Physiology of Autonomic Nervous System PDF

# Introduction to the Autonomic Nervous System (ANS) ## Objectives At the end of this lecture the student should be able to describe the following: - General arrangement and higher controlling centers of the sympathetic and parasympathetic systems. - Actions of the sympathetic and parasympathetic...

# Introduction to the Autonomic Nervous System (ANS) ## Objectives At the end of this lecture the student should be able to describe the following: - General arrangement and higher controlling centers of the sympathetic and parasympathetic systems. - Actions of the sympathetic and parasympathetic system on different body organs. - Autonomic neurotransmitters and receptors, and their effects. - The adrenal medulla. ## The Autonomic Nervous System (ANS) - ANS has two subdivisions: Sympathetic and Parasympathetic - Higher (CNS) controller of ANS is the Hypothalamus (part of the Limbic System). - Moreover, the Hypothalamus itself is influenced by the Cingulate Gyrus. - Posterior hypothalamus controls Sympathetic NS - Anterior hypothalamus controls Parasympathetic NS - These hypothalamic centers exert their effects via the motor centers of he brainstem & spinal cord. ## Autonomic Nervous System (ANS) - Controls smooth and cardiac muscles, both of which are involuntary. They are parts of the visceral organs. - There are 2 lower motor neurons arranged in series: - (1) Preganglionic neuron: whose cell-body is situated inside the CNS, arising from: - (a) spinal cord (thoraculumbar segments) in case of sympathetic system. - (b) brain (cranial nerve autonomic nuclei) + spinal cord (sacral segments) in case of parasympathetic system. - (2) Postganglionic nerve: whose cell-body is situated outside the CNS, in a ganglion. - Axons of preganglionic nerves are myelinated, but those of postganglionic nerves are unmyelinated. - That is why postganglionic nerves are slower-conducting than preganglionic nerves. ## Difference Between Autonomic and Somatic Nervous Systems The document includes a diagram showing a comparison between the autonomic and somatic nervous systems. - **Autonomic nervous system**: includes a preganglionic neuron (myelinated) and a postganglionic neuron (unmyelinated) which innervates smooth muscle or glands in a visceral organ. An autonomic ganglion is located between the neurons. - **Somatic nervous system**: includes a somatic motor neuron (myelinated) which innervates skeletal muscle. ## Sympathetic (Thoracolumbar) System - Diagram shows the schematic of the sympathetic nervous system, highlighting the cholinergic preganglionic neuron, the autonomic ganglion, and the adrenergic postganglionic neuron. - Preganglionic nerves exit the CNS from the thoracic and lumbar segments of the spinal cord, therefore, the sympathetic system is also called "Thoracolumbar Outflow" - Preganglionic fibers are myelinated. - The postganglionic neuron cell-bodies are located in paravertebral sympathetic ganglia, distant from their target tissues. - Postganglionic fibers are unmyelinated & longer than preganglionic fibers. - There is much divergence. The ratio (pre/post) being = 1:10 - This considerable divergence results in generalized and widespread effects on body functions when this system is activated. ## Parasympathetic (Craniosacral) System - Diagram shows the schematic of the parasympathetic nervous system, highlighting the cholinergic preganglionic neuron, the autonomic ganglion, and the cholinergic postganglionic neuron. - Preganglionic nerves exit the CNS from the cranium and sacral segments of the spinal cord. Therefore, the parasympathetic system is also called “Craniosacral Outflow”. - Preganglionic fibers are myelinated. - The postganglionic neuron cell-bodies are located in parasympathetic ganglia, close to their target tissues or embedded in their walls. - Postganglionic fibers are unmyelinated & shorter than preganglionic fibers. - There is little divergence, the ratio of pre- to postganglionic fibers is 1:3 - This little divergence results in more specific, discrete and localized effects in the body when this system is activated. ## General Effect of Sympathetic Activation (Stimulation) - Diagram shows a seesaw, with parasympathetic activity on one side and sympathetic activity on the other. - The sympathetic system is active during stress and physical activity, such as physical exercise (as in sport), and during increased mental & emotional