Download The GIT - UoD Moodle

Gastro-intestinal Tract
(GIT)
The GIT provides the body
with water, electrolytes &
nutrients.
1
In general the wall of GIT composes of mucosa,
submucosa, inner circular & outer longitudinal
layer of smooth muscles & serosa.
In some parts there are additional muscle layer
between the mucosa & submucosa called the
muscularis mucosae.
The smooth M. of GIT act as a synsytium, they
are connected by tight junctions allowing the
spread of excitement from one M. fiber to
another by movement of ions between the cells,
leading to spread of AP through the GIT.
2
Factors affect RMP to become more +ve
leading to depolarization:
1- Stretch of M. fiber.
2- Ach.
3- parasympathetic stimulation.
4- GIT hormones.
Factors affect the RMP to more –ve leading to
hyperpolarization:
1- NE or Epinephrine.
2- Sympathetic stimulation.
3
Control of GIT functions
There are 2 control systems;
1- The nervous system.
2- The hormones.
4
The neural control
I) The enteric nervous system.
The GIT has its own NS which can function
independently.
It consists of 2 plexus:
a- The myenteric (Auerbach plexus);
it composes of neurons in between the
longitudinal & circular M. layer extending from
the esophagus to the anus.
It controls the movement of GIT, i.e M.
contraction.
5
b- Submucosal (Meissners plexus);
It composes of neurons in the submucosa.
It controls the secretions of GIT.
When Ach is secreted from the enteric neurons
it stimulates both contraction & secretion.
While if NE is secreted from the enteric neurons
it will inhibit both contraction & secretion.
6
II) The extrinsic innervation ( autonomic NS).
The parasympathetic fibers to GIT run through vagus
N. which supply the esophagus, stomach, pancreas,
small intestine & 1st half of large intestine.
Also parasympathetic fibers from S2, S3 & S4 pass
through the pelvic Ns to supply the 2nd half of large
intestine down to the anus.
Parasympathetic stimulation increases GIT motility &
secretion because it stimulates the enteric NS to
secrete Ach.
The sympathetic innervation originates from T5 to L2
spinal segments.
7
8
Movements of the GIT
9
Propulsive movement (peristalsis).
Mixing movement.
10
Movements of the GIT
There are 2 types of movements:
1- Propulsive movement or peristalsis.
It occurs at any site from the esophagus
downward.
Accumulation of food in any portion causes
distension of that part & stimulation of
myenteric plexus & so contraction of the circular
Ms starts 2-3 cm above the distension, then the
ring of contraction moves downward to push the
food down.
11
2- Mixing movement.
It occurs when there is peristalsis but the
sphincter is closed, or due to local
contractions of some parts of GIT.
This movement leads to mixing of the
luminal contents with the secretions of the
GIT to digest the contents and to make
contact of the content with GIT mucosa for
absorption.
12
13
All the absorbed nutrients reaches the liver (except fat,
which is absorbed into intestinal lymphatics).
In the liver ½ - 2/3 of the nutrients are stored.
Also any bacteria or harmful substances that enters
the circulation from the GIT are removed by the
14
Kuppfer cells.
Transport & mixing of food
The time which food stays in different parts of
GIT is very important & any part of this time is
properly controlled by several neural &
hormonal factors so that the passage of food
should not be too rapid nor too slow.
15
16
The intrinsic desire for food is
called hunger, while the type of
food which person seeks is called
the appetite.
17
Mastication (Chewing)
Chewing is important to break large food
particles into small ones & mix them with
the saliva, so facilitating easy swallowing
& further digestion.
Most of the muscles of chewing are
innervated by the motor branches of the
5th cranial nerve.
18
Swallowing (Deglutition)
1- The voluntary stage.
2- The pharyngeal stage.
3- The esophageal phase.
19
Swallowing (Deglutition)
1- The voluntary stage.
The tongue pushes the food upward &
backward against the palate while the lips are
closed, after that swallowing becomes
involuntary.
2- The pharyngeal stage.
a- The soft palate moves upward
to close the posterior nares
preventing reflux of food to the nose.
