AUTONOMIC NERVOUS SYSTEM

ARCHANA

Introduction

• ANS also k/a the vegetative , visceral or involuntary nervous system, serves as the functions of maintaining ongoing organ function and adopting to environmental changes.

• It serves as one of component of a larger sensory integrative motor system made up of the nervous, endocrine and immune system that ensures inside & outside forces affecting the body.

Anatomy

• Has both peripheral and central components.• Peripheral ANS : 3 components• 1) sympathetic n. system (SNS)• 2) parasympathetic system (PSNS)• 3) enteric n. system (ENS)• ANS efferents consist of two neurons • i) preganglionic neurons : whose soma is in the brain

stem or sc • ii) postganglionic : soma is in ganglion, a cluster of

neuron cell bodies outside the CNS.

SNS

PSNS

• The oculomotor nerve (iii) innervates the eye• Facial and glossopharyngeal nerves innervate

face , oropharynx and salivary glands• Vagus nerve innervates thoracic and

abdominal organs down to the descending colon.

• Sacral parasympathetic outflow (pelvic nerves) controls the pelvic organs.

ENS • The myenteric plexus, located between the

longitudinal and circular muscle layers, is responsible for the segmentation of gut contents and peristalsis.

• That involves reciprocal innervation of the two muscle layers : inhibited & other is excited with propulsion of gut contents.

• Submucosal plexus , located between the circular muscle and mucosa , regulates ion & water transport across the epithelia, controls crypt cell secretions & feeds information to regulate the gut motility.

Physiology

Neurotransmitters and their receptors

• Based on NT release , the ANS can be divided into 3 divisions: 1) the cholinergic system

• 2) the adrenergic system • 3) non adrenergic non cholinergic• NANC division overlaps SNS & PSNS.• Cholinergic fibres innervate ganglia (both SNS & PSNS) , PSNS

postganglionic synapses, SNS postganglionic synapses on sweat glands.

• SNS postganglionic adrenergic fibres release NE.• Dopamine is released by SNS postganglionic adrenergic fibres in

the renal, mesenteric and coronary vascular beds & by interneurons in sympathetic ganglia.

Effects on specific structures

• SNS & PSNS generally shows antagonistic effects.• HEART: SNS activation produces +ve chronotropic

,inotropic & dromotropic effects.• Parasympathetic activation reduces heart rate &

force.• Blood vessels: SNS activation produces

constriction of renal, mucosal, cutaneous, splanchnic arteries and some veins.

• PSNS activation produces opposite effects.

• Gut : SNS activation produces relaxation of smooth muscle, contraction of sphincter muscle & reduced secretions.

• PSNS effects Inc. peristalsis & secretions, relaxation sphincter muscles.

• Lungs : SNS activation produces relaxation of smooth muscle, bronchodilation & reduced secretions.

• PSNS produces bronchoconstriction & mucus secretion.• Eye : SNS activation produces dilation of pupil &

retracting of nictitating membrane.• PSNS activation causes miosis, lacrimation, contraction

of ciliary muscle, helps in accommodation of the lens for near vision.

• Bladder : SNS activation produces relaxes the detrusor muscle of bladder, contracts trigone, leads to urine retention.

• PSNS acts oppositely ,results in urine evacuates.• Sweat gland: eccrine type glands are activated by sympathetic

cholinergic innervation.• Apocrine glands are activated by sympathetic adrenergic innervation.• Sweat gland don’t receive PSNS innervation.• Salivary glands: SNS activation produces limited saliva, decreases

blood flow.• PSNS activation produces release of profuse saliva.• Reproductive organs: SNS activation produces ejaculation, contraction

of muscle in epididymis, ductus deferens, prostate glands.• PSNS innervation is for venoconstriction in penis & clitoris, producing

erection & for mucus secretions.

Effect of exercise

• ANS makes compensatory changes to exercise , inc. SNS activity, dec. PSNS activity & stimulated respiratory activity.

• SNS signals to the heart helps to inc. cardiac output to meet the tissue demand.

• Max. cardiovascular changes occurring with exercise is due to central resetting of baroreceptor reflex, inc. sympathetic activity.

• Stimuli to chemoreceptor eliciting inc. SNS activity include inc. in H+ , K+ , BRADYKININ.

EFFECTS ON AGEING

• Normal ageing in reduced in ANS function in response to stress .

• Baroreceptor reflex function is reduced with Inc. in age, Inc. in BP, Dec. in cardiac function, reduced exercise tolerance.

• Vagal parasympathetic regulation of heart rate is Dec.

• But circulating NE levels may be increased, with reduced receptor responsivity.

Pharmacology

• Major classes of drugs acting on the ANS are usually classified by the transmitter system affected.

• Alpha adrenergic agonist drugs such as phenyl propanolamine are used to Inc. BP & treat urinary incontinence by activating receptors on appro. Tissues.

• While its antagonists i.e. phenoxybenzamine are used as antihypertensive agents by blocking alpha receptor.

• Beta adrenergic agonist drugs such as isoproterenol are used as bronchodilator agents, while beta antagonists such as metaprolol are used to treat cardiac arrhythmias.

Disorders

• Trauma & diseases affecting the ANS constitute a major portion.

• Several number of autonomic disorders exist i.e. diabetes neuropathies, anhydrosis, sympathectomy following cervical cord trauma, various dysautonomias, where diffuse degeneration of SNS & PSNS postganglionic or preganglionic neurons occurs.

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