general principles function of GIT Flashcards
gastrointestinal tract smooth muscle is excited by?
almost continual slow intrinsic electrical stimuli.
two basic types of electrical waves:
slow waves and spikes.
describe the contractions of the gastrointestinal tract
occur rhythmically.
the rhythm is determined mainly by the frequency of slow waves of smooth muscles membrane potential.
slow wave
are not action potentials but undulations in the resting membrane potential of smooth muscle ( cyclic waves of depolarization and repolarization)
interstitial cells of Cajal- cat as electrical pacemakers for smooth muscle cells.
interstitial cells of Cajal ion channels
periodically open and generate inward (peacemaker) currents that producing a slow wave activity- cyclic changes in membrane potential.
slow waves do not stimulate muscle contraction by themselves except where?
in the stomach.
slow waves also trigger generation of what?
spike potentials
spike potentials
are action potentials that stimulate muscle contraction
they are prolonged than those of skeletal muscles.
slow waves propagate and spike occurs at the peak of slow waves.
each muscle layer function as what ?
Syncytium.
that is when an action potential is elicited anywhere within the muscle mass, it generally travels in all directions in the muscle.
the normal resting membrane potential of spike wave in the smooth muscle fibers of the gut
-50 and -60
spike waves appears where ?
slow waves peak
MP is more positive than -40 millivolts
higher the slow wave potentials rise, the greater the frequency of the spike potentials.
The frequency range of the spike wave
between 1 and 10 spikes per second
higher spike frequency in small intestine
the factors that stimulate depolarization / depolarize GIT smooth muscle membrane potential.
stretching of the muscle.
stimulation of acetylcholine released from the endings of parasympathetic nerves.
stimulation of specific gastrointestinal hormones.
Factors that hyperpolarize the membrane.
the effect of norepinephrine or epinephrine on the fiber membrane
stimulation of the sympathetic nerves that secrete mainly norepinephrine at their nerves endings.
the gastrointestinal tract has an intrinsic nervous system called ?
enteric nervous system.
the enteric nervous system lies where?
beginning of the oesophagus and ending of the anus.
and is controlled by movement and secretion.
the enteric nervous system is composed of wo interconnected plexuses.
- outer plexus lying between the longitudinal and circular muscle layers called the myenteric plexus ( Auerbach’s plexus).
- Inner plexus called the submucosal plexus (Meissner plexus) and lies in the submucosa.
myenteric plexus
controls the gastrointestinal movements
submucosal plexus
control gastrointestinal secretions and blood flow.
Both myenteric and submucosal plexuses are connected to the ?
parasympathetic and sympathetic fibers.
The enteric nervous system can function ………….of these extrinsic nerves
independelty
sensory nerve endings that originate in the gastrointestinal epithelium
send afferent fibers to the enteric system.
send impulses to the spinal cord and the brain stem
what is created by the sensory nerves
short reflexes within the gut
long reflexes that relayed to the gut from brain stem and prevertebral ganglia
GIT sensory system nerves can be stimulated by:
- irritation of mucosa
- excessive gut distention
3.chemical substances in gut.
stimulation of the myenteric plexus can result in?
Increased tonic contraction or tone of the gut wall
increased intensity of the rhythmic contractions
increased rate of the rhythmic of contraction
increased velocity of conduction of excitatory waves along the gut wall cause rapid movement of the gut peristaltic waves.
myenteric plexus neurons also
send inhibitory signals
e.g nerve endings secrete vasoactive intestinal polypeptide (VIP)
inhibitory signals
s inhibit contraction of the intestinal sphincter muscles and impede movement of food along
the gastrointestinal tract.
Pyloric sphincter inhibition
allows emptying of the stomach into the duodenum
➢Inhibition of the ileocecal valve muscle
allows emptying from the small intestine into the cecum
submucosal plexus to control local :
intestinal secretion
absorption
contraction of the submucosal muscle.
*Submucosal muscle control the degree of infolding of the gastrointestinal mucosa
*Parasympathetic supply to the gastrointestinal Tract has 2
divisions.
*Cranial division
➢cranial parasympathetic nerve fibers are almost entirely
in the vagus nerves
*Sacral division
➢The sacral parasympathetic originate in the sacral
segments of the spinal cord.
➢Sacral parasympathetic function - execute the
defecation reflexes.
which c stimulation generally increases activity
of the entire enteric nervous system - which in turn
enhances activity of gastrointestinal functions
parasympathetic stimulation
Sympathetic nervous system stimulation exerts its effects in two ways:
secretion of norepinephrine inhibits intestinal tract smooth muscle
and inhibits the entire enteric nervous system
strong stimulation of sympathetic system can inhibit? what?
motor movements
impending movements of food through the gastrointestinal tract.
. Reflexes integrated with the enteric nervous system and gut wall.
control GIT secretion, peristalsis, mixing contractions, local inhibitory effects.
reflexes between the GIT and the sympathetic ganglia.
i. Gastrocolic reflex - transmit signals from the stomach cause evacuation of the colon.
ii. Enterogastric reflexes - signals transmitted from the colon and small intestine to inhibit stomach motility
and stomach secretion.
iii.Colonoileal reflex - inhibit emptying of ileal contents into the colon.
