L3 GI Motility

Overview

This lecture covers how the GI tract moves. It starts from the smooth muscle basis of gut motility (structure of the gut wall, muscle layers, and the slow-wave electrical activity that drives spontaneous contraction), then builds up the two motility regimes: the interdigestive (fasting) pattern built around the migrating motor complex, and the fed-state patterns of peristalsis and segmentation. It finishes by walking motility region by region from mouth to colon, ending with a clinical example (ileostomy after colectomy).

GI Muscle Type and Gut Wall Structure

• The GI tract’s core functions: it moves, secretes, digests, and absorbs.
• The gut is almost exclusively smooth muscle, with striated muscle only at the pharynx and anus, where it provides voluntary control.
• Gut wall layers, from lumen outward: epithelial cells (with endocrine and receptor cells) -> lamina propria -> muscularis mucosae -> submucosal plexus -> circular muscle -> myenteric plexus -> longitudinal muscle -> serosa.
  • The submucosal plexus sits between the muscularis mucosae and the circular muscle.
  • The myenteric plexus sits between the circular and longitudinal muscle layers.
• Function of each muscle layer:
  • Muscularis mucosae: increases surface area.
  • Circular muscle: reduces luminal diameter.
  • Longitudinal muscle: produces shortening.
  • Together they generate motility programmes, e.g. peristalsis.

A histology slide shows three columns (A, B, C) of muscle micrographs, each with a longitudinal and a cross-sectional view, but the panels are not labelled with muscle-type names on the slide. Given the preceding slide's statement that the GIT is almost exclusively smooth muscle with striated muscle only at the pharynx/anus, this is likely a skeletal/cardiac/smooth muscle histology comparison, but that identification is not stated on the slide and is not recorded as fact.

Electrical and Mechanical Basis of Contraction

• Smooth muscle shows spontaneous activity from pacemaker cells.
• Sequence: slow waves -> reach threshold -> action potential -> contraction.
  • On the slow-wave/tension graph, action potential bursts occur where slow waves cross threshold, and phasic contractions in the tension trace coincide with those AP bursts.
• Activity is modified and coordinated by stretch, nerves, hormones, and reflex loops (e.g. the enterogastric reflex).
• Some GI activities are voluntary and complex, and may involve skeletal muscle in addition to smooth muscle.

Motility: Definition and Adaptability

• Motility moves food through the GI tract, causes mechanical breakdown, mixing, and contact (with the mucosa), via contraction/relaxation of smooth muscle.
• Motility must adapt to:
  • presence or absence of food
  • the type and amount of food
  • absorptive capacity
• Two motility regimes:
  • Between meals: the interdigestive motor cycle.
  • In the presence of food: peristalsis and segmentation.

Interdigestive Motility: The Migrating Motor Complex

• The interdigestive motor cycle is a “housekeeping” pattern, coordinated along the gut.
• The migrating motor complex (MMC):
  • Is coordinated, propagating from stomach -> small intestine -> large intestine.
  • Begins around 4-5 hours after a meal and repeats roughly every 2 hours.
  • Has three periods: inactivity, intermittent activity, and intense activity.
• Contractile force recordings along the jejunum, mid small bowel, and ileum show periodic bursts of activity during fasting, switching to continuous, denser contractile activity after feeding.
• Function: clears cells, food residue, and secretions from the gut.
• Controlled by the enteric nervous system and the hormone motilin.

Peristalsis and Segmentation

• Peristalsis:
  • Function: propulsion (with or without some mixing).
  • Occurs in the oesophagus, stomach, small intestine, and large intestine.
  • Mechanism: proximal circular muscle contracts (squeeze) while the segment just distal to the bolus relaxes, propagating the bolus forward along the tube.
  • Fluoroscopic recordings in the jejunum confirm forward movement of a bolus/marker consistent with this mechanism.
• Segmentation:
  • Function: mixing.
  • Occurs in the small and large intestine.
  • Mechanism: circular muscle contracts and relaxes at intervals along the tube, producing mixing and contact with the mucosa; contents move back and forth within a segment as the pattern of constrictions shifts location, rather than progressing along the tube as in peristalsis.
  • Fluoroscopic recordings in the jejunum show segmented regions consistent with this back-and-forth mixing.

Region-Specific Motility

• Mouth: mastication (chewing) reduces particle size, mixes food, and allows tasting; it is voluntary but with reflex control.
• Oesophagus:
  • Driven by the swallowing reflex, then oesophageal peristalsis.
  • Swallowing sequence: (A) at rest; (B) soft palate elevates, involving the epiglottis and upper oesophageal sphincter; (C) epiglottis lowers further to cover the airway; (D) the bolus passes into the pharynx.
  • Clinical points noted: problems can affect solids and/or liquids differently; reflux relates to the lower oesophageal sphincter.
• Stomach:
  • Functions: storage, mixing, and mechanical breakdown.
  • The proximal stomach is specialised for storage; the distal stomach for peristalsis, which predominates over retropulsion.
  • Gastric peristaltic wave: a contraction wave moves from the body of the stomach toward the pylorus, pushing particles ahead of it, with some partial retrograde flow at the pylorus.
  • Retropulsion: particles reaching the pylorus are forced backward (retrograde) into the antrum instead of passing through, giving net retrograde movement of that content.
  • Solids empty from the stomach more slowly than liquids.
  • Gastric motility/secretion is organised into cephalic, gastric, and intestinal phases.
  • Vomiting is listed as a stomach-related function/event in this section.
• Small intestine: mixes, absorbs, and propels contents (via peristalsis); also shows segmentation and pendular movements; regulated by hormonal, nervous, and reflex mechanisms.
• Large intestine/colon:
  • Functions: storage and water absorption.
  • Shows slow segmentation and mass movements.
  • Mass movements drive defaecation, which involves both reflex responses and voluntary control, coordinated via the brain, brain stem, and spinal cord.

