Gut motility Flashcards
Describe Ca2+ dependent contraction
[Ca2+]increases in the cytosolic smooth muscle
Ca2+ binds to calbidin = buffer
Activates MLCK
This enzyme phosphorylates light chains of myosin heads
Binds to actin filaments for cross bridge formation
Describe electromechanical coupling
Slow waves and membrane depolarisation opens Ca+ volted gated channels
Ca2+ influx -> contraction
Describe pharmacomechanical coupling
Chemical messengers ( NT + hormones) induce contraction
Binds to GPCRS + activate PLC-> DAG : Receptor operatred Ca2+ channels allowing for Ca2+ influx
Bind to GPCRS + activate IP3 signalling molecule
Ca2+ released from sarcoplasmic reticulum
STIM1 detects Low [Ca2+] in SR
Opening of store operated channels for Ca2+ influx
What is the latch bridge state?
Why is it important?
Muscle remains tension at low ATP expenditure
Slow detachment of dephosphorylated myosin heads
May be due to decreased ATPase activity
Prevents fatigue
What is unitary smooth muscle?
One source of stimulation triggers slow steady contraction to move food through the GI tract
What is multi unit smooth muscle?
Fine contraction Multiple sources of stimulation cause contraction in individual cells
Cells are not electrically coupled
Describe peristalsis
What controls it?
Intrinsic local relfex to move food along the GI tract
Controlled by myenteric plexus
Slow waves and depolarisation cause circular muscles form Contractile rings on oral side of distended segment
Longitudinal muscles relax
What are the interstitial cells of cajal?
Pacemaker cells allowing for slow waves, depolarisation and Ca2+ entry
Ca2+ entry activates ANO1 channel so CL- efflux ( contributes to depolarisation)
RyR + IP3 channels release Ca2+ fro m SR so more ANO1 activation
What is the migrating motor complex?
Occurs in fasting state from stomach -> terminal ileum
Series of contractions to sweep up any residual material or debris to clear GI tract before next meal
Phase 1: = Quiesence, rare contractions
Phase 2: Low grade peristaltic contractions increasing in frequency
Phase 3: Strong intense peristaltic contractions which are elicited by motilin released from M cells
What are long range reflexes?
Describe the 6 main ones?
Neural reflexes coordinating GI activity
Intestino intesinal: One segment of intestine distends and next portion relaxes
Enterogastric: Distension of the bowel inhibits stomach motility and secretion
Ileocolic: Distension of colon prevents ileal emptying
Gastro colic/ dudodeno colic: Food/stretch in the stomach or duodenum increases colonic motlity
Gastroileal: Activity in the stomach promote opening of the ileocoecal sphincter
Describe the 3 stages of deglutition ( swallowing)
Oral phase: Food enters mouth and lubricated with salvia to form a bolus. Hits roof of mouth and activates somatosensory centre to contract/close upper oesophageal sphincter
Pharyngeal phase: Bolus is propelled from the pharynx to oesophagus
Contraction of epiglottis to prevent aspiration
Opening of upper oesophageal sphincter
Oesophageal phase: Bolus is propelled from oesophagus to the stomach via peristalsis. Upper oesophageal sphincter closes to prevent relflux
What is the gastric accomodation reflex?
Stomach fills, increases compliance to prevent pressure building
Describe the churning actions of the stomach
Slow waves allow for peristalsis so food is propelled towards the antrum
Pyloric sphincter contraction
Secondary peristaltic waves grind food smaller
Retropulsion : Increased pressure in stomach pushes food back into the corpus
Chyme released slowly into the duodenum
Describe intestinal motility
Peristalsis
Segmentation - mixing of contents with GI secretions to bring in close contact with the mucosa
Segmentation: Alternating contractions forming a contractile ring separated by a relaxed area
Describe colonic motility
Peristalsis - poor for maximal absorption
Haustration: Presence of segmented bowel haustra to allow mixing
