GI Motility Flashcards
Where is the GI Tract?
Located between the oesophagus and anal canal
What is the GI tract primed by?
Pacemaker cells which initiate a spreading basal electrical rhythm
-These pacemaker cells are now known as Interstitial Cells of Cajal
How does the GI Tract cause movement of chyme?
When Chyme presses against the membranes, it causes a release of ACh (acetylcholine) and VIP (vasoactive intestinal peptide)
- ACh binds to the smooth muscle cells surrounding the ICC and cause a contraction to the previous section of GI Tract already travelled through
- VIP binds to the ICC and causes a relaxation, increasing the area and pushing the chyme along the tract
How do parasympathetic and sympathetic nerve activation affect the GI Tract?
- Parasympathetic Nerve Activation Promotes increased motility
- Sympathetic Nerve Activation:
- Decreases motility directly via beta-adrenoceptors and indirectly by decreasing ACh release via alpha2-adrenoceptors
What is GI activity also influenced by?
Locally produced chemicals and hormones
What are the stages of gastrointestinal motility?
1) mastication (oropharyngeal phase)
2) deglutition (oesophageal phase)
What is an oesophagus?
A hollow tube separated at either by physiological sphincters
What creates a sphincter?
A zone of elevated pressure (ZEP) from muscle myogenic tone
What is the primary wave of oesophageal perstalsis?
mechanoreceptors in pharynx detect food bolus which initiates peristaltic wave controlled by vagal nerves
- The rate of movement is proportional to the viscosity of the bolus
- Contraction speed is 5cm/sec and whole wave lasts 7-10secs
If food is not projected to stomach, secondary wave of peristalsis is initiated by local vago-vagal reflexes
What is the freuency of the spreading basal ecetrical rhythm (BER)?
3-5min
-Not always associated with the peristaltic wave, only when the underlying smooth muscle is at its most excitable
What is smooth muscle excitability controlled by?
1) myogenic properties of smooth muscle cells
2) activity of intrinsic nerves
3) activity of extrinsic nerves
4) hormones or locally produced chemicals
What occurs on a full stomach?
- 1st hour, peristaltic waves are weak (ZEP is closed)
- After 1st hour, peristaltic waves are more powerful
How is the Control of Gastric Motility achieved?
Neural Control
- Vagal relaxatory fibres induce relaxation in corpus and fundus which are mediated by VIP
- Sympathetic (NA) stimulation inhibits via inhibiton parasympathetic ganglion transmission
- Vagal fibres releasing ACh stimulate gastric motility
Hormonal Control
- Stimulatory hormones are gastrin & motilin
- Inhibitory hormones are gastrin, secretin and CCK
What factors affect the rate at which contents pass into the duodenum?
- Gastric Motility
- Meal Composition
- Meal Volume
- Fragment Size
- Osmolarity
- Acid
- Fat
How does Small intestine motility work?
Intestinal contents are mixed with the secretions of mucosal cells and with the pancreatic juice and bile
What are the two types of muscular contractions in the small intestine?
- Segmenting contractions
- Peristaltic contractions
What are segmenting contractions?
Only occur in circular muscle and moves chyme both directions to increase exposure to mucosal surface
What are peristaltic contractions?
- more longitudinal muscle
- normally over short distances
- after meals
-occasional propulsive movement propogating over long distances called migrating motility complex (MMC)
How is small intestine motility controlled?
1) Myogenic mechanisms
2) Myenteric plexus (local regulation)
3) Extrinsic nerves
4) Local chemicals
How does large intestine motility take place?
Principle functions of large intestine are to reabsorb water and store faeces
- movement mostly slow and non-propulsive (similar to segmentation contractions but more infrequent 2/hr)
- 3 to 4 times per day there are powerful contractions that drives contents to distal portion of large intestine prior to defecation
How long does it take food to reach the terminal ileum?
12 hours
What is defecation?
- rectum usually empty
- arrival of faecal material induces desire to defaecate dues to peristaltic waves in colon
