Advan. Physiol. Edu. 29: 131-132, 2005;
doi:10.1152/advan.00011.2005
1043-4046/05 $8.00
ADV PHYSIOL EDUC 29:131-132, 2005
© 2005 American Physiological Society
ILLUMINATIONS
STUDENT DEMONSTRATION OF RELATIONSHIP BETWEEN INTESTINAL SLOW WAVES AND PHASIC CONTRACTIONS
Kathy J. LePard, PhD
Department of Physiology
Midwestern University
555 W. 31st Street
Downers Grove, IL 60515
E-mail: klepar{at}midwestern.edu
The rate of intestinal phasic contractions is determined by the frequency of the slow wave or basic electrical rhythm, an alternating depolarization and hypopolarization (i.e., oscillation) of the membrane potential of smooth muscle cells. Interstitial cells of Cajal, specialized cells located within the wall of the gastrointestinal tract, network together and communicate with smooth muscle cells to establish the slow-wave frequency.
Excitatory and inhibitory neurotransmitters increase and decrease the motility of the intestine. When the local concentration of excitatory neurotransmitters increases, smooth muscle depolarizes past threshold during the peak of the slow wave, resulting in action potentials followed by contraction. During the trough of the slow wave, the membrane potential remains below threshold and an action potential does not occur. As a result, the smooth muscle returns to basal tone. This establishes a phasic contraction of the smooth muscle. Alternately, when the local concentration of inhibitory neurotransmitters increases, the membrane potential at the peak of the slow wave is too far below threshold for an action potential to occur and the smooth muscle relaxes.
Students frequently confuse the frequency of the slow wave and the frequency of phasic contractions. Therefore, I have developed a student demonstration to help differentiate between them. Two student volunteers come to the front of the classroom. One student must have "strong thighs" and the other student a "sense of rhythm." The student with the strong thighs represents the slow wave. The second student with a sense of rhythm represents the phasic contractions or motility of the smooth muscle. Physical positions for the students representing the slow wave or motility are shown in Fig. 1, A and B, respectively.
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Fig. 1. A: positions for student representing slow wave. B: positions for student representing motility.
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The student representing the slow wave alternately squats and stands at a slow frequency with hands at the side (Fig 2A). This student never stops squatting and standing throughout the entire demonstration because a slow wave always occurs at the same frequency. As the person representing the slow wave continues, the student representing the motility of the smooth muscle remains standing still and upright (basal tone, no contraction or relaxation) (Fig. 2B).
Phasic contractions occur after an action potential is generated in smooth muscle due to excitatory neurotransmitter release. During the peak of the slow wave, the student representing the slow wave stands on tiptoes, lifts the hands over the head, and waves the hands to represent that the threshold was crossed and an action potential was generated in the smooth muscle (Fig. 3A). After the action potential occurs, the student representing the motility squats to represent the contraction or shortening of the smooth muscle (Fig. 3B). As the student representing the slow wave squats to represent the trough of the slow wave, the hands return to the side. Then the person representing the motility stands up, representing a return to basal tone.
As the students continue this pattern at a steady rate, it is pointed out that the maximal frequency of phasic contractions cannot be greater than the maximal frequency of the slow wave. The maximal frequency of phasic contractions can be less than the maximal frequency of the slow wave because, even in the continued presence of excitatory neurotransmitters, not every peak of the slow wave may cross the threshold to generate an action potential.
Smooth muscle relaxes below basal tone when the membrane potential is more negative. After sympathetic nerves to the gut are stimulated, norepinephrine is released and smooth muscle relaxes. To demonstrate this, the student representing the slow wave squats lower than before and does not stand up straight, showing that the oscillation in membrane potential is still occurring, but is farther from threshold (Fig 4A). During this time, the student representing motility stands on the toes to be taller, representing a relaxation or lengthening of the muscle (Fig 4B).
This demonstration has been used for pharmacy, physical therapy, physician assistant, and medical students with positive comments on evaluation. The humor that accompanies the demonstration often assists students in remembering the information during examination.
Copyright © 2005 by the American Physiological Society.
