NERVE SUPPLY TO GASTROINTESTINAL TRACT
GI tract has two types
of nerve supply:
I. Intrinsic nerve supply
II. Extrinsic nerve supply.
INTRINSIC NERVE
SUPPLY – ENTERIC NERVOUS SYSTEM
Intrinsic nerves to GI tract form the enteric nervous system that controls all the secretions and movements of GI tract. Enteric nervous system is present within the wall of GI tract from esophagus to anus. Nerve fibers of this system are interconnected and form two major networks called
1. Auerbach plexus
2. Meissner plexus.
These nerve plexus contain nerve cell
bodies, processes of nerve cells and the receptors. The receptors
in the GI tract are
stretch receptors and chemoreceptors. Enteric nervous system is controlled by extrinsic
nerves.
1. Auerbach Plexus
Auerbach plexus is
also known as myenteric
nerve plexus. It is present in between the inner circular
muscle
layer
and the outer longitudinal muscle layer.
Functions of Auerbach
plexus
Major function of this
plexus is to regulate the movements of GI tract. Some nerve fibers of this
plexus
accelerate the movements by secreting
the excitatory neurotransmitter substances like acetylcholine,
serotonin
and substance P. Other fibers of this plexus inhibit
the GI motility by secreting the inhibitory neurotransmitters such as vasoactive
intestinal polypeptide (VIP), neurotensin and enkephalin.
2. Meissner Nerve
Plexus
Meissner plexus is otherwise
called submucus nerve plexus. It is situated in between the muscular layer and submucosal layer of
GI tract.
Functions of Meissner
plexus
Function of Meissner plexus is the
regulation of secretory functions of GI tract. These nerve
fibers cause
constriction of blood vessels of GI
tract.
EXTRINSIC NERVE SUPPLY
Extrinsic nerves that control the
enteric nervous system are from autonomic nervous system. Both sympathetic and
parasympathetic divisions of autonomic nervous system innervate the GI tract.
Sympathetic Nerve
Fibers
Preganglionic
sympathetic nerve fibers to GI tract arise from lateral horns of spinal cord
between fifth thoracic and
second lumbar segments (T5 to L2). From here, the fibers leave the
spinal cord, pass through the ganglia of sympathetic chain without having any
synapse and then terminate
in the celiac
and mesenteric ganglia. The postganglionic
fibers from these ganglia are distributed throughout the GI tract.
Functions
of sympathetic nerve fibers
Sympathetic nerve
fibers inhibit the movements and decrease the secretions of GI tract by secreting
the neurotransmitter noradrenaline. It
also causes constriction of sphincters.
Parasympathetic Nerve
Fibers
Parasympathetic nerve fibers
to GI tract pass through some
of the cranial
nerves and
sacral
nerves. The preganglionic and postganglionic
parasympathetic nerve fibers to mouth and salivary glands pass through
facial and
glossopharyngeal nerves. Preganglionic parasympathetic nerve fibers to
esophagus,
stomach, small intestine and upper
part of large intestine pass through vagus nerve. Preganglionic nerve
fibers to lower part
of large intestine arise from second, third and fourth sacral segments (S2, S3 and
S4) of spinal
cord and pass through pelvic nerve. All these preganglionic parasympathetic
nerve fibers synapse with the postganglionic nerve cells in the myenteric and submucus plexus.
Functions
of parasympathetic nerve fibers
Parasympathetic nerve
fibers accelerate the movements and increase the secretions of GI tract. The
neurotransmitter secreted by the
parasympathetic nerve fibers is acetylcholine (Ach).
Preganglionic
parasympathetic nerve fibers to esophagus, stomach, small intestine and upper
part of large intestine pass through vagus nerve. Preganglionic nerve fibers
to lower part of large intestine arise from second, third and fourth sacral
segments (S2, S3 and S4) of spinal cord and pass through pelvic nerve. All
these preganglionic parasympathetic nerve fibers synapse with
the postganglionic nerve cells in the myenteric and submucus plexus.
Functions
of parasympathetic nerve fibers
Parasympathetic nerve
fibers accelerate the movements and increase the secretions of GI tract. The neurotransmitter
secreted by the parasympathetic nerve fibers is acetylcholine
(Ach).
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