Dr.Nusrat Tariq (M.Phill., Ph.D Scholar)
Associate Professor
Department of physiology
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Gastrointestinal physiology:
A branch of human physiology that addresses the
physical function of the gastrointestinal (GI)
system.
Dr. Alzoghaibi
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.
Gastrointestinal system:
1-The gastrointestinal (GI) tract— a series of
hollow organs:
Mouth
Esophagus
stomach
small intestine
large intestine— rectum and anus.
2- Solid accessory organs:
Liver
Pancreas
Gallbladder
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Primary Functions of Digestive System
1. Ingestion - getting food into the GI tract (eating)
2.Propulsion - moving food along the tract by swallowing and peristalsis
(wave-like motion)
3. Mechanical Digestion - the physical grinding and churning of foodstuffs to
breakdown and expose to enzymes and the surface of the GI tract
4.Chemical Digestion - breakdown of larger molecules into absorbable parts
by enzymatic action
5.Absorption - transport of digested molecules, vitamins, minerals, water, into
blood
6. Excretion - elimination of unused foodstuff, heavy metals, toxins,
alkaloids.(feces).
7. Helps Erythropoises by secreting intrinsic factor needed for Vitamin B12
absorption
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.
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Dr. Alzoghaibi
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Typical cross section of the Gut
SMOOTH MUSCLE OF G.I.T
Unitary type,visceral or syncytial smooth muscle.
Contract spontaneously in the absence of neural or
hormonal influence but in response to stretch (such as in
stomach and intestine)
Cells are electrically coupled via gap junctions so each
muscle layer functions as a syncytium.
Multiunit type smooth muscle.
Do not contract in response to stretch or without neural
input (such as in esophagus & gall bladder).
Composed of discrete independently working smooth
muscle fibers ,each of which is innervated by single nerve
ending.
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Slow Waves & Spike potentials are Forms of
Electrical Activity in GI Muscles
Slow waves
 They are not action potential but are slow undulating changes
in membrane potential(-56mv)
 Frequency of slow waves determines rhythm of gastrointestinal
movements.
 They do not cause Ca++ to enter the smooth muscles so by
themselves cause no muscle contraction.They mainly excite
the appearance of intermittent spike potentials.
 Occur at different frequency
stomach (3/min)
small intestine (duodenum, 12/min)
ileum & colon (8-9/min).
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.
 Caused by complex interactions among smooth muscle cells and
specialized cells called Interstitial Cells Of Cajal (Electrical
Pacemaker).
 These interstitial cells form a network with each other and are
interposed between the smooth muscle layers, with synapticlike contacts to smooth muscle cells.
 The interstitial cells of Cajal undergo cyclic changes in
membrane potential due to unique ion channels that
periodically open and produce inward (pacemaker) currents that
may generate slow wave activity.
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Spike Potentials
 The spike potentials are true action potentials.
 They occur automatically when the resting membrane
potential of the gastrointestinal smooth muscle becomes more
positive than about −40 millivolts (the normal resting
membrane potential((−50 −60 ml.volts).
 Spike potentials appear on the peaks of slow waves.
 The higher the slow wave potential rises, the greater the
frequency of the spike potentials, usually ranging between 1
and 10 spikes per second.
 The spike potentials last 10 to 40 times as long in
gastrointestinal muscle as the action potentials in large nerve
fibers, with each gastrointestinal spike lasting as long as 10 to
20 milliseconds.
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Each time the peaks
of the slow waves
temporarily become
more
than The positive
higher the
40slow
millivolts,
wave spike
potentials
on
potentialappear
rises, the
these
peaks
greater
the
frequency of the
spike potentials,
usually ranging
between 1 and 10
spikes per second.
Figure 62-3; Guyton & Hall
.
 Factors that depolarize the membrane:
 Stretching of
the muscle
 Ach
 Parasympathetic stimulation
 Hormonal stimulation
 Factors that hyperpolarize the membrane:
 Norepinephrine
 Sympathetic stimulation
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Contractions in Gastrointestinal Smooth Muscles
 Phasic contractions
 Periodic contractions followed by relaxation; such as in gastric
antrum, small intestine and esophagus
 Tonic contractions
 Maintained contraction without relaxation; such as in orad
region of the stomach, lower esoghageal, ileocecal and internal
anal sphincter
 Not associated with slow waves .
- Caused by:
• Continuous repetitive spike potential
• Hormonal effects
• Continuous entery of Ca ions.
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The Musculature of the Digestive Tract

Two Main Muscle Layers:

Longitudinal muscle layer

Circular muscle layer

Oblique muscle layer (stomach only)
Longitudinal Muscle:
 Contraction shortens the segment of the intestine and expands the lumen
 Innervated by ENS, mainly by excitatory motor neuron
 Ca influx from out side is important.
Circular muscle:
 Thicker and more powerful than longitudinal.
 Contraction reduces the diameter of the lumen and increases its length .
 Innervated by ENS, both excitatory and inhibitory motor neurons.
 More gap junctions than in longitudinal muscle.
 Intracellular release of Ca is more important
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