20
21
b- The palatopharyngeal folds are approximated so
that only well chewed food can pass in between them.
c- The vocal cords are strongly approximated, the
larynx is pulled up & the epiglottis closes the larynx, all
these occur to prevent the passage of food to the
larynx.
d- The upward movement of the larynx pull & enlarges
the opening of the esophagus to receive the bolus of
food.
e- The pharyngoesophageal sphincter is relaxed, with
rapid & strong peristaltic waves start in the upper part
of the pharynx & spreads down to the middle & lower
part then to the esophagus.
22
Pharyngeal phase
The sensory stimuli from the
posterior portion of the mouth
& pharynx passes via the
trigeminal & glossopharyngeal
Ns to the swallowing center in
the medulla oblongata & pons,
then efferent impulses return
via the 5th , 9th, 10th & 12th
cranial Ns.
23
3- The esophageal phase.
The esophagus have 2 types of peristaltic
movements; primary & secondary.
The primary is continuation of the peristaltic
waves begins in the pharynx, it is facilitated by
the gravity in upright position.
24
If the primary peristalsis fails to push the food
to the stomach, then the secondary peristaltic
waves result from distention of the esophagus
by the retained food & they continue until all
the food passes to the stomach.
The Ms of the pharynx & upper esophagus are
striated & they are supplied by the
glossopharyngeal N.
While the Ms of the lower 2/3rd of the
esophagus are smooth & supplied by the
vagus N in connection with the myenteric
plexus.
25
The lower esophageal sphincter
(Gastroesophageal sph.)
2 factors prevent reflux of gastric content
into the esophagus:
1- contraction of the circular Ms of the
lower esophageal end, which relaxes only
when peristaltic waves passes down to the
esophagus.
2- The esophagogastric angle, which acts
as a valve-like mechanism to prevent
reflux.
26
27
Motor functions of the stomach
1- Storage of food:
When food enters the stomach, the stomach
wall relaxes until it accumulates about 1.5 L.
This is mediated by the vagovagal reflex i.e
sensory stimuli arise from the stomach travel
through the vagus to the brainstem, then
inhibitory orders return through the vagus to the
stomach to inhibit stomach contractions.
28
2- Mixing of food with gastric secretions until it
becomes semifluid material called Chyme.
When food reaches the stomach, the slow
waves of stomach initiates weak peristaltic
contractions that push the content of stomach
toward the antrum, but these contractions are
weak & cannot overcome the resistance of the
pyloric sphincter, so the food is mixed with the
secretions of gastric glands.
Hunger contractions: These are strong peristaltic
contractions that arise 12 - 24 hours after last meal,
also they appear when the level of blood sugar is low,
they cause pain in the pit of the stomach this pain is
called hunger pangs..
29
3- Emptying of the stomach:
The stomach contractions becomes stronger so
that they overcome the resistance of pyloric
sphincter & with each contraction few milliliters
of chyme pushed into the duodenum.
Stretching of the wall of stomach stimulates
myenteric plexus so that strong contractions will
be elicited.
In addition, the products of meat digestion
stimulates the antral cells (G cells) to secrete
gastrin which stimulates gastric motility.
30
Movements of the small intestine
1- Mixing movement (segmentation contraction):
When the chyme distends the small intestine
wall, many concentric contractions appear, then
after these segments relax, other sets of
contraction starts at new points.
31
2- Peristalsis:
- Each peristaltic wave progress to only 3-5
cm, therefore, the chyme requires about
3-5 hours to pass from the pylorus to the
ileocecal valve.
- These peristaltic waves greatly increase
after meal & they cause progression of the
chyme & spread it to be well digested &
absorbed.
32
Peristaltic rush
It is strong & powerful waves that wash
down the contents of small intestine to the
large intestine.
It occurs when there is irritant chyme or
when there is excessive distention of the
small intestine.
33
Migrating motor complex
These are moderately powerful waves
occur in fasting person, starting from the
stomach sweeping any excess digestive
secretions or other intra intestinal debris
into the colon, thereby preventing their
accumulation in the upper GIT.
34
Functions of the ileocaecal valve
This valve is important to prevent
regurgitation of the fecal material from
the caecum into the ileum.
The ileocecal sphinctor slightly resist
the emptying of ileum, so, prolongs
the stay of the chyme, thereby
facilitates its absorption.
35