Parasympathetic Nervous System

PARASYMPATHETIC DIVISION

Parasympathetic division of ANS is otherwise called the craniosacral outflow because, the fibers of this division arise from brain and sacral segments of spinal cord.

CRANIAL OUTFLOW OR CRANIAL PORTION OF PARASYMPATHETIC DIVISION

Cranial outflow or cranial portion of parasympathetic division arises from brainstem. It innervates the

blood vessels of head and neck and many thoracoabdominal visceral organs. Cranial outflow

includes the following cranial nerves:

1. Oculomotor (III) nerve

2. Facial (VII) nerve

3. Glossopharyngeal (IX) nerve

4. Vagus (X) nerve.

Preganglionic fibers of these cranial nerves arise from neurons situated at two different levels:

1. Tectal or midbrain outflow (III cranial nerve)

2. Bulbar level or bulbar outflow (VII, IX and X cranial

nerves).

Preganglionic fibers are longer and reach the postganglionic neurons, which are situated within the

organs or close to the organs innervated by these nerves. Preganglionic fibers are myelinated, but the

postganglionic fibers are non-myelinated.

1. Tectal or Midbrain Outflow

Group of cells forming Edinger-Westphal nucleus of III cranial nerve gives rise to tectal fibers. Fibers from this nucleus end in ciliary ganglion. Postganglionic fibers from here supply the sphincter pupillae and ciliary muscle.

2. Bulbar Level or Bulbar Outflow

Preganglionic fibers are the fibers of VII, IX and X cranial nerves, which arise from the nuclei present in the medulla oblongata. Fibers of VII cranial nerve supply the lacrimal, nasal, submaxillary and sublingual glands. Preganglionic fibers of this nerve end in sphenopalatine ganglion and submaxillary ganglion. Postganglionic fibers from sphenopalatine ganglion supply lacrimal and nasal glands. Postganglionic fibers from submaxillary ganglion supply sublingual and submaxillary glands Fibers of IX cranial nerve supply the parotid gland. Preganglionic fibers synapse with neurons of otic ganglion. Postganglionic fibers from otic ganglion supply the parotid gland. Fibers of X cranial nerve supply visceral organs of the body. Preganglionic fibers terminate in the ganglia, which are situated on or near the organs. Postganglionic fibers from the ganglia supply the organs. Vagus nerve supplies almost all the organs in the thorax and abdomen, but not the pelvic organs.

SACRAL OUTFLOW OR SACRAL PORTION OF PARASYMPATHETIC DIVISION

Sacral outflow or sacral portion of parasympathetic division arises from the sacral segments of spinal

cord. It innervates smooth muscles forming the walls of viscera and the glands such as large intestine, liver, spleen, kidneys, bladder, genitalia, etc. Preganglionic fibers arise from anterior gray horn

cells of 2nd, 3rd and 4th (from 1st also in some cases) sacral segments of spinal cord and form the pelvic

nerve (nervi erigens). Fibers end on postganglionic neurons, which are situated on or near the visceral

organs. Fibers from postganglionic neurons supply descending colon, rectum, urinary bladder, internal

sphincter, urethra and accessory sex organs. Sacral parasympathetic fibers supply those visceral organs which are not supplied by vagus.

FUNCTIONS OF ANS

Autonomic nervous system is concerned with the regulation of functions, which are beyond voluntary

control. By controlling the various vegetative functions, ANS plays an important role in maintaining constant internal environment (homeostasis). Almost all the visceral organs are supplied by both

sympathetic and parasympathetic divisions of ANS and the two divisions produce antagonistic effects on

each organ. When the fibers of one division supplying to an organ is sectioned or affected by lesion, the

effects of fibers from other division on the organ become more prominent.

NEUROTRANSMITTERS OF ANS

Different nerve fibers of ANS execute the functions by releasing some neurotransmitter substances.

SYMPATHETIC FIBERS

1. Preganglionic fibers: Acetylcholine (Ach)

2. Postganglionic noradrenergic fibers: Noradrenaline

3. Postganglionic cholinergic fibers: Ach

Postganglionic sympathetic cholinergic nerve fibers supply sweat glands and blood vessels in heart and in skeletal muscle.

PARASYMPATHETIC FIBERS

1. Preganglionic fibers: Ach

2. Postganglionic fibers: Ach

SYMPATHOMIMETIC DRUGS

Sympathomimetic drugs or adrenaline-like drugs are the drugs, which produce the effects of sympathetic

stimulation. Adrenaline and noradrenaline produced in the body act only for a short duration of about 1 to

2 minutes. Whereas, sympathomimetic drugs injected.

 PARASYMPATHOMIMETIC DRUGS

 Parasympathomimetic drugs or Achlike drugs are drugs, which produce the effects of parasympathetic

stimulation. Ach produced in the body acts only for a short period, whereas the injected Ach acts for a long time. Similarly, parasympathomimetic drugs also exhibit their actions for a longer time. Parasympathomimetic drugs are:

1. Drugs which Act on Muscarinic Receptors

Pilocarpine and methacholine produce their effects by acting on the muscarinic receptors.

2. Drugs which Prolong the Action of Ach

Action of Ach can be prolonged by preventing its destruction. Drugs like neostigmine and physostigmine

inhibit the activity of acetylcholinesterase and so the Ach is not destroyed quickly.

PARASYMPATHETIC BLOCKERS

Parasympathetic blockers are drugs, which prevent the actions of parasympathetic neurotransmitter. The drugs atropine, homatropine and scopolamine inhibit the actions of Ach by blocking the muscarinic receptors.

GANGLIONIC BLOCKERS

Ganglionic blockers are the drugs that prevent the transmission of impulses from preganglionic neurons

to postganglionic neurons. Tetraethyl ammonium ion, hexamethonium ion and pentolinium are some

of the ganglionic blockers. These drugs block both sympathetic and parasympathetic ganglia. However,

ganglionic blockers are commonly used to block sympathetic ganglia, rather than the parasympathetic

ganglia because sympathetic blockade overshadows the parasympathetic blockade.

Sympathomimetic drugs are:

Drugs Stimulating the Receptors Directly

1. Phenylephrine (alpha receptors)

2. Isoproterenol (beta receptors)

3. Albuterol (beta2 receptors).

Drugs Inducing the Release of Noradrenaline

1. Ephedrine

2. Tyramine

3. Amphetamine.

SYMPATHETIC BLOCKERS

Sympathetic blockers are the drugs that prevent

actions of sympathetic neurotransmitter. Sympathetic

blockers act on all levels.

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