2. GI Part 8 Flashcards
what are the 3 types of GI reflexes essential to GI function
- reflexes that are integrated entirely within the gut wall
- reflexes form the gut to the prevertebral sympathetic ganglia and then back to the GI tract
- reflexes form the gut to the spinal cord/brainstem and then back to the GI tract
reflexes that are integrated entirely within the gut wall – what does it do
control of GI secretions, peristalsis, formation of mixing contractions, local inhibitory effects
reflexes form the gut to the prevertebral sympathetic ganglia and then back to the GI tract – what do they do
transmit signals long distances to other areas of the GI tract
reflexes form the gut to the prevertebral sympathetic ganglia and then back to the GI tract – 3 different types
- gastrocolic reflex – signals from stomach that cause evacuation of colon
- enterogastric reflex – signals from the colon and small intestine inhibit stomach motility and stomach secretion
- colonoileal reflex – signals from the colon that inhibit emptying of ileal contents into the colon
reflexes form the gut to the spinal cord/brainstem and then back to the GI tract – 3 different types
- reflexes from the stomach and duodenum to the brainstem and then back to the stomach to control gastric motor and secretory activity
- pain reflexes that cause general inhibition of the entire GI tract –> happens in colic
- the defecation reflex which travels from the colon and the rectum to the spinal cord and back again to produce the colonic, rectal, and abdominal contractions required for defecation
describe the peristaltic reflex
stretching of the intestinal wall during passage of bolus triggers a reflex that constricts the lumen behind the bolus and dilates the lumen ahead of it
coordination of longitudinal and circular musculature
propulsive movement of the chyme
what muscles are contracted and what muscles are relaxed in the constricted and dilated areas during peristalsis
constricted – longitudinal muscle relaxation and circular muscle contraction
dilated – longitudinal msucle contraction and circular msucle relaxation
what does stretch of the intestinal wall stimulate in the peristaltic reflex
afferent neurons (mechanoreceptors) –> interneurons –> motor neurons
what is the direction of propulsion in the peristaltic reflex
oral to aboral
what neurotransmitters do excitatory motor neurons release and what do they cause
release Ach and SP
causes smooth muscle contraction
what neurotransmitters do inhibitory motor neurons release and what do they cause
release NO, ATP, VIP
cause smooth msucle relaxation
what does the term vagovagal reflex refer to
gastrointestinal tract reflex circuits where afferent and efferent fibers of the vagus nerve coordinate responses to gut stimuli via the dorsal vagal complex in the brain
what stimulates the vagus afferent fibers in the vagovagal reflex
stimulation of the mechanical receptors located in the gastric mucosa
what does the vagovagal reflex control
motility the gastrointestinal muscle layers in response to distension of the CI tract by chyme like receptive relaxation of the stomach in response to mastication of food and deglutition
describe the distal region of the stomach in terms of motility
the gastric pump
grinding and sieving function
breaks solid pieces of food down into particles small enough for small intestine digestion
describe the proximal region of the stomach in terms of motility
gastric reservoir
stores and retains food as it awaits eventual entry into the small intestine
what are the 4 main steps of the inflow into the gastric pump
- gastric reservoir (tonic relaxation and tonic contraction)
- a strong peristalsis wave in the corpus
- passage into the grinder (mixing and braking down into small particles)
- pylorus
motoric of the stomach – gastric reservoir (tonic relaxation and tonic contraction)
maintain tone without increasing pressure – allows storage of lots of food
motoric of the stomach – a strong peristalsis wave in the corpus
food moves to distal part for mixing and grinding
motoric of the stomach – passage into the grinder (mixing and braking down into small particles)
passage into the grinder, might need to be mixed/ground multiple times
motoric of the stomach – pylorus
movement to the small intestine
inflow into the gastric pump happens due to the following sequence (4)
- tonic contractions of the fundus (gastric store)
- strong peristalsis wave in the corpus
- passage into the grinder and then into the pylorus
- emptying of fluid and predigested particles into the duodenum (fluids empty faster than solids)
increased pressure (moderate) after stomach filling induces a reflective relaxation of the stomach (3)
- receptive relaxation (vagovagal reflexes) – through mastication and deglutition, mechanical stimulation of the pharynx
- adaptive relaxation (gastro-gastric reflexes) – food, expanding stomach
- feed back relaxation – nutrients
functions of intestinal motility (3)
- mixing food with digestive juices (GI secretions)
- enhancing contact between intestinal wall and food
- peristalsis, the propulsive movement of the chyme to the distal (aboral) direction
motility of the small intestine occurs in 2 distinct phases
- digestive period – food is present in the gut
2. interdigestive period – little food is present in the gut
what are the 2 patterns in the digestive period of small intestine motility
