Gastrointestinal Physiology 11-14 Flashcards
Characteristics of smooth muscle cells in the GI tract
- function as a synctitium
- cells are arranged in bundles of fibers
- muscle fibers are electrically connected to eachother by gap junctions
Components of extrinsic and intrinsic control systems that regulate GI tract functions
- systems located in the wall of the GI tract (intrinsic systems)
- nerves- enteric nervous system
- endocrine secretions- secretin, gastrin, CCK, GIP, and motilin
- systems located outside the wall of the GI tract (extrinsic systems)
- nerves- vagus and splanchnic nerves
- endocrine secretions- aldosterone
Functions of the enteric nervous system (ENS)
a component of the autonomic nervous system, located within the GI wall–> autonomic regulation of GI elementary functions
Plexus Myentericus
in the muscularis externa, between longitudinal and circular muscle–> control of the muscular activity, muscle tone and contraction rhythm
Plexus submucosus
between submucosa and circular muscle–> mucus secretion and reabsorption
enteric neurons
secrete their neurotransmitter from varicosities or bulge-like structures located on often lengthy axonal collaterals or branches. the effect of an enteric neuron is spread to affect a wide area
dogiel type 1 enteric neuron
small cell body with short dendrites. motor neurons
dogiel type 2 enteric neuron
large cell bodies with one or 2 long dendrites. sensory neurons
dogiel type 3 enteric neuron
multiple shapes and functions
sensory nerve cells
detect changes or stimulus and regulates function
axons go to other enteric nerve cells and to the CNS
mechanoreceptors
recognize stretching of intestinal wall or volume changes by intraluminal pressure
chemoreceptors
detect nutrients, osmolarity, pH
neurites–> decodification of the stimulus–> sensibilization of the endocrine cell
interneuron
process signals coming from other nerve cells or from the CNS
muscle motor neurons
primarily located in the plexus myentericus
stimulatory: Ach–> M3–> increase motility
inhibitory: NO, ATP, VIP, NANC
secretomotor neurons
primarily located in the plexus submucosus
stimulatory: Ach, Substance P and VIP–> increase secretion
inhibitory: SOMA and NPY
vasomotor neurons
in both plexuses
ach and VIP–> increase vasodilation
under physiological conditions, is the GI tract under inhibition or stimulation?
inhibition
GI motility disorders
hypoganglionosis
hirschsprung’s disease (congenital megacolon)
ATP
non-peptide, in postganglionic sympathetic neurons, function is to contract muscle cells
NO
non-peptide, in gastric nerves, function is to reduce motility
vasoactive intestinal polypeptde
peptide, in parasympathetic ganglia, function is vasodilation, relaxation of smooth muscle and sphincters
substance P
peptide, in sympathetic ganglia, function is to increase secretion and motility
neuropeptide Y
peptide, in postganglionic sympathetic neurons, function is to inhibit secretion
slow wave
rhythm of GI contraction frequency controlled by these slow waves of smooth muscle membrane potential
not action potentials, slow, undulating changes in the resting membrane potential
origin not completely understood, may come from interstitial cells of Cajal
spike potentials
true action potentials
occur automatically when the resting membrane potential of the GI smooth muscle becomes more positive than about 40mV
the higher the slow wave potential rises, the greater the frequency of the spike potentials
three types of GI reflexes essential to GI control
- reflexes integrated entirely within the gut wall enteric nervous system–> control GI secretion, peristalsis, mixing contractions, local inhibitory effects
- reflexes form the gut to the prevertebral sympathetic ganglia and then back to the GI tract–> transmit signals long distances to other areas of the GI tract
- —gastrocolic reflex: signal form the stomach that causes evacuation of the colon
- —enterogastric reflex: signal from the colon and small intestine to inhibit stomach motility and secretion
- —colonoileal reflex: signal from the colon to inhibit emptying of ileal contents into the colon - reflexes from the gut to the spinal cord or brain stem and then back to the GI tract
- —reflexes from the stomach and duodenum to the brain stem and back to the stomach to control gastric motor and secretory activity
- —pain reflexes that cause general inhibition of the entire GI tract
- —defecation reflexes that travel from the colon and rectum to the spinal cord and back again to produce the colonic, rectal, and abdominal contractions required for defecation
peristaltic reflex
stretching of the intestinal wall during the passage of a bolus triggers a reflex that constricts the lumen behind the bolus and dilated the lumen ahead of it–> coordination of longitudinal and circular musculature–> propulsive movement of the GI content
a property of many syncytial smooth muscle cells
proximal to the site of distention: excitatory motor neurons are stimulated–> Ach and Sp–> smooth muscle contraction
distal to the site of distention: inhibitory motor neurons are stimulated–> NO, ATP, VIP–> smooth muscle relaxation
vagovagal reflex
afferent and efferent fibers of the vagus nerve coordinate responses to gut stimuli via the dorsal vagal complex in the brain
