4.2 Lower GIT

Question 1

Primary functions of the small intestine

Answer 1

Absorption and digestion

Question 2

Segments of the small bowel

Answer 2

1. Mucosa
2. Submucosa
3. Muscularis externa
4. Serosa or adventitia

Question 3

Extrinsic control of intestinal function?

Answer 3

Para: excitatory = contraction
Symp: inhibitory = relaxation

Exception to the rule: GI sphincters (opposite effects)

Question 4

Intrinsic control of intestinal function

Answer 4

By the Enteric Nervous System

1. Myenteric Plexus - controls mainly GI movements
2. Submucosal Plexus - controls mainly secretion and local blood flow in the GI

Question 5

T or F. Only the brain controls the gut behavior independently.

Answer 5

NO. The ENS can control gut behavior independent of the brain since it is equipped with its own senses and reflex.

Question 6

The smooth muscle of the GI is excited by?

Answer 6

Almost continuous slow intrinsic electrical activity along the membranes of the muscle fibers

2 basic types of electrical waves/membrane potentials

1. Slow waves
   - aka basic electrical rhythm or pacesetter potential
   - not an AP
   - appear to be caused by the complex interactions among smooth muscle cells and interstitial cells of Cajal (latter are the electrical pacemakers for the smooth muscle)
2. Spikes
   - True AP for GIT smooth ms
   - Significant amt of Ca2+ enter the membrane to cause contractions
   - may be caused by the intrinsic motor output from the ENS to the elec pacemakers (interstitial cells of Cajal)
   - likely modulated by extrinsic motor supply

Question 7

Motor pattern in the interdigestive state?

Answer 7

1. Interdigestive (fasting) state
   -in between meals
   -distinct pattern of electromechanical activity: Migrating Myoelectric Complex (MMC), the basis for motility of small bowel during fasting
   -starts in the stomach antrum until the distal ileum
   -3 phases:
   Phase 1: Quiescent period, rep by slow waves, little to no mechanical activity, intestinal transport is limited
   Phase 2: Irregular contractile activity, intermittent appearance of spikes superimposed on slow waves, has bile secretion and mucus discharge
   Phase 3: regular contractions with varied amplitudes; contractions are induced by Motilin

-MMC occurs at a rate of 5 cm/min; at inervals of abt 90-120 min
-Frequency of slow waves and velocity of aboral migration declines from the proximal to the distal small bowels
-

Question 8

Motility during postprandial state

Answer 8

Postprandial (Fed) state

• MMC inhibited
• Pattern of motility: Mix of segmentation and propulsive contractions
• Portion of small intestine becomes distended with chyme -> elicits localized concentric contractions

Question 9

Peristaltic rush

Answer 9

> Characterized by powerful and rapid peristalsis due to intense irritation of intestinal mucosa such as in diarrhea

> Initiated partly by ANS and brain stem and partly by enhancement of myenteric plexus reflexes w/in the gut wall