GI Motility & Regulation Flashcards
Segmentation
Mixing action
Type of motility
No net movement forward
Just back and forth
Peristalsis
Propulsive movements
Moves food from mouth to the anus
Smooth muscle contraction
Extracellular Ca enters cell or Ca is released from SR
Ca+ Calmodulin activates Myosin Light Chain Kinase which then phosphorylates Myosin which can then bind with Actin and contract
Myosin Light Chain Phosphatase removes the phosphate group from myosin, inactivating it
Smooth Muscle in the GI Tract
Unitary (Single Unit) Cell Type
- Held together by adherens junctions - Communicates electrically via gap junctions - Pacemaker cells with spontaneous activity - Intrinsically produces BER & muscle tone without tension (myogenic properties)
Tension comes from neurotransmitters acting on muscle – Role of the ANS
Basic Electrical Rhythm
The smooth muscle of the GI tract has a baseline rhythm of depolarization, but it’s not enough on it’s own to cause contraction. But it is pulsitile and highly regulated. When you get ACh on top of that, then you get enough for regular contractions and changes in tension.
Which parts of the GI tract have what BERs?
Stomach has 3 cycles/min
Duodenum has 12 clycles/min
Want it to be slower at the top
Phases of digestion
Cephalic- neural control
Gastric phase- Neural (early) and hormonal
Intestinal- Mostly hormonal but some neural
Stages of swallowing (and which are voluntary and involuntary)
Stage 1. Voluntary- oral cavity then bolus pushed by tongue to oropharynx
Stage 2. Involuntary- glottis covers trachea, UES relaxes
Stage 3. Involuntary- esophageal peristalsis
Esophagus
Transports and adjusts food temperature
Has upper & lower esophageal sphincters (UES, LES): basically just thickening of smooth muscle
Muscular walls transition from skeletal to smooth muscle
Thick muscular walls produce strong peristatic waves
Release of substances to prepare intestine for food that’s coming
NO, VIP, and ATP are released downstream of the food in the GI tract, warning it to relax and prepare for food so get good peristalsis
Lower esophageal sphincter (and what problems happen when it’s not working properly)
Gatekeeper between the esophagus and the stomach
Failure to relax due to damage/loss of the enteric nerves of the LES wall is called achalasia and can make swallowing difficult
Inappropriate LES relaxation can cause acid reflux and damage to the inner lining of the esophagus
Stomach
Stores food (up to 3-4 L)
HCl disinfects food, denatures and digests proteins and produces intrinsic factor
Resulting chyme in released in peristatic spurts into the duodenum
Pyloric sphincter
At end of the stomach
Regulates food going to duodenum
The pyloric sphincter serves as a sieve (prevents passage of particle >1-2 mm in size)
Intrinsic factor
Important for Vit B12 absorption
Gastric acid
Need low pH and pepsin to digest food
Gastric motility patterns
Mixing and churning with retropulsion and receptive relaxation
A little bit is allowed to empty but most gets kicked back towards the fundus and gets mixed again
Rate of emptying of different foods in the stomach
Carbs- fastest (few hours to leave stomach)
Protein- more slowly
Fat- slowest
Vomiting
Centrally regulated by vomiting center in brain
Steps:
- Salivation (HCO3-) and sensation of nausea (helps offset the acidic stomach contents)
- Reverse peristalsis from upper small intestine to stomach
- Ab muscles contract and UES and LES relax
- Gastric contents are ejected
Small Intestines
Liquification, pH adjustment, release of hormones and addition of enzymes in response to food entering the duodenum
Enzymatic digestion of food occurs in the lumen and the enterocyte surface
Absorption of nutrients, water, and ions occurs by cellular and paracellular pathways
Both peristaltic and segementation motility
Key hormones released by small intestines
CCK
Secretin
GIP
Gastroileal reflex
stomach activity stimulates movement of chyme through the ileocecal sphincter
Mediated by both ENS & external innervation
Gastrocolic reflex
food in stomach stimulates mass movement in colon
Mediated by both ENS & external innervation
Migrating Motor Complex (MMC)
Occurs in the absence of feeding (during fasting) – Housekeeping!
Occurs every 90-100 minutes with 3 phases starting from the stomach & propagating aborally to ileocecal valve. The hormone Motilin appears to initiate but appears to have a neural component as well.
Forces anything that wasn’t previously digested to get cleared out.
Phases of MMC
Phase I
Quiescence occurs for 40-60% of the 90 min duration
Phase II
Motility increases but contractions are irregular
Fails to propel luminal content
Lasts 20-30% of MMC duration
Phase IIII
5-10 minutes of intense contractions
From body of stomach to pylorus to duodenum to ileocecal valve
Pylorus fully opens
Ileocecal valve
normally closed (e.g. to prevent reflux of bacteria from colon into ileum)
Opened by distension of end of ileum (local reflex)
Closed by distension of proximal colon (local reflex)
Large intestines
Serves as reservoir for undigested foodstuff
Main function is to reabsorb water and ions
Elimination of waste is controlled by two sphincters – internal (involuntary) and external (voluntary) anal sphincters
Two types of motility: haustration & mass movement
NO MMC
Types of motility in large intestines and describe them
Haustration: Muscles of the colon wall are contracted intermittently to divide the colon into functional segments known as “haustra”
Mass movements: wave of contraction that usually follows a meal and that moves content over larger distance than with regular peristalsis; colon remains contracted for a while; strong peristaltic waves 1-3 X/day; slow
Defecation
Filling of the rectum causes relaxation of the internal anal sphincter via release of VIP and NO from intrinsic nerves. At same time, the external anal sphincter contracts - rectoanal inhibitory reflex
Defecation (evacuation) occurs when the external anal sphincter is voluntarily relaxed and is enhanced by an increase in intra-abdominal pressure
