Smooth muscle tissue in the wall of the gastrointestinal (GI) tract differs from other types of muscle tissue in a number of important ways. For example, smooth muscle is slow compared to skeletal muscle. Because the sliding of myofilaments proceeds at a much slower pace than in skeletal muscle, smooth muscle often exhibits a prolonged contraction phase. Because the same amount of energy is used for slow contraction as for fast contraction, GI muscle can sustain tension for long periods without fatigue—exactly what is needed for the long process of digestion.

Another unique functional characteristic of smooth muscle is its ability to maintain basal tone. The basal tone is a continuous state of minimal contraction. Continuous tension is maintained in cells that have enough calcium ions free in the sarcoplasm to generate the contraction response. The force of contraction can increase above the basal tone by way of the effects of an action potential, which causes rapid influx of extracellular calcium.

Gastrointestinal sphincter muscles, as you recall from the previous chapter, are ringlike formations of smooth muscle in the GI wall that act as gateways or valves that regulate movement of chyme from one part of the tract to the next part. Sphincter muscles generally have a higher basal tone than does surrounding smooth muscle—thus constricting the lumen and “keeping the gate closed.” Usually, stimuli in a section preceding the sphincter trigger a decrease in basal tone to allow material to pass through. Some of these reflexes are discussed later in this chapter.

Most of the smooth muscles in the GI wall are electrically coupled by gap junctions—so-called single-unit muscles. Such electrical coupling allows for intrinsic control of smooth muscle contraction, as in cardiac muscle. Smooth muscle fibers exhibit an intrinsic, rhythmic fluctuation in membrane voltage that is sometimes called basic electrical rhythm (BER). As part A of the figure shows, the peaks of these slow waves sometimes reach the threshold potential—thus triggering bursts of action potentials. Because the fibers are electrically coupled, action potentials generated in one fiber spread rapidly to many surrounding fibers. This phenomenon is called pacemaker activity (as we have already seen in the heart with cardiac muscle fibers). Part A of the figure shows the effect of pacemaker activity on the force generated in a local area of the GI muscle. Between action potentials, the muscle cells exhibit the basal tone—but when stimulated by pacemaker potentials, the contractile force increases dramatically. The end result of pacemaker activity is a rather rhythmic increase and decrease in smooth muscle tension.

In part B of the figure, you can see that the small intestine shows an unusual pattern of this rhythmic pacemaker activity during the fasting state. In the fasting state, the electrical rhythm (and therefore the rhythm of contraction) is relatively quiet, except for a coordinated wave of motor activity every 1½ to 2 hours. Each wave of rhythmic contractions is called a migrating motor complex (MMC). One function of the MMC is thought to be that of clearing out any remaining material, such as larger, indigestible particles, bile and other secretions, bacteria, and sloughed off epithelial cells. The MMC pattern is largely triggered by the hormone motilin released from endocrine cells in the duodenum.

Once material enters a segment of the digestive tract, the slow basic rhythm of contraction changes to the rapid rhythms of segmentation (mixing actions) and peristalsis (progressive movement).