15. GI Tract Introduction Flashcards
What are the layers of the gut tract wall from outer to inner?
- serosa
- longitudinal smooth muscle
- myenteric plexus
- circular smooth muscles
- meissner’s corpuscle
- submucosa
- mucosa
What is the serosa layer?
- outer layer of connective tissue and simple squamous epithelium
- continuous with the mesentery (pathway for blood to gut tube)
Where is the serosa layer missing in the GI tract?
- esophagus, where it is replaced with adventitia to connect to abd wall
What is the submucosa?
- lamina propria but thicker
2. incorporates the blood vessels and submucosal plexus
What layers does the mucosa include in order to form a continuous sheet that lines the entire GI tract?
- lamina propria: loose CT with nerve, blood vessel, glands
2. muscularis mucosa: thin smooth muscles, form mucosal ridge
What type of cells help form the mucosa layer of gut?
- simple columnar epithelium with goblet cells
2. line entire gut tract
What forms the enteric nervous system?
- myenteric and meissners plexuses
2. neurons are supported by intrinsic glial cells
How does the enteric nervous system operate?
- intrinsic regulation and sensory reflexes
- parasympathetic increase GI activity
- sympathetic decrease GI activity
Where are myenteric plexus and what does it control?
- in between longitudinal and smooth muscles
- tonic and rhythmic contraction of smooth intestinal muscles
- excitatory (Ach, NO)
- inhibitory (VIP)
Activation of the myenteric plexus will lead to what effects?
- increased tonic contraction of gut wall
- more intense rhythmic contractions
- increased rate of contractions
- increased velocity
What does the Meissner’s plexus mostly control?
- GI secretion, absorption, and local blood flow
2. mostly local effects not long distant.
What is peristalsis?
- propulsion of food along the gut tract forcing bolus from one direction to the other
How does peristalsis work?
- circular muscles contract to prevent backflow
- longitudinal muscle will propel the bolus forward, causing distension of the tube
- distension will cause contraction of next circular ring, and the next segment of longitudinal muscle
What is receptive relaxation?
- vagovagal reflex, which activated by food and casues distension of the proximal region of stomach
- CCK is then released causing distal distention of stomach
What is the role of the distal stomach with mixing food and beginning digestion?
- contraction of distal region mixes food with gastric secretions
- contraction seals off distal end and propels food back into the “body” of stomach for mixing
What will increase gastric contractions, to improve mixing?
- stimulation of vagus nerve (parasympathetic)
Stimulation of what nervous system leads to decreased gastric mixing?
- stimulation of sympathetic nervous system, and inhibition of signals to vagus nerve
When is gastric emptying the fastest?
with isotonic solutions, occur with contraction of distal end of stomach to duodenum
What will slow the emptying of the stomach contents into the duodenum?
- hypo-/hypertonic solutions
2. fat (increases CCK that slows emptying)
Why would acid in the duodenum slow gastric emptying?
- the duodenum is a basic environment, that can be damaged by too low a pH, therefore gastric secretions will be reduced to protect the duodenal lining
What does food in the stomach continue to activate?
- the gastroileal reflex
2. mediated by extrinsic ANS and gastrin
What increases mixing and digestion in the small intestine?
- vagus nerve (parasympathetic) increases digestion
2. sympathetic NS, inhibits vagus nerve signaling. decreasing digestion
What is the role of enterochromaffin cells?
- sense food in intestine
- release serotonin
- serotonin activate primary afferent neurons for peristalsis
What system is in control of the peristalsis in the small intestine?
- enteric nervous system
What causes the ileocecal sphincter to close, and prevent fecal backflow into the small intestine?
- distension of the proximal colon
2. site of mass colonic water absorption
What creates haustra?
segmented contraction in proximal colon
What occurs in the distal colon?
- semisolid, slow moving fecal material
2. fecal material propelled into rectum in mass movement
Defecation
- internal anal sphincter must relax allowing movement into rectum, and urge begins with 25% filling of rectum
What prevents defecation?
external anal sphincter under voluntary control
What maneuver will increase the ability to defecate?
- valsalva maneuver which increase intra-abdominal pressure
What is the gastrocolic reflex?
- stretching of stomach increases parasympathetic stimulation and frequency of mass movements along the GI tract.
- CCK and gastrin react but at slower rate.
What are slow waves?
- slow oscillating potentials inherent to smooth muscles of GI tract
Where do slow waves originate from?
- interstitial Cajal cells
- can vary intensity from 5-15mV
- frequency from 3-12/min
What cause muscle contraction of the smooth muscles in the GI tract?
- spike potentials
What do spike potentials generate?
- open Ca channels, and Na channels in GI smooth muscles
2. have a longer time of activation than normal action potential
What is a spike potential?
- action potential of smooth muscle GI
- frequency= the amplitude of the signal
- last 10-40x longer than normal action potential
What factors cause depolarization of GI smooth muscles cells?
- muscle stretching
- Ach stimulation (parasympathetic)
- GI specific hormone stimulation
What factors cause hyperpolarization of smooth muscles of GI tract?
- norepi/epi
2. sympathetic nerve stimulation
What cause tonic contractions of GI tract?
- continuous repetitive spike potential
- hormones
- continuous entry of Ca ion
What are the major three different types of GI reflexes?
- reflex integrated within gut wall enteric nervous system
- from gut to prevertebral sympathetic ganglia and back to GI
- from gut to spinal cord/brainstem and back to GI
What are the different type of reflexes that are associated with the enteric nervous system?
- GI secretions
- peristalsis
- mixing contractions
- local inhibitory effects
What reflexes send signals from the gut to the prevertebral sympathetic ganglia and back to the GI?
- gastrocolic (evacuation of colon)
- enterogastric (inhibit stomach motility/secretion)
- colonoileal (empty ileal content into colon)
