Download Physiology of GIT

Physiology of GIT:
Gastro intestinal wall including many layers :
- Serosa
-Longtudinal muscle layer& Circular m. layer
- Submucosa
- Mucosa , a layer of smooth muscle lies in the deeper layer of
mucosa called (muscularis mucoasa).
The motor function of the gut performed by different layer of
smooth muscle..The smooth muscle fibers arranged in
bundles , they extends longitudinally down the intestinal tract
in the longitudinal m. layer, & arranged around the gut in the
circular m. layer .Within each bundle the muscle fiber
electrically connected with one another through large number
of gab junction that allow low resistance movements of ions
from one cell to the next. therefore electrical signals can
travel readily from one fiber to the next within each bundle
but more rapidly along the length of the bundle than
sideways.
Each bundle of smooth separated from the next by
loose connective tissue but the bundle fuse With one
another at many points , therefore each muscle layer
function as syncytium , that is when an action
potential is elicited anywhere within the muscle mass ,
it generally travels in all directions in the muscle.the
distance that it travels depends on the excitability.
 Autonomic control of GIT:
 The parasympathetic supply of GIT is divided into cranial
& sacral divisions, except for few parasympathetic fibers
to the mouth & pharangeal regions , the cranial
parasympathetic transmitted almost entirely in the vagus
nerve, these fibers provide innervation to the esophagus
,stomach pancreas & somewhat less to the intestine
down through the 1st half of large intestine
 ,the sacral parasympathetic originate in the S2, S3 &S4
segment of spinal cord .
 The parasympathetic neurone of parasympathetic
system located in the myenteric &submucosal plexuse,
stimulation of parasympathetic nerve generally cause
increase in activity of enteric nervouse system.
 The sympathetic fiber of the of the GIT originate from
segment T-5 & L-2 of spinall cord .
Hormonal control of GIT motility:
1- cholecystokiniin:(cck): secreted by the mucosa of
duodenum& jejunum in response to the presence
of some substances in the intestinal contents. it
has a potent effect in increasing the contractility
of GB, also it inhibit the stomach motility.
2- secretin secreted by S cell in the mucosa of
duodenum in response to gastric acid juice , it has
inhibitory effect on most of GIT.
3- gastric inhibitory peptide:secreted by the mucosa
of upper S.I. ,it has mild effect in decreasing motor
activity of stomach .
Transport & mixing of food in the GIT:
Mastication (chewing):
The teeth are designed for chewing , the
anterior (incisors) provide strong cutting
action & the posterior (molars)provide
granding action . Most of chewing process
caused by chewing reflex which can be
explained as that : the presence of bolus of
food in the mouth initiates reflex inhibition to
the muscle of mastication which allow the
lower jaw to drop, the drop in turn initiates a
stretch reflex of the jaw muscle that leads to
rebound contraction & , this automatically
raises the jaw to cause closure of the teeth,
but it also compress the bolus again
against the lining of the mouth, which
inhibit the jaw muscle once again
allowing the jaw to drop &rebound again
Chewing is important for digestion of all food
but it especially important for most fruit ,raw
vegetables because these have undigested
cellulose membrane around their nutrient
portions that must be broken before the food
can be used , the digestive enzyme act only
on the surface of food particles,the rate of
digestion is highly depenndent on the total
surface area exposed to the intestinal
secretion .
Swallowing (deglutition):
1- voluntary stage
When the food is ready for swallowing it is
voluntary squeezed or rolled posteriorly in
to the pharynx by pressure of tongue
upward & bachward against the palate.
2- pharangeal stage of swallowing (involuntary)
When the bolus of food enter the pharynx it
stimulate the swallowing receptors arround
the opening of pharynx & send signals to the
brain stem to initiate swallowing
3- pharangeal stage of swallowing .
Function of lower esophageal sphincter
(gasrtoesophageal sphincter GOS)
:at the lower 2 -5 cm of esophagus above its
junction with stomach , the circular muscle
function as sphincter , this sphincter
remain tonically consrticted in contrast to
the mid part of esophagus which remain
relaxed .
when peristaltic wave pass down the
esophagus ,receptive relaxation relax the
LOS ,& allow propulsion of swallowed food
to the stommach.if LOS faild to relax result
in condition known as achalassia.