L41. GI Tract Motility

Question 1

What is the smooth muscle like in the GI tract?

Answer 1

• GI tract (visceral or enteric) smooth muscle is different from other types of smooth muscle
• GI smooth muscle cells are connected:
• Electrically via gap junctions
• Physically via adherens junctions
• Because of electrical and physical connections, cells act together as a single unit. GI tract smooth muscle is therefore called unitary smooth muscle

Two major contraction patterns are seen:
1. Phasic contractions, contract/relax over seconds (e.g. peristalsis)
2. Tonic contractions, sustained from minutes to hours (e.g. sphincters)

Question 2

Tell me more about phasic contractions?

Answer 2

• Phasic contractions called slow wave contractions are triggered by depolarisation and repolarisation of the smooth muscle
• Cyclic depolarisation and repolarisation in GI tract smooth muscle is called the basic electrical rhythm (BER) and is caused by Pacemaker cells called Interstitial cells of Cajal

How does the BER cause slow wave contractions?
- When the membrane potentials of muscle cells depolarize and reach threshold action potentials (APs) occur
- The APs trigger contractions (excitation contraction coupling)

Question 3

What are Interstitial cells Cajal and how do they work?

Answer 3

Interstitial cells of Cajal (ICC) are the pacemakers of the GI tract which generate cyclic slow waves of depolarisation and repolarisation

Cyclic changes in:
- The activity of Na+/K+-ATPase changes cyclically
- Membrane K+ conductance (opening and closing K+ channels)

Locations:
- Stomach and small intestine = boundaries between the smooth muscle layers and the myenteric plexus
- In the large intestine = boundary between the muscularis and submucosal layer

Interstitial cells of Cajal have multiple branching processes extend to surrounding smooth muscle cells. The branching processes are electrically linked to muscle cells by gap junctions