7. Smooth Muscle Pacemakers

Question 1

What is the difference between a smooth muscle slow wave and pacemaker potential?

Answer 1

(both of these are classed as single unit smooth muscle)

SLOW WAVE POTENTIALS:

• wave like fluctuations in membrane potential; cyclically bring membrane potential closer or further away from threshold
• physiological cause - cyclical variation in rate at which NaK pump pushes Na+ out of SM cell
• not able to cause an AP individually, however when combined with other factors are able to induce regular (cyclical) AP’s
  
  • ‘other factors’ - mechanical/ nervous stimulation, hormonal input
• AKA Basic Electrical Rhythm

PACEMAKER POTENTIALS:

• Able to induce AP’s; and thus cause subsequent muscle contraction
• Physiological cause - due to changes in ion channel permeabilities

Question 2

Describe smooth muscle motility in the small intestine.

Answer 2

• pacesetter cells initiating action potentials occur along the whole length of the small intestine, and their activity declines along the length of the small intestine (therefor those with the highest freq of 11-13/min are in duodenum, and the slowest are in the terminal ileum 8-9/min)
• Peristalsis, a forward-propelling movement sue to the progressive contraction of the circular muscles and relaxation of the longitudinal muscles
• Pacemaker potentials of smooth muscle cells in ther muscularis externa determine the time and periodicity of these movements
• Segmentation: the most common movment, involving mixing cause by closely spaced alternate contraction and relaxation of the circular muscle layer. Local mixing of chyme with intestinal juices.
• Enteric reflexes: respond to presnece of chyme to increase intestinal motility. Segmentation dpends on distention which sends impulses to the enteric plexus and CNS.

Question 3

Describe the migrating myo-electric complex (MMC) in humans

Answer 3

• When fasting (several hours after a meal)
• Bursts of intense electrical and contractile activity, seperated by periods of quiescence
• Propagate from duodenum and progress to terminal ileum
• Repeat every 75-90 mins

Phase 1: quiescent phase: slow waves, few AP’s, few contractions
Phase 2: Irregular AP’s and contraction. gradual increase in intensity and frequency.
Phase 3: Intense electrical/contractual activity which lasts for 3-6 mins
Phase 4: Rapid decline in activity which eventually blends into phase 1

Question 4

What is the function of the MMC (migrating myoelectric complex)?

Answer 4

• During fasting small intestine motility follows a different pattern. There are bursts of intense electrical thus contractile activity then nothingness. The cycle repeats every 75 to 90 minutes (which is the time for 1 wave to pass). 4 phases:
  1. Quiescent phase: Slow wave, few action potential, few contractions
  2. Irregular AP phase: Gradual increase in frequency and intensity
  3. Intense phase: Lasts for 3 to 6 minutes and obviously has the most intense and frequent AP’s
  4. Rapid decrease phase: Rapid decrease of all activity.

The collective function is to sweep the bowel clean, and inhibits the movement of large intestine bacteria into the small intestine.
Motilin is the hormone responsible for MMC
Eating abolishes the MMC