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Introduction
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Digestive System
consists of:
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Objectives
After studying this chapter,
The student should be ableto:
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Explain how food is swallowed &transported to the stomach.
Describe digestion in thestomach.
How gastric secretion isregulated.
i.e. stimulated & inhibited.
List the factors that govern
emptying of the stomach.
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List major components ofpancreatic secretion.
Give their functions.
Explain how secretion iscontrolled.
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Give the functions of theliver.
Explain the role ofbile in thedigestive process & itscontrol.
Explain functions ofgallbladder& control.
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Intestinal secretion &motility.
Regulation of intestinalwall.
Factors affectingintestinal absorption.
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List the functions of thecolon.
List the various types &functions of motility.
Explain howdefecation occurs.
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Digestive system
Gastro intestinal tract Digestive glands
1.salivary glands
2.liver
3.Pancreas
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General Structure ofthe Digestive
System
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Layers of Alimentary Canal Organs
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Cross section of the gut.
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Functions of GIT
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Functions of digestive System:
It is the normal site for:Food & fluids enterance to body:
so it supplys & provides' body with :
Nutritive subs.Vitamins.
Minerals.
Fluids
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1.digestion of food: break down of
complex: Protein, fat & carbohydrate
2.Secretion of digestive juice: variousenzymes & hormones.
3. Motility: Propulsion + mixing' contents
(mostly anal wards)4.Absorption: mainly in small intestine:
absorption of products of digest &
vitamins & minerals: all pass from m.m toblood or lymphatics.
5.Excretion: of undigested & unabsorbed
substances.
6.it has immune function.
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2) Absorption
Transfer of the
contents of the gutfrom lumen into the
mucosa to blood.
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:3) SecretionTransport of fluid,
electrolytes, peptides, .etc
from blood to GIT lumen.( about 7500 ml/day)
Usually isotonic with
plasma (except saliva ishypotonic.)
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4) RES (Immune System)
Humoral antibodies &cellularimmune system
(lymphocytes in Peyerspatches in SI ).
Protect the body against
micro-organisms in lumen ofgut.
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5) Endocrine function
Release of
somatostatin, intestinalglucagon, gastrin,
secretin, CCK etc
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Regulation of GIT functions
(1)Nervous:
neural
Regulation
(2) Hormonal
regulation
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Musculature of the digestive tract is
composed ofsmooth muscle fibers.
Except for the striated muscles of:1. Mouth.
2. The upper part of the oesophagus.
3. The external anal sphincter.
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All smooth muscles are involuntary,controlled by:
Autonomic nerves:
a) Extrinsic: (symp & parasymp)b) Intrinsic: (enteric nerve plexus)
1) The myenteric (Aurbach's plexus)
2) The submucosal (Meissner'splexus)
a) Intr insic innervat ion o f the gu t
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a)Intr insicinnervat ion o f the gu t
The enteric nerve plexus comprises:
1) The myenteric (Aurbach's plexus):
lies in the musculosa between thecircular & longitudinal muscle layers.
It is concerned with controlling the motoractivity of the alimentary canal.
It is mainly excitatory to the tonic &
rhythmic contractions, but it also contains some inhibitory
neurons.
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2) The submucosal
(Meissner's plexus):
lies in the submucous layer.
It is concerned with controlling the local
secretory function of the GIT by
stimulating the local exocrine &
endocrine secretory cells.
It contains the neurons of the sensory
afferent nerves which arise from themucosal layer.
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Innervation of the gut.
b) Extr insic innervat ion o f the
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b)Extr insicinnervat ion o f the
gu tThe intrinsic neural plexuses of
the ENS are:
connected together & modulated by extrinsic
autonomic nerves (sympathetic ¶sympathetic).
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The parasympathetic supply is
generally excitatory to the enteric
nervous system:
It stimulates the motility and secretion.
However, it contains also some
inhibitory vagal fibers to the stomachand the gastrointestinal sphincters.
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The sympathetic nerve
supply generally inhibits
secretion and motility.However, it contains also
some excitatory fibers.
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Sensory fibers arise from
the luminal epithelium and
wall of the gut to the entericplexuses and from there to
the prevertebral ganglia,spinal cord and brain stem.
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Gastro intest inal Reflexes1) Local enteric reflexes:integrated in the ENS.
e.g. reflex gastrin secretion & myenteric
(peristaltic) reflex & gastrocolicreflex.
2) Ganglionic reflexes:
integrated at the prevertebral sympganglia (celiac, mesenteric &hypogastric).
e.g. enterogastric reflex.
