Electrical Basis of GI Rhythmic Contractions
- Like all muscle, contraction by GI smooth muscle cells is initiated by electrical activity via excitation-contraction Coupling. However, there are some unique mechanisms that generate the characteristic slow, rhythmic contractions performed by GI smooth muscle and these are discussed here.
- The electrical basis for the slow, rhythmic contractions of GI smooth muscle is 'Slow Waves' which refer to the rhythmic undulations of membrane potential observed in GI smooth muscle cells. They occur at characteristic frequencies in different parts of the alimentary tract; for example, cells in the duodenum display about 12 undulations/minute whereas those in the stomach display roughly 3 undulations/minute. At the peaks of the slow waves the membrane potential becomes sufficiently depolarized to initiate action potentials within GI smooth muscle cells. Thus, a recording of the smooth muscle cell membrane potential will display slow undulations with spikes of depolarization, representing action potentials, over-laying their gentle peaks. The resultant action potentials induce an influx of calcium into the smooth muscle cell cytosol resulting in the actual contraction of smooth muscle fibers via mechanisms of excitation-contraction coupling.
- The frequency of slow waves appears to be largely determined by pacemaker cells within the myenteric plexus. Although the autonomic nervous system cannot modulate the frequency of the slow waves, it can control whether the peaks of the slow waves reach the threshold potential required to elicit an action potential.
- Influence of Sympathetic Nervous System
- Sympathetic tone slightly hyperpolarizes the GI smooth muscle cell membrane potential. Thus the slow waves undulate at more negative membrane potentials and thus are less likely to reach the threshold potential required to elicit action potentials and thus contraction. In this way sympathetic activity generally reduces GI motility.
- Influence of Parasympathetic Nervous System
- Parasympathetic tone slightly depolarizes the GI smooth muscle cell membrane potential. Thus the slow waves undulate at higher membrane potentials and thus are more likely to reach the threshold potential required to elicit action potentials and thus contraction. In this way parasympathetic activity generally enhances GI motility.