Clinical correlate: after a colectomy (removal of the colon), an ileostomy may be formed, an opening from the ileum (small intestine) brought to a stoma site on the lower abdominal wall, bypassing the large intestine entirely.

Self-test

1. List the four core functions of the GI tract.
2. What type of muscle makes up almost all of the GI tract, and where does striated muscle occur instead?
3. List the layers of the gut wall in order from the epithelium to the serosa, and state where the submucosal and myenteric plexuses sit relative to the muscle layers.
4. Describe the function of each of the three GI smooth muscle layers (muscularis mucosae, circular muscle, longitudinal muscle).
5. Describe the sequence of events, from pacemaker activity to contraction, that underlies spontaneous smooth muscle activity in the gut.
6. List the factors that modify or coordinate GI smooth muscle activity.
7. List the three factors that GI motility must adapt to.
8. Distinguish the interdigestive motor cycle from the motility patterns seen in the presence of food.
9. Describe the migrating motor complex: its direction of propagation, its timing relative to a meal, and its three periods.
10. What controls the migrating motor complex, and what is its main function?
11. Distinguish peristalsis from segmentation in terms of function, gut regions involved, and mechanism.
12. Describe the mechanism of peristaltic propulsion of a bolus.
13. Describe the sequence of events during swallowing, from the resting position to the bolus entering the pharynx.
14. Distinguish the motility roles of the proximal and distal stomach.
15. Distinguish a gastric peristaltic wave from retropulsion.
16. Describe the motility patterns of the small intestine and the large intestine, and explain what triggers and controls defaecation.
17. A patient has had a colectomy. What procedure is typically performed as a result, and what does it involve?

Answers

Reveal answers

1. Moves, secretes, digests, and absorbs.
2. Almost exclusively smooth muscle; striated muscle occurs only at the pharynx and anus, where it provides voluntary control.
3. Epithelial cells (with endocrine/receptor cells) -> lamina propria -> muscularis mucosae -> submucosal plexus -> circular muscle -> myenteric plexus -> longitudinal muscle -> serosa. The submucosal plexus lies between the muscularis mucosae and the circular muscle; the myenteric plexus lies between the circular and longitudinal muscle layers.
4. Muscularis mucosae increases surface area; circular muscle reduces luminal diameter; longitudinal muscle produces shortening; together they generate motility programmes such as peristalsis.
5. Spontaneous pacemaker activity generates slow waves in the membrane potential; when a slow wave reaches threshold it triggers a burst of action potentials, which produces a phasic contraction (rise in tension).
6. Stretch, nerves, hormones, and reflex loops (e.g. the enterogastric reflex).
7. The presence or absence of food, the type and amount of food, and absorptive capacity.
8. The interdigestive motor cycle occurs between meals and is a coordinated “housekeeping” pattern (the migrating motor complex). In the presence of food, motility instead shows peristalsis (propulsion) and segmentation (mixing).
9. The MMC propagates in a coordinated way from the stomach to the small intestine to the large intestine, begins about 4-5 hours after a meal, and repeats roughly every 2 hours. It has three periods: inactivity, intermittent activity, and intense activity.
10. It is controlled by the enteric nervous system and the hormone motilin; its function is housekeeping, clearing cells, food residue, and secretions from the gut.
11. Peristalsis propels content (± mixing) via proximal contraction and distal relaxation that propagates along the oesophagus, stomach, small intestine, and large intestine. Segmentation mixes content in the small and large intestine via circular muscle contracting/relaxing at intervals, moving contents back and forth as the constriction pattern shifts location rather than progressing along the tube.
12. Circular muscle contracts (squeezes) just proximal to the bolus while the segment just distal to it relaxes; this pattern propagates along the tube, driving the bolus forward.
13. At rest the airway is open; the soft palate then elevates, involving the epiglottis and upper oesophageal sphincter; the epiglottis lowers further to cover the airway; the bolus then passes into the pharynx.
14. The proximal stomach is specialised for storage; the distal stomach mainly produces peristalsis, which predominates over retropulsion.
15. A gastric peristaltic wave moves from the body of the stomach toward the pylorus, pushing particles forward (with some partial retrograde flow at the pylorus). Retropulsion instead forces particles at the pylorus backward into the antrum rather than letting them pass through, giving net retrograde movement.
16. The small intestine mixes, absorbs, and propels contents via peristalsis, and also shows segmentation and pendular movements, regulated by hormonal, nervous, and reflex mechanisms. The large intestine stores content, absorbs water, and shows slow segmentation and mass movements; mass movements drive defaecation, which involves both reflex responses and voluntary control coordinated via the brain, brain stem, and spinal cord.
17. An ileostomy is typically formed: the ileum (small intestine) is brought to a stoma opening on the lower abdominal wall, bypassing the large intestine.