Smooth muscle cells in the body can exhibit different types of contractions. Some smooth muscle cells contract rhythmically or intermittently, while others are continuously active and maintain a level of "tone." The smooth muscle that exhibits rhythmic or intermittent activity is called phasic smooth muscle, which can be found in the walls of the gastrointestinal and urogenital tracts. This type of smooth muscle corresponds to the single-unit category because the cells contract in response to action potentials that spread from cell to cell. On the other hand, smooth muscle that is continuously active is called tonic smooth muscle. Examples of continuously active smooth muscle include vascular smooth muscle, respiratory smooth muscle, and certain sphincters. Tonic smooth muscle does not generate action potentials but its activity is proportional to the membrane potential. Phasic and tonic contractions of smooth muscle are caused by the interactions between actin and myosin filaments. However, during tonic contraction, there is a change in the cycling kinetics of cross-bridges, allowing the smooth muscle to maintain force with low energy cost.

Smooth muscle cells have different structures depending on their location in the body. For example, blood vessels and airways have a simple tubular structure where the smooth muscle cells are arranged in a circular manner. When these cells contract, the diameter of the tube decreases, increasing resistance to the flow of blood or air. In the gastrointestinal tract, the organization of smooth muscle cells is more complex. Layers of smooth muscle arranged both circularly and longitudinally provide mechanical actions for mixing food and propelling its contents through the digestive tract. The coordination between these layers relies on a complex system of autonomic nerves connected by plexuses. The presence of these plexus cells contributes to the ability of these organs to reduce internal volume during urination. In organs like the urinary bladder or rectum, the smooth muscle in the walls allows for expansion as urine or feces accumulate. The arrangement of smooth muscle cells in hollow organs varies depending on their function and mechanical loads.

In all hollow organs, smooth muscle is separated from the organ's contents by other cellular elements. These elements can range from simple vascular endothelium to complex mucosa in the digestive tract. The walls of hollow organs also contain a significant amount of connective tissue, which bears a larger portion of the wall stress as the organ volume increases.
Fig. 14.1 ). In some organs the smooth muscle cells contract rhythmically or intermittently, whereas in other organs the smooth muscle cells are continuously active and maintain a level of “tone.” Smooth muscle exhibiting rhythmic or intermittent activity is termed phasic smooth muscle and includes smooth muscles in the walls of the gastrointestinal (GI) and urogenital tracts. Such phasic smooth muscle corresponds to the single-unit category described earlier because the smooth muscle cells contract in response to action potentials that propagate from cell to cell. Smooth muscle that is continuously active, on the other hand, is termed tonic smooth muscle.
Vascular smooth muscle, respiratory smooth muscle, and some sphincters are continuously active. The continuous partial activation of tonic smooth muscle is not associated with action potentials, although it is proportional to membrane potential. Tonic smooth muscle would thus correspond to the multiunit smooth muscle described earlier. Phasic and tonic contractions of smooth muscle result from interactions of actin and myosin filaments, although as discussed later in this chapter, there is a change in cross-bridge cycling kinetics during tonic contraction, such that the smooth muscle can maintain force at low energy cost.
•Fig. 14.1 Somecontractileactivitypatternsexhibitedbysmoothmuscles.Tonicsmoothmusclesarenormallycontractedandgenerateavariablesteady-stateforce.Examplesaresphincters,bloodvessels,andairways.Phasicsmoothmusclescommonlyexhibitrhythmiccontractions(e.g.,peristalsisintheGItract)butmaycontractintermittentlyduringphysiologicalactivitiesundervoluntarycontrol(e.g.,voidingofurinefromurinarybladder,swallowing).
Structure of Smooth Muscle Cells
Fig. 14.2 ). Blood vessels and airways exhibit a simple tubular structure in which the smooth muscle cells are arranged circumferentially, so contraction reduces the diameter of the tube. This contraction increases resistance to the flow of blood or air but has little effect on the length of the organ. Smooth muscle cell organization is more complex in the GI tract. Layers of smooth muscle in both circumferential and longitudinal orientations provide the mechanical action for mixing food and also propelling the luminal contents from the mouth to the anus. Coordination between these layers depends on a complex system of autonomic nerves linked by plexuses. These plexuses cells in the walls of these organs contributes to their ability to reduce internal volume to almost zero during urination are located between the two muscle layers. The smooth muscle in the walls of saccular structures such as the urinary bladder or rectum allows the organ to increase in size with accumulation of urine or feces. The varied arrangement of
Normally partially contracted (tone) Blood vessels, airways Stomach, intestines Esophagus, urinary bladder Phasically active or defecation. Smooth muscle cells in hollow organs occur in a spectrum of forms, depending on their function and mechanical loads.
In all hollow organs the smooth muscle is separated from the contents of the organ by other cellular elements, which may be as simple as vascular endothelium or as complex as the mucosa of the digestive tract. The walls of hollow organs also contain large amounts of connective tissue that bear an increasing share of the wall stress as organ volume increases.