DRS - smooth muscle and motility Flashcards
Where is smooth muscle found in the GIT?
In most regions of GIT
Where is skeletal muscle found in the GIT?
pharynx
top third of oesophagus
external anal sphincter
What are the two types of smooth muscle in the GIT and where are they?
Phasic: - rapid contraction and relaxation - body of oesophagus, stomach antrum, small and large intestines Tonic: - sustained contractions - sphincters, upper stomach
What are the pacemaker cells of smooth muscle in the GIT?
Interstitial cells of CAJAL
How does smooth muscle contract?
An action potential spreads along the sarcolemma down the T-tubules. Depolarisation by Na+ entry causes L-type calcium channels to open. Calcium will bind to RyR2 on the SR causing them to open to release more calcium from stores in the SR. The calcium binds to calmodulin to activate myosin light chain kinase. This phosphorylates MLCs allowing myosin-actin cross bridges to form.
What does the activation of the myenteric plexus cause?
Increases tonic contraction
Increases intensity of rhythmic contractions
What causes muscles of the GIT to relax?
VIP and NO
What causes muscles of the GIT to contract?
ACh
What are the sensory neurones in the smooth muscle of the GIT stimulated by?
Mechanoreceptors in the submucosal layer detect stretch due to the presence of food
Chemoreceptors in the myenteric plexus detect the chemical composition of food
What is Hirschprung’s disease?
Lack of neuronal ganglionic cells in the ENS plexi
Results in a megacolon
Submucosal and myenteric plexi are absent
What is Chaga’s disease?
Infectious disease of a parasitic nature
significant reduction in number of ganglionic cells in the ENS
What is achalasia?
Dramatic reduction in number of neuronal cells in the lower oesophageal segment
failure of gastrooesophageal shpincter to relx
disease of the oesophagus
Due to lack of inhibitory motor neurones in the lower part of the oesophagus
What is regulation in the fundus regulated by?
The vagovagal reflex
Where does sieving occur and what does it do?
Sieving in the pylorus ensures only small particles can enter the duodenum
Where does grinding occur and what does it do?
The antrum wall is thick because this where grinding occurs
Liquids and small particles are emptied
Large particles retained in bulge of terminal antrum
Grinding involves the churning of a bolus to reduce its size
Functions of the pylorus
- allows regulated emptying of gastric contents
Prevents reflux of duodenal contents into the stomach - Regulates amount of contents going into the small intestine
Phases of gastric emptying
- Cephalic
- sight, smell
- food in mouth
- relaxation of stmach enables it to store larger volumes - Gastric
- food in stomach
- empties at rate proportional to the volume in it
- due to myogenic reflex (stretching of smooth muscle causes contraction), activation of pressure receptors send impulses in nerve plexi and vagus nerve, gastrin release in response to peptides - Intestinal
- inhibitory phase, allows more time to digest and absorb nutrients
- fats - increase CCK
- low pH - secretin
- amino acids - gastrin
- carbohydrates - GIP
3 functions of motor activity in the GIT
- segmental contractions
- mixes chyme with secretions
- non-propulsive - peristaltic contractions
- propulsion in caudal direction - Allows some organs to acts as reservoirs for holding luminal content
- stomach
- allows stomach to function effectively
Types of motility in small intestine
Segmentation
- rings of circular movement, muscles contract and relax
- results in mixing
Peristalsis
- sequential contractions of longitudinal and circular muscle
- results in propulsion of chyme
What is the migrating motor complex
A pattern of electromechanical activity that occurs in GI smooth muscle during periods between meals
Phases of MMCs
- Prolonged quiescent period
- Period of increasing action potential frequency and contractility
- Period of peak electrical and mechanical activity that lasts a few minutes
- Period of declining activity that merges into next quiescent period
What are the functions of MMCs?
- propels particles greater than 2mm from stomach to duodenum
- cleansing of gut
- clears small intestine of residual content
- ensures cells that have been shed are flushed out
- ensures bacteria don’t migrate up into small intestine
- allows non-digestible material to be expelled
Where do MMCs originate and where do they travel?
The stomach and often travel to the distal end of the ileum
What does feeding cause?
Termination of MMCs and initiation of fed motor pattern segmentation and peristalsis
