Slow Waves and Action Potentials Are Forms of Electrical Activity in GI Muscles

Electrical slow waves are omnipresent and responsible for triggering action potentials in some regions, whereas in other regions (e.g., the gastric antrum and large intestinal circular muscle) they represent the only form of electrical activity (Fig. 26.2). They are always present in the small intestine where they decrease in frequency along a gradient from the duodenum to the ileum. In the gastric antrum, the terms slow wave and action potential are used interchangeably for the same electrical event. When action potentials are associated with electrical slow waves, they occur during the plateau phase of the slow wave (see Fig. 26.2).

Action potentials in GI smooth muscle are mediated by changes in calcium and potassium conductances. The depolarization phase of the action potential is produced by an all-or-nothing increase in calcium conductance, with the inward calcium current carried by L-type calcium channels. The opening of potassium channels as the calcium channels are closing at or near the peak of the action potential accounts for the repolarization phase. The L-type calcium channels in GI smooth muscle are essentially the same as those found in cardiac and vascular smooth muscle. Therefore, disordered GI motility may be a adverse effect of treating of cardiovascular disease with drugs that block L-type calcium channels.

Smooth Muscle Slow Waves

Electrical slow waves. In GI muscles, slow waves occur in four phases determined by specific ionic mechanisms. Phase 0: Resting membrane potential; outward potassium current. Phase 1, the rising phase (upstroke depolarization), activates voltage-gated calcium channels and voltage-gated potassium channels. Phase 3, the plateau phase, balances inward calcium current and outward potassium current. Phase 4, the falling phase (repolarization), inactivates voltage-gated calcium channels and activates calcium-gated potassium channels.

Electrical slow waves. In GI muscles, slow waves occur in four phases determined by specific ionic mechanisms. Phase 0: Resting membrane potential; outward potassium current. Phase 1, the rising phase (upstroke depolarization), activates voltage-gated calcium channels and voltage-gated potassium channels. Phase 3, the plateau phase, balances inward calcium current and outward potassium current. Phase 4, the falling phase (repolarization), inactivates voltage-gated calcium channels and activates calcium-gated potassium channels.

Stomach

Small intestine

Colon

Stomach

Small intestine

Colon

Slow Waves Intestine

30 sec

30 sec

Electrical slow-wave frequencies. Slow waves with similar waveforms occur at different frequencies in the stomach, small intestine, and colon.

(Fig. 26.5). The ICCs are interconnected into networks by gap junctions that impart the properties of a functional electrical syncytium to the network. Gap junctions also electrically connect the ICCs to the circular muscle. Electrical current flows from the ICC network across the gap junctions to depolarize the membrane potential of the circular muscle fibers to the threshold for action potential discharge.

Pacemaker networks of ICCs are located surrounding the small intestinal circular muscle at the border with the longitudinal muscle (myenteric border) and at its border with the submucosa. Slow waves generated by the ICC network at the submucosal border spread passively across gap junctions into the bulk of circular muscle, and those at the myenteric border spread passively into both longitudinal and circular muscle. Muscle fibers of the circular muscle are interconnected by gap junctions that transmit the slow-wave electrical current from fiber to fiber throughout the bulk of the muscle.

Electrical Slow-Wave Frequencies in the Stomach, Small Intestine, and Colon. Electrical slow waves with essentially the same waveform occur at different frequencies in the gastric antrum and small and large intestinal circular muscle when recorded with intracellular electrodes (Fig. 26.3). Slow waves occur at 3/min in the antrum, as high as 18/min in the duodenum, and 6 to 10/min in the colon. The maximum contractile frequency of the muscle does not exceed the frequency of the slow waves, but it may occur at a lower frequency because all slow waves may not trigger contractions. The nervous system determines the nature of the contractile response during each slow wave in the integrated functional state of the whole organ.

Electrical Slow Waves Without Action Potentials in the Small Intestine. As a general rule, slow waves in the small intestinal circular muscle trigger action potentials and action potentials trigger contractions. Slow waves are omnipresent in virtually all mammalian species and may or may not be accompanied by action potentials. Contractions do not occur in the absence of action potentials. The electrical slow waves in Figure 26.4 were recorded with an extracellular electrode attached to the serosal surface of the intestine. This method records from many circular muscle fibers. Shallow contractions appearing in the absence of action potentials on the slow waves reflect the responses of a few of the total population of muscle fibers under the electrode (Fig. 26.4A). In this case, the action potential currents from the small number of fibers are too small to be detected by the surface electrode. With this method of recording, the size of an action potential appears larger when larger numbers of the total population of muscle fibers are depolarized to action potential threshold by each slow wave. The amplitude of phasic contractions associated with each electrical slow wave increases in direct relation to the number of muscle fibers recruited to firing threshold by each slow-wave cycle (Fig. 26.4B).

Electrical Slow Waves and Interstitial Cells of Cajal. Interstitial cells of Cajal (ICCs) are the generators of electrical slow waves in the stomach and small and large intestine

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Responses

  • falco
    How intestinal slow waves produced?
    8 years ago
  • Mike Gruenewald
    What are the types of electrical activity of the gastrointestinal smooth muscle?
    2 years ago
  • Yolanda
    Is slow wave a type of action potential?
    2 years ago
  • Mariam
    How do slow waves affect action potential?
    2 years ago
  • Marco
    Are slow waves and iccs different?
    21 days ago

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