134b - GI Motility

Question 1

Which cells are the electrical pacemakers for smooth muscle cells?

Answer 1

Interstitial cells of Cajal

• Produce slow waves in GI muscle
  
  • Sub-threshold
  • No contraction unless there is an additional stimulus

Question 2

A patient suffers a spinal cord injury at C7

Will they be able to digest food?

Answer 2

Yes

• The enteric NS receives input from the autonomic NS, but it can function (mostly) normally even without the autonomic NS

Question 3

A region of the intestine contracts weakly upon stimulation of its extrinsic nerves. Distention of the region elicits a peristaltic reflex, but with weak contractions. Slow wave activity is absent. Taken together, these findings suggest a disorder of which of the following?

a. enteric nerves
b. parasympathetic nerves
c. release of motilin
d. smooth muscle cells
e. sympathetic nerves

Answer 3

d. smooth muscle cells
* Peristaltic reflex is intact => nerves, motilin are not the problem

Question 4

When the stomach senses stretch, what happens in the:

Upper stomach?

Lower stomach?

Answer 4

• Upper stomach: receptive relaxation
  
  • Stretch -> Vagal and enteric neurons acitvated -> NO release

[Similar to how the LES is myogenic controlled by sensing distension/stretch, so releases NO to relax itself; this also releases and takes in NO to relax the upper stomach (cardia and fundus area)

• Lower stomach: more contractions
  
  • Slow wave contractions increase in amplitude, resulting in action potentials
  • Increases mixing, pushes a small amount into the duodenum at a time
    
    • Depends on pyloric sphincter tone

Question 5

What is the primary stimulus for defecation?

Which parts of the nervous system control the defecation reflex?

Answer 5

Primary stimulus = distension

Which causes:

• Enteric NS
  
  • Peristalsis
  • Relaxation of the internal sphincter
• Somatic NS
  
  • Relaxation of the external sphincter (voluntary)
  • Sensation of the urge to defecate

Question 6

What is responsible for the basal tone of the lower esophageal sphincter?

What changes must happen to allow us to swallow?

Answer 6

Basol tone is under myogenic control (smooth muscle)

When we swallow, nitric oxide (NO) is released, causing the LES to relax

Too much NO -> Transient LES opening -> reflux

Question 7

In a patient in whom resting tone of the internal anal sphincter is normal, distention of the rectum induces normal relaxation of the internal anal sphincter, no change in tone of the external anal sphincter, and no sensation of the urge to defecate. These findings are consistent with the finding of damage to which of the following?

a. enteric nerves
b. internal anal sphincter
c. vagus nerve
d. spinal cord
e. transverse colon

Answer 7

d. spinal cord
* Sensation of the urge to defecate adn the external anal sphincter are controlled by spinal nerves

Question 8

What is the most common movement of the GI tract in the fed state?

Answer 8

Segmentation

Purpose = mixing

Much more common than peristalsis

Note: both segmentation and peristalsis are activated by distension

Strong distension -> peristalsis

Moderate distension -> mixing

Question 9

What is the major stimulus during each phase of digestion?

• Cephalic
• Gastric
• Intestinal
• Inter-digestive

Answer 9

• Cephalic: Neural via CNX (seeing, smelling, thinking of food)
• Gastric: Distension
• Intestinal: Chyme entering the SI
• Inter-digestive: Migrating motor complex

Question 10

Which part of the nervous system controls blood flow to the gut?

Answer 10

Sympathetic NS

• Increased sympathetic tone -> decreased blood flow to the gut
  
  • This is why you get cramps if you run right after eating

Question 11

What is the major movement pattern of the large intestine?

Answer 11

Haustral contractions

• Like segmentation, but contents move forward AND back instead of just being churned around in the same place
• This allows more fluid to be absorbed