stimulation of mechanical receptors located in the gastric mucosa stimulates vagus afferents
controls contraction of the GI muscle layers in response to distention of the tract by the chymus
active during the receptive relaxation of the stomach in response to swallowing food
when food enters the stomach, this reflex goes from the stomach to the brain, then back again to the stomach causing active relaxation of the smooth muscle in the stomach wall
motility of the stomach
- the gastric store- tonic relaxation and tonic contraction
- the gastric pump- strong peristaltic wave in the antrum
- passage into the grinder and then into the pylorus
- emptying of fluid and pre-digested particles into the duodenum
increased pressure after stomach filling induces a reflectory relaxation of the stomach
- receptive relaxation– vagovagal reflexes
- adaptive relaxation– gastro-gastric reflexes
- feedback relaxation
intestinal motility functions
- mixing of food with digestive juices
- enhancing contact between intestinal wall and food
- peristalsis, the propulsive movement of the chymus distally
digestive period of intestinal motiility
propulsive pattern: peristaltic wave–> fast aboral migrating contractions
nonpropulsive pattern: segmentation–> localized contractions of circular muscle, small segments of the intestine contract tightly, dividing the gut into 2 segments of constricted and dilated lumen
interdigestive period of intestinal motility
time in which stomach and small intestine are empty
between meals–> migrating motoric complex, typically lasts 80-120 minutes
phase 1: motoric rest, no contractions
phase 2: intermittent and irregular contractions, sometimes isolated stronger ones
phase 3: strong peristaltic contractions starting from the stomach and migrating distally to reach the colon
migrating motoric complex
helps push indigested material out of the intestine
control of the bacterial population
motility of the large intestine
mixing activity is prominent
in horses and pigs, colonic segmentation is more pronounced and results in the formation of sacculations or haustra
antiperistaltic waves: oral migrating contractions that impeded the movement of ingesta, causing a more intense mixing activity
pathologic contractions
giant contractions with high amplitude and long lasting contraction
oral migrating: vomitting
aboral migrating: diarrhea
vomitting
defense mechanism and important clinical sign activated in order to eliminate GI content
complex reflex involving many striated muscle groups and other structures outside the GI tract
coordinated in the brainstem
vomiting stimuli
before food intake
after food intake–> visceral afferents to vomiting center
after absorption–> stimulate chemoreceptor trigger zone in the area postrema
vomiting sequence of events
antiperistaltic wave originates in duodenum
propulsion of ingesta toward stomach
contraction of abdominal musculature increasing intra-abdominal pressure
expansion of chest cavity while glottis remains closed in order to lower intrathoracic pressure
relaxation of lower esophageal sphincter
opening of upper esophageal sphincter
Concerning GI smooth muscle cells, which one is true?
A. their contraction cannot be extrinsically influenced
B. spike potentials do not conduce to a contraction
C. they show spontaneous undulations in the membrane potential
C
Which kinds of sensory neurons will be stimulated during the peristaltic reflex?
A. chemosensitive neurons
B. mechanosensitive neurons
B
Inhibitory muscle motor neurons use:
A. acetylcholine
B. substance P
C. NO, ATP, VIP
C
The proximal stomach serves a storage function in retaining food
A. True
B. False
A
Vomiting contains only ingesta of gastric origin
A. True
B. False
B
Diarrhea
increase in frequency of defecation or fecal volume
volume increase is often due to increased water content
water in the gut results from ingested water, water secreted by glands of the GI tract, or water secreted or lost directly through mucosal epithelium
diarrhea occurs when there is a mismatch between secretion and absorption
malabsorptive diarrhea
absorption is inadequate to recover all secreted water
viral, bacterial, protozoan infections
destruction of the villi and reduced length
shortened villi cause a loss of absorptive intestinal surface area
more affected are mature enterocytes that possess the enzymes of the brush border and transport proteins
secretory diarrhea
rate of intestinal secretion increases and overwhelms the absorptive capacity
some pathogenic bacteria produce enterotoxins which bind to enterocytes and stimulate the adenylyl cyclase activity and the production of cAMP–> opening of chloride channels, water and other electrolytes follow
defectation
internal and external anal sphincters
internal is normally tonically contracted–> anal continence
internal is parasympathetically innervated by the sacral spinal segment and sympathetically innervated by the lumbar segment
in most species: sympathetic stimulation–> constriction of the sphincter; parasympathetic stimulation–> relaxation
voluntary constriction of the external anal sphincter will block the reflector activation of defecation
retrosphincteric reflex
feces accumulates in the rectum
peristaltic movement of feces into the rectum and relaxation of internal sphincter
urge to defecate