propulsive pattern
non propulsive pattern
describe the propulsive pattern of the digestive period of small intestine motility
peristaltic waves
fast aboral migrating contractions
faster in duodenum, medium in jejunum, slow in ileum
describe the non propulsive pattern of the digestive period of small intestine movement
segmentation contractions
localized contractions of circular msucle
small segments of the small intestine contract tightly dividing the gut into 2 segments of constricted and dilates lumen
doesn’t contribute significantly to the net aboral propulsion of ingesta, but is important when nutrient concentration is high
what is the interdigestive period of small intestine motility
time in which stomach and small intestine are empty between meals – typically 80-120 minutes
what is the motility pattern of the interdigestive period of small intestine motility called and what does it do
migrating motoric complex (MMC)
helps push undigested material out of the intestine
control of the bacterial population
what are the 3 phases of MMC of the interdigestive period of small intestine motility
phase 1 – motoric rest, no contractions (GI all quiet, 60-70 minutes)
phase 2 – intermittent and irregular contractions, sometimes isolated stronger ones (20-30 minutes)
phase 3 – strong peristaltic contractions starting from stomach and migrating distally to reach colon (3-10 minutes)
describe the motility pattern of the large intestine
mixing activity is prominent in colon of all species
horses and pigs – colonic segmentation more pronounced and results in formation of sacculations (haustra)
main motility patterns observed in large intestine
peristaltic waves
anti-peristaltic waves – oral migrating contractions that impede movement of ingesta, causing a more intense mixing activity
pathologic contractions
giant contractions – high amplitude and long lasting contractions
oral migrating – vomiting
aboral migrating – diarrhea
describe vomiting
defense mechanism and important clinical sign activated in order to eliminate gastrointestinal content
complex reflex involving many striated muscle groups and other structures outside the GI tract
coordinated in the brainstem
stimuli of vomiting an be classified according to when/where they originate (3)
- before food intake – color, smell, emotions, appearance
- after food intake (particles in intestinal lumen) – visceral afferents, vomiting center
- after absorption (particles in blood) – drugs, toxins – stimulate Chemoreceptor trigger zone (CTZ) in the area postrema
vomiting sequence of events (6)
- antiperistaptic wave originates in duodenum (vomiting may contain ingesta of intestinal origin)
- propulsions of ingesta toward stomach
- contraction of abdominal musculature increasing the intrabdominal pressure
- expansion of the chest cavity while the glottis remains closed in order to lower the intrathoracic pressure
- relaxation of the lower esophageal sphincter
- opening of the upper esophageal sphincter
describe diarrhea
diarrhea refers to an increase in the frequency of defecation or fecal volume
this increase is often due to increased water content
what does water in the gut result from that can lead to diarrhea
ingested water
water secreted by glands of the GI tract
water secreted or lost directly through the mucosal epithelium
why does diarrhea occur
occurs when there is a mismatch between secretion and absorption of water
what are the 2 types of diarrhea
malabsorptive diarrhea
secretory diarrhea
when does malabsorptive diarrhea occur and what can cause it
occurs when absorption is inadequate to recover secreted water
viral, bacterial, protozoan infections
what happens to villi in malabsorptive diarrhea and which cells are more affected
destruction of the villi and reduced length of villi
shortened villi causes a loss of absorptive intestinal surface area
more affected are mature enterocytes that possess the enzymes of the brush border and transport proteins
when does secretory diarrhea occur
occurs when the rate of intestinal secretion increases and overwhelms the absorptive capacity
how can bacteria cause secretory diarrhea
some pathogenic bacteria produce enterotoxins
enterotoxins bind to enterocytes and stimulate adenylyl cyclase activity and cAMP production
leads to opening of chloride channels – water and other electrolytes follow
what are involved in the defecation reflex and what type of msucle are they
internal sphincter – smooth muscle
external sphincter – striated muscle
what kind of innervation does the internal sphincter have in regards to the defecation reflex
both parasympathetic (sacral spinal segment) and sympathetic (lumbar) innervated
what do parasympathetic and sympathetic stimulation of the internal sphincter case in regards to the defecation reflex
parasympathetic stimulation – relaxation of the sphincter
sympathetic stimulation – constriction of the sphincter
which sphincter is normally tonically contracted (continence)
internal sphincter
what does voluntary constriction of teh external anal sphincter in trained animals do
blocks the reflective activation of defecatin
describe the recto sphincteric reflex
feces accumulate in the rectum (reservoir)
peristaltic movement of feces into the rectum and relaxation of the internal anal sphincter
urge to defecate – contraction of descending colon and rectum as well as increase of intraabdominal pressure
