GI Motility

Question 1

What causes most GI contractions (by way of exciting spike potentials)? What is the “electrical pacemaker” believed to control these contractions?

Answer 1

Slow waves. Not actually action potentials, but undulating changes in resting membrane potential. Interstitial cells of Cajal are thought to be the pacemaker.

Question 2

What are spike potentials?

Answer 2

True action potentials that occur when resting membrane potential is more than -40mV, and cause muscle contraction.
Spike potentials occur at the peak of slow waves and is when a significant amount of Ca ions enter the fiber causing contraction.

Question 3

What is the main difference between how action potentials in nerve fibers for GI smooth muscle are generated?

Answer 3

Nerve fibers: Action potentials caused by rapid entry of sodium ions through Na channels.
GI: allow large numbers of Ca Ions with small number Na ions (Ca-Na channels). Channels are slow to opn and close accounting for long duration of action potentials. Ca entry also causes muscle fibers to contract

Question 4

What happens during depolarization of the membrane? What causes it?

Answer 4

The potential becomes less negative, and muscle fibers are more excitable.

Stretching of the muscle, stimulation by Ach released from parasympathetic nerves, stimulation by several GI hormones.

Question 5

What happens during hyperpolarization of the membrane? What causes it?

Answer 5

The potential becomes more negative, and muscle fibers less excitable.

Effect of norepinephrine or epinephrine and stimulation of sympathetic nerves that secrete mainly norepi.

Question 6

Which ions enter smooth muscle fibers during slow waves?

Answer 6

Only sodium; not calcium. Therefore slow waves do not cause muscle contraction.

Question 7

What are the two types of movements that occur in the GI tract?

Answer 7

Propulsive movements to move food along tract (mainly peristalsis) and mixing movements to keep contents thoroughly mixed.