Download LEC.1 17/4/2016 Dr.Baybeen Alselevany Gastrointestinal (GI

LEC.1
17/4/2016
Dr.Baybeen Alselevany
Gastrointestinal (GI) System
Objectives:
 Identify structures and organs that make up the GI system
 Identify functions of GI system
 identify the tissue layers that compose the majority of the GI system.
 identify basic electrical activity of GI smooth muscle.
 Neural control of GI system.
Structures of gastrointestinal system
The gastrointestinal system is a muscular tube about 9 meters long.GI system is
a group of interconnected organs and glands that run continuously from mouth to
anus .Its divided into two parts:
1. gastrointestinal tract (GIT) or Alimentary canal or tubular alimentary tract
extending from the mouth to the anus .(mouth , pharynx , esophagus ,
stomach , small intestine , large intestine , and anal canal(anus).. GIT is
subdivided into:
 .upper GIT includes the mouth, esophagus and stomach.
 Lower GIT is composed of small intestine, large intestine, rectum and
anus.
2- Outlying glands or accessory or associated organs such as: Salivary glands,
Pancreas, liver and Gallbladder .These accessory organs release their
secretion into GIT.
Functions of GIT
The gastrointestinal system or gut is the portal through which nutritive
substances, vitamins, minerals, and fluids enter the body. Proteins, fats,
and complex carbohydrates, are broken down into absorbable units
(digested).
 Ingestion: intake, in this case in the form of food and fluids.
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 Digestion: the breakdown of foods into smaller basic parts, to be used by
body for growth and repair. Its occurs principally in the small
intestine. Mechanical digestion is the process of changing the
physical structure of food while chemical digestion which occurs
by digestive enzymes in GIT system and food particles are
converted into simpler forms appropriate for absorption.
 absorption :is the mechanisms by which the digestive end products as
well as water , electrolytes , vitamins and minerals cross the mucosa and
enter the blood stream and ultimately to tissue and cells.
 Elimination: removal of waste from the body.
Tissue layers that compose the majority of the GI system
Physiologic Anatomy of the Gastrointestinal Wall
Walls of GIT have various layers, beginning at esophagus and
extending to the anus .The GIT is composed of the five basic tissue layers
1. Mucosa (innermost layer) .2.submucosa containing blood and lymph
vessels .3. Circular muscle layer 4. Longitudinal muscle layer. 5. Serosa.
Gastrointestinal smooth muscle functions as a "syncytium"
Basic Electrical Activity of GIT smooth muscle
The smooth muscle of the GIT has spontaneous rhythmic fluctuations in
membrane potential between about -50 and -60 mV.The smooth muscle
of the GIT is excited by continual slow, intrinsic electrical activity along
the membranes of muscle fibers
1. Slow wave's potential or basic electrical rhythm (BER) or pace-maker
waves. .This BER) is initiated by the specialized cells, called interstitial
cells of Cajal, that are believed to act as electrical pacemakers for
smooth muscle cells.. The interstitial cells of Cajal undergo cyclic
changes in membrane potential. The intensity of BER varies between 5
and 15 mv and their frequency ranges in different parts of the human GIT
from 3-12 pulse per minute..
 In the stomach about 3 pulse / minute,
 duodenum 12 pulse / minute ,
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 terminal ileum 8 pulse / minute,
 Cecum 9 pulse /minute, and
 Sigmoid 16 pulse /minute.
Therefore the rhythm of contraction of the:
 body stomach is 3 contractions /min.
 ,Duodenum 12 contractions /min,
 Terminal ileum 8 contractions / min. ,
 Cecum 9 contractions /min
 sigmoid 16 contractions/ min.
2- Spike potentials (SP) or action potential (AP)
In GIT smooth muscle fibers the channels responsible for (AP) are: calcium sodium channels, they allow large numbers of calcium ions to enter along with
smaller numbers of sodium ions. These channels are much slower to open and
close.
3- Changes in voltage of the resting membrane potential (RMP):
Under normal conditions the RMP averages -56mv, multiple factors can change
these levels which are:
A- Depolarization of the membrane: Depolarization is due to calcium ion
influx. Factors that depolarize the membrane increases the tension of intestinal
smooth muscle are:. 1- Stretching of the muscle. 2- Stimulation by acetylcholine
(ACH). 3- Stimulation by Parasympathetic that secret acetylcholine at their never
endings.4- Cold (colic pain). 5- Stimulation by several specific gastrointestinal
hormones.
B- Hyperpolarization of the membrane: hyperpolarizing portion is due to
K+ efflux. Factors that hyperpolarization membrane and decreases the tension
of intestinal smooth muscle are: .1- Norepinephrine (NE) or epinephrine
.2.Heat
3- Stimulation of sympathetic nerves that secrete mainly NE at their
never endings.
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NEURAL CONTROL
1. Enteric (intrinsic) nervous system.
2. Autonomic (extrinsic) nervous system.
1. ENTERIC NERVOUS SYSTEM (ENS): Enteric nervous system is important in
controlling GIT motility and secretion. The GIT has a nervous system all its own
called the enteric nervous system. It lies entirely in the wall of the gut beginning
in the esophagus and extending all the way to the anus .There are two major
networks of nerve fibers that are intrinsic to GIT:
a- myenteric plexus (Auerbach’s plexus which controls mainly the GIT
movements (Motility) or peristalsis.
b- an inner plexus called submucosal or Meissner’s plexus .This plexus
controls GI secretion, local blood flow, and local absorption. Together these two
plexuses constitute the intramural (within the wall) plexuses or ENS.
Neurotransmitters secreted by ENS: different neurotransmitter substances
those are released by the nerve endings of different types of enteric
neurons. :
(1) ACH which excites GI activity. (2) NE almost inhibits GI activity. Epinephrine secreted from adrenal medulla and secreted into the circulation
which reaches GIT by the way of the blood. 3-serotonin 4- Dopamine.
5. Cholecystokinin (CCK) 8- somatostain. 9-gases: nitric oxide (No) and
carbon monoxide (CO) .10- Vasoactive intestinal peptide (VIP). Which inhibits
GI activity.( receptive relaxation )
2. Autonomic control of GIT or Extrinsic innervations
The intestine receives dual extrinsic innervations
nervous system (A.N.S) .
from the autonomic
1. Parasympathetic nervous system 2. Sympathetic nervous system
Nerve fiber from both sympathetic and parasympathetic enters the GIT
and synapse with both plexuses of enteric nervous system.
A. parasympathetic innervations: The parasympathetic supply to the
gut is divided the: 1. cranial outflow. 2. Sacral outflow.
Cranial outflow, which innervating salivary. About 75% of all
parasympathetic nerves fibers are in the vagus nerve which innervates
esophagus, stomach, pancreas, liver, gallbladder, less to the small
intestine, and proximal half of the colon.
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2- Sacral outflow: originated in the second, third, and forth sacral
segments of spinal cord and pass through the pelvic nerves to the distal
half of large intestine and all the way to the anus. These fibers function
especially to excite the defecation reflexes. The parasympathic fibers are
cholinergic (release acetylcholine). . So stimulation of parasympathetic
nerves causes increase in the activity (motility& secretion).
.b. Sympathetic outflows. In general stimulation of the sympathetic will
inhibit motility and secretion.
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