activity such as in fear, anger, worry, anxiety, severe pain preparing the body for fight or flight. - And promoting mechanisms which increase energy production & accelerate metabolism. ## General Effect of Parasympathetic Stimulation - Diagram shows a seesaw, with parasympathetic activity on one side and sympathetic activity on the other. - Parasympathetic system activities are related to the relaxed state and rest. - Conservation of body energy, and - Vegetative functions (nutritive, body-building, restorative anabolic functions & tissue repair), & is more active during feeding, digestion & rest & sleep. ## Autonomic Neurotransmitters - Diagram shows a comparison between the sympathetic and parasympathetic neurotransmitters, highlighting the release of acetylcholine and norepinephrine at the ganglia and blood vessels. - All preganglionic fibres (sympathetic and parasympathetic) secrete acetylcholine at the ganglia. - All postganglionic parasympathetic fibers secrete acetylcholine at target organs. - Most postganglionic sympathetic fibers secrete norepinephrine. - However, 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. ## Adrenergic Receptors (Receptors for Norepinephrine & Epinephrine) - Alpha (a) adrenergic receptors are found in : - Iris radial muscles (pupil dilation) - Blood vessels (especially arterioles) - GIT - Beta (β) adrenergic receptors can be beta one (β1) or beta 2 (β1) found in: - Heart (β1) - Bronchioles (β2) - Skeletal muscle (β2) (NB vasodilation in skeletal muscle is partly by cholinergic muscarinic receptors & partly by β2 adrenergic receptors). - GIT (β2) - Norepinephrine mainly excites a (and β to a lesser extent) - Epinephrine excites both a & β equally ## Effects of Adrenergic Receptor Stimulation - Alpha (a) receptors: - (1) Vasoconstriction (raised BP) - (2) Pupil dilation - Beta (β) receptors: - (1) Increased heart rate, HR (β1) - (2) Increased heart force of contraction (β1) - (1) and (2) above increase the amount of blood pumped by the heart, & contribute to raising the BP. - (3) Vasodilatation (β2) - (4) Bronchiolar dilation (β2) - (5) Intestinal wall relaxation (β2) - (6) Bladder wall relaxation (β2) ## Cholinergic Receptors - Are divided into - (1) Nicotinic found in all ganglia (i.e., the synapses between pre- & postganglionic of both sympathetic & parasympathetic divisions of the ANS). - (2) Muscarinic found on all effector cells innervated (& stimulated) by: - (1) postganglionic parasympathetic fibers, & - (2) postganglionic sympathetic cholinergic nerves (to sweat glands & blood vessels in skeletal muscle). - Cholinergic Receptor Blockers - Atropine block muscarinic receptors. - Hexamethonium (block both muscarinic & nicotinic receptors). ## Effect of Sympathetic & Parasympathetic Stimulation The document presents a table summarising effects of sympathetic and parasympathetic stimulation on different organs. | Organ | sympathetic | parasympathetic | |-----------------------------------|---------------------|-----------------------| | Pupil | Dilatation of pupil | Constriction of pupil | | Heart | Increased heart rate | Decreased heart rate | | | Increased force of contraction | No effect | | Systemic blood vessels | constriction | Little or no effect | | Blood Pressure | Raised | Little or no effect | | Lung (Bronchioles) | Dilation | Constriction | | Glands: Nasal, Lacrimal, Salivary, | | | | Gastric, Intestinal, Pancreatic | Slight (small volume) secretion | Copious (large volume) secretion | | Sweat Glands | Increased secretion | No effect | | Urinary bladder | Promote retention | Promotes emptying: | | | | contraction of wall) | | | | relaxation of (sphincter | | Blood sugar | raised | No effect | | Gastrointestinal Tract (GIT) | | | | secretions | Decreased | Increased | | GIT motility (contraction of | | | | muscles in walls, Peristalsis) | Decreased | Increased | | Sphincters | Constriction | Relaxation | | | | | | | Therefore, | Therefore, | | | sympathetic | parasympathetic | | | system to GIT | system to GIT | | | promotes retention | promotes digestion | | | | & excretion | | Blood vessels to skeletal muscles | Dilatation (cholinergic) | None | | Genital System | Ejaculation | Erection | | Adrenal medulla | Secretion of | No effect | | | epinephrine & | | | | norepinephrine | | | Metabolism | Increased | No effect | | Blood: Coagulation | Increased | No effect | ## Autonomic Neurotransmitter - The document contains a thank you slide.