganglionic neurons are long and postgang fibers are short ( because the parasympathetic ganglia are located either close to the arget organ or embedded in its wall. 2. Pregang fibers are myelinated and postgang fibers unmyelinated. 3.Pregang fibers secrete ACh in ganglia 4. The cholinergic receptor in ganglia is nicotinic 5.The neurotransmitter secreted by postgang fibers is ACh 6. The receptors on the target organ is cholinergic muscarinic ( can be blocked by Atropine ). 7.There is little divergence. The ratio (pre/post) = 1/3. The limited divergence results in more specific and selective actions. 11/14/2024 32  It should be noted that Under physiological conditions , nearly all sympathetic and parasympathetic activities are opposite ( contradictory ) to each other ; however , the two systems cooperate ( & may act in unison/accord ) in (1) salivary secretion ( sympathetic → scanty secretion rich in enzymes , parasympathetic → Watery , copious secretion ) , (2) Emotional stress : parasympathetic → increased tear and nasal secretions , sympathetic → increased heart- rate , BP , etc (3) Sexual intercourse : parasympathetic → erection of the penis or clitoris , sympathetic → ejaculation or orgasm 11/14/2024 33  Autonomic Neurotransmitter All preg. Fibres (sympathetic and parasympathetic ) secrete acetylcholine at the ganglia. All postganglionic parasympathetic fibers secrete acetylcholine at target organs. Most postganglionic sympathetic fibers secrete norepinephrine at target organs. Postganglionic sympathetic fibers to sweat gland & blood vessels of skeletal muscles release acetylcholine All epinephrine in the bloodstream comes from the adrenal medulla. Postganglionic sympathetic nerves can not synthesize epinephrine from its precursor which is norepinephrine. 11/14/2024 34 Further reading –Tutorial 1]Describe the autonomic reflexes integrated at the level of spinal cord. 2]Outline the functions of the autonomic nervous system 3]Describe the location of postganglionic sympathetic and parasympathetic neurons and the pathways they take to the visceral structures they innervate. 4] How does sympathetic discharge prepare the individual for flight of fight? Does the sympathetic nervous system have any other functions? 25  References 1.Dee Silverthorn. Human Physiology. 6 th ed. Pearson, 2013, Boston. 2.Guyton A C and Hall J E. Textbook of Medical physiology. 12th ed. W. B. Saunders & Co, 2008. Philadelphia. 3.Ganong W F. Review of Medical Physiology. 24th ed. Simon Schuster Asia Pvt. Ltd, 2012. Singapore.