Reflexes between the gastrointestinal tract and the spinal cord or brain stem.
Vagovagal reflex - signals from the stomach and duodenum to the brain stem and back to the stomach—
via the vagus nerves—to control gastric motor and secretory activity.
ii. Pain reflexes that cause general inhibition of the entire gastrointestinal tract.
iii.Defecation reflexes - signals travel from the colon and rectum to the spinal cord and back again to produce
the powerful colonic, rectal, and abdominal contractions required for defecation
the effect of major GIT hormones on GIT motility
- Hormones control gastrointestinal secretion and motility.
- Hormones act on target cells with specific receptors for the hormone and do not require nervous
connections to control gastrointestinal function
Hormonal control of Gastrointestinal motility.
Gastrin is secreted by G cell of the antrum of the stomach.
The primary actions of Gastrin
stimulate gastric acid formation or secretion.
stimulate growth of the gastric mucosa
Gastrin is secreted in response to the following stimuli:
- Ingestion of a meal.
- Stomach distention.
- Products of proteins
hormone that is secreted in the mucosa of small intestine by I cells response to fat, fatty acids and monoglycerides in the intestinal contents
cholecystokinin
cholecystokinin stimulate…….to contract and whta?
gallbladder to contract and release bile into the small intestine.
function of bile
Bile emulsify fatty substances – fat digestion and absorption.
why does emptying of food slow or takes longer?
CCK slows the emptying of food from the stomach to allow adequate time for digestion of the fats in
the upper intestinal tract.
cholecystokinin inhibits what?
appetite
e by stimulating sensory nerve in the duodenum and send signal via the vagus nerve to
inhibit feeding centers in the brain
the first gastrointestinal hormone to be discovered.
Secretin
secreted by S cells in the mucosa of the duodenum
Secretion of secretin occur in response to acidic gastric juice emptying into the duodenum from the
pylorus of the stomach.
secretin stimulate what?
pancreatic secretion of bicarbonate to neutralize the acid in the small intestine.
and secretin has a mild effect on GIT tract motility.
What is secreted by K cells
the gastric inhibitory peptide is a glucose-dependent insulinotropic peptide. secreted in the mucosa of upper small intestine.
is secreted mainly in response to fatty acids and amino acids (response less to carbohydrate).
Gastric inhibitory peptide
GIP slows emptying of gastric contents into the duodenum when the upper small intestine is already
overloaded with food.
also GIP stimulate insulin secretion.
Motilin
is secreted by the stomach and upper duodenum by Mo cells during fasting.
the function of motilin
to increase gastrointestinal motility.
Motilin is released periodically and stimulates waves of gastrointestinal motility called ?
interdigestive myoelectric complexes
occurs in the stomach and small intestine every 90 min during fasting.
ingestion of food inhibits GITs….
motility
Two types of movements occur in the gastrointestinal tract:
propulsive -Move food forward along the tract and rate of propulsion allow digestion and
absorption.
mixing-Keep the intestinal contents until thoroughly mixed
Propulsive movements — Peristalsis
Peristalsis mechanism: A contractile ring is formed by the circular muscle around the gut and then moves
forward
peristalsis stimulation:
➢ Large amount of food collects in the gut cause distention (stretching) of the gut wall stimulates the
enteric nervous system to contract the gut wall.
➢ Chemical or physical irritation of the epithelial lining in the gut.
➢ Parasympathetic nervous signals to the gut triggers a strong peristalsis.
congenital absence of the myenteric plexus result in what ?
in weak or absence of peristalsis in affect portion of the GIT tract.
congenital absence of the myenteric plexus result in what ?
in weak or absence of peristalsis in affect portion of the GIT tract.
paralyze the cholinergic nerve endings of the myenteric plexus
results in weak or blocked peristalsis in the entire gut is due to ?
Atropine
The “Law of the Gut”
- peristaltic waves move toward the anus with downstream receptive relaxation.
Law of the Gut” - peristaltic waves move toward the anus with downstream receptive relaxation
* Peristalsis can “theoretically” occur in either direction from a stimulated point.
* Peristalsis toward the mouth is inhibited while continuing toward the anus.
* Gut wall distention initiates peristalsis on the orad side and moves toward the anal direction.
* The gut smooth muscles relax downstream toward the anus - receptive relaxation allow food to be propelled
more easily toward the anus than toward the mouth
Segmentation Contractions—Mixing Movements
Mixing movements differ in different segments of the alimentary tract.
- Peristaltic contractions cause most of the mixing when intestinal content is blocked by a sphincter.
- Local intermittent segmentation contractions lasting between 5 to 30 seconds occur in the gut wall
Blood passes through liver sinusoids is (lined by
reticuloendothelial cells
leaves vai hepatic veins to IVC
Sympathetic stimulate
vasoconstriction of the arterioles and decreased blood flow