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3) Central nervous reflexes:
integrated in the CNS (the brain
and spinal cord).
e.g. peristaltic reflex in the
upper half of the oesophagus (vagovagal reflex) & the spinal
defecation reflexes. R t i
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Receptors in
'wall: (short
reflexes)
Mechanoreceptors
Stim. By stretch
Chemoreceptor
Stimulated by:Products of food digestion &
acids & Osmotic changes
Send impulses to
Local n. plexus
Or through afferent
autonomic n. as
vagus nerve
Receptors out side the gut
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Receptors out side the gutstimulation of:
visual, alfactory or taste receptors can lead to GIT
responses through :- long reflexes
They affect the gut function through efferent
autonomic nerves.
Or leads to:
- Hormonal release:
Release of local hormones: act only on GIT,
affect secretion and motility.
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Release of GIT hormones is regulated by:
1- Nerve impulses :- Local
- Autonomic
2- Chemical factors:
- food digestion products.
- pH changes.
-osmolarity ..
-all can stimulate ' mucosal cells
to produce : hormones..
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Smooth m of GIT functions as a
syncytium
*The longitudinal m.fs are arranged in bundles &extends down the Intestinal tract.
*m.fs. are electrically connected through hugenumber ofgap junctions with low resistance forion movement, so electrical signals travel fromone cell to the next very rapidly along the bundles.
*The bundles fuse with one another at many pointsso they function as a syncytium that allow spreadof action potential in all directions in the muscles.*N.B.
(there are few connections between longitudinal &circularm layers.)
Electrical activity of GIT smooth
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Electrical activity of GIT smoothmuscles: it is of2 types :
1-slow wave (BER):
it is not action potential, just
small changes in RMP.2- Spike potential:
they are true action potentials ,they occur on peaks of slow
waves
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1) Basic electrical rhythm (BER)
The smooth muscle of the GIT
has spontaneous rhythmic
fluctuations in membranepotential between about65 &
45 mV.
Except in the oesophagus & the
proximal portion of the stomach.
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BER
Initiated by the interstitial cells of Cajal,
stellate mesenchymal pacemaker cells
with smooth muscle-like features that send
long branched processes into the intestinalsmooth muscle.
The BER itselfrarely causes muscle
contraction, but spike potentialssuperimposed on the most depolarizing
portions of the BER waves do increase
muscle tension.
BER f GIT th l & it l ti
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BER of GIT smooth muscle & its relation
to muscle contraction.
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The rate of the BER is
about 4/min in the stomach.
It is about 12/min in the duodenum
about 8/min in the distal ileum.
about 2/min at the cecum.
about 6/min at the sigmoid.
The function : to coordinate peristaltic and other
motor activity.Contractions occur only during the depolarizing
part of the waves.
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2) The Spike
The depolarizing portion(Ca2+ influx). The repolarizing portion (K+ efflux).
Many polypeptides &
neurotransmitters affect the BER: e.g.Ach increases the number of
spikes & the tension of the smooth
muscle epinephrine decreases the number of
spikes and the tension
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Stimulatory effect of acetylcholine andinhibitory effect of epinephrine.
Th P i t lti M t
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The Peristaltic Movement
(Peristalsis)
Propels the food or chyme along the
GIT & It has a mixing function.
Peristalsis is a wave ofconstriction -usually preceded by relaxation- of the
gut which proceeds analwards .
It is a local enteric reflextriggeredby distension of the gut.
(called the myenteric orperistaltic
reflex .
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Peristaltic reflex
Stimulus: distension of the gut
(e.g. by a bolus of food)
Receptors: receptors in the gutmucosa.
send impulses to neurons of themyenteric plexus.
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Effect
Ring constriction proximal to the
bolus:
(contraction of the circular m fibers &
relaxation of the longitudinal m fibers).
Receptive relaxation distal to the
bolus:
(relaxation of the circular m fibers &
contraction of the longitudinal m fibers).
This pulls the gut wall over the bolus
hel in its ro ulsion analwards.
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Peristalsis
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The Law o f the Gut:
A distension of the gut
produces a peristaltic wave
that starts at the site ofdistension & proceeds
analwards.
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Electrical activity of GIT smooth
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ymuscles: it is of2 types :
1-slow wave (BER): it is not action potential,just smallchanges in RMP due to
cyclic changes of electrcal pace maker cells whice rspecialised cells,present among smooth muscles, itcauses opening of ion channels with inward current
that generate the slow wave activity.2- Spike potential: they r true action potentials ,theyoccur on peaks of slow waves, they r due to muchCa++& little Na+ influx.
Factors that make the membrane more excitable r:
strech , acetylcholine,parasympathetic nerves &several GIT hormones. The sympathetic,noradrenaline & certain hormones r inhibitory factors.