Parasympathetic Nervous System

$**Parasympathetic nervous system:**$ subdivision of the autonomic nervous system

• ‘rest and digest’ system
• discrete activation

<<Distribution of Parasympathetic Nervous System<<

1. Cranial division
2. Sacral division

}}1. Cranial division}}

Oculomotor nerve (III)

@@arises from:@@ oculomotor nucleus

@@relays in:@@ ciliary ganglion

@@affects:@@

• ciliary muscles
• medial rectal muscles
• constrictor pupillae muscle

Facial nerve (VII)

@@arises from:@@ superior salivary nucleus

@@relays in:@@ sphenopalatine ganglion

@@affects:@@

• tears
• saliva

Glossopharyngeal nerve (IX)

@@arises from:@@ inferior salivary nucleus

@@relays in:@@ otic ganglion

@@affects:@@

• saliva

Vagus nerve (X)

• represents ==75% of parasympathetic fibers== of the body
• has %%afferent%% and %%efferent%% fibers
• can %%stimulate%% and %%inhibit%%
• widely distributed

@@arises from:@@ dorsal nucleus

@@relays in:@@ terminal ganglion (==no post-ganglionic fibers==)

@@affects:@@

• heart (ventricles are not supplied)
• lungs
• gastrointestinal tract

}}2. Sacral division}}

Pelvic splanchnic nerve (nervi erigentes)

@@arises from:@@ lateral horn cells of S2 - S3

@@relays in:@@ hypogastric ganglia

@@affects:@@

• urinary bladder
• distal 2/3 of large intestine
• male and female sex organs
• blood vessels

<<Parasympathetic Effect on Organs<<

<<Organs supplied by Parasympathetic Fibers Only<<

• constrictor pupillary muscles
• esophagus
• gastric glands
• erectile tissue

<<Secretion of Acetylcholine and Norepinephrine by Postganglionic Nerve Endings<<

Many of the parasympathetic nerve fibers and almost all sympathetic fibers merely touch the effector cells of the organs that they innervate.

Where these filaments touch or pass over or near the cells to be stimulated they usually have varicosities (bulbous enlargements).

The %%varicosities%% have vesicles where acetylcholine or norepinephrine are synthesized and stored.

When an action potential spreads over the terminal fibers, the depolarization process increases the permeability of the fiber membrane to calcium ions, allowing calcium influx into the nerve terminals or nerve varicosities.

The calcium ions in turn cause the varicosities to empty their contents to the exterior. Thus the transmitter substance is secreted.

}}Synthesis of Acetylcholine, its destruction after secretion, and its duration of action}}

@@Synthesized in@@: terminal endings and varicosities of cholinergic nerve fibers (where it is stored in vesicles in highly concentrated form until its release)

• Acetylcholine secreted into tissue by cholinergic nerve ending
• persists in tissue for few seconds
• then, split into %%acetate ion and choline,%% catalyzed by ==acetylcholinesterase== enzyme
• choline formed is transported back into terminal nerve ending
• used again for synthesis of new acetylcholine

}}Synthesis of Norepinephrine, its removal, and its duration of action}}

@@Synthesis begins in@@: axoplasm of the terminal nerve endings of adrenergic nerve fibers

@@Completed:@@ inside secretory vesicles

Basic steps:

1. hydroxylation of tyrosine to DOPA
2. decarboxylation of DOPA into dopamine
3. transport of dopamine into the vesicles
4. hydroxylation dopamine to norepinephrine

Note: In the ^^adrenal medulla^^, 80% of the norepinephrine is transformed into epinephrine via ==methylation of norepinephrine==

After secretion of norepinephrine by the terminal nerve endings, it is removed from the secretory site in three ways:

1. %%reuptake%% into the adrenergic nerve endings by an active transport process (major pathway)
2. %%diffuse%% away from the nerve endings into the surrounding body fluids and then into the blood.
3. %%destruction%% of small amounts by tissue enzymes, such as ==monoamine oxidase==

<<Cholinergic (Acetylcholine)<<

• Chemical transmitter
• autonomic ganglion
• all parasympathetic post-ganglionic fibers
• some sympathetic post-ganglionic fibers
• motor end plate (neuromuscular juntcion)
• adrenal medulla
• central nervous system

Receptors:

@@Pre-ganglionic neurotransmitter:@@ Acetylcholine

^^Pre-ganglionic receptor:^^ Nicotinic receptors

@@Post-ganglionic Neurotransmitters:@@ Acetylcholine

^^Post-ganglionic Receptors:^^ muscarinic receptors

muscarinic:

• %%stimulated by:%%
  • reversible: neostigmine
  • irreversible: organic phosphorus compounds (Diisopropyl fluorophosphate (DFP))
• ^^blocked by:^^ atropine

nicotinic:

• %%stimulated by:%% small dose of nicotine
• ^^blocked by:^^ large dose of nicotine

<<Drugs that Act on Cholinergic Effector Organs - Parasympathomimetic Drugs (Cholinergic Drugs)<<

$**Parasympathomimetic drugs:**$ medications that activate the parasympathetic nervous system by mimicking or modifying the effects of acetylcholine.

@@Types:@@

• agonist: a drug or substance that binds to a receptor inside a cell or on its surface and causes the same action as the substance that normally binds to the receptor.
• antagonist: a substance that stops the action or effect of another substance.

Hemicholinium

• ==blocks the reuptake of choline== by the high-affinity choline transporter at the presynapse
• indirect acetylcholine antagonist

Vesamicol

• acting presynaptically by ==inhibiting acetylcholine uptake== into synaptic vesicles and reducing its release
• cholinergic physiological antagonist

Botulinum toxin

• ==inhibits the release of acetylcholine==

Intravenous Acetylcholine Injection

Acetylcholine injected intravenously doesn’t usually cause the same effect throughout the body as parasympathetic stimulation, because most of the acetylcholine is destroyed by cholinesterase in the body.

Other Drugs

E.g. ==pilocarpine==, ==methacholine==

• not rapidly destroyed
• produce widespread parasympathetic effects

Certain drugs have a parasympathomimetic action by potentiating effects of naturally secreted acetylcholine

E.g. ==neostigmine==, ==pyridostigmine==