Smooth Muscle Biology

C. Organization of Smooth Muscle: Unlike striated-muscle fibers, which abut end-to-end, smooth muscle fibers overlap to various degrees and attach to one another by fusing their endomysial sheaths. The sheaths are interrupted by many gap junctions, which transmit the ionic currents that initiate contraction. Smooth muscle fibers form fascicles that vary in size but are usually smaller than those in striated muscle. The fascicles, each surrounded by a meager perimysium, are often organized in layers separated by the thicker epimysial connective tissue. Fibers in adjacent layers often lie perpendicular to one another.

D. Mechanism of Contraction: The mechanism of smooth muscle contraction is a modification of the sliding-filament mechanism. At the beginning of the contraction, the myosin filaments appear and the actin filaments are pulled toward and between them. Continued contraction involves forming more myosin filaments and further sliding of the actin filaments. The sliding actin filaments pull the attached dense bodies closer together, shortening the cell. Unlike striated muscle fibers, individual smooth muscle fibers may undergo partial peristaltic, or wavelike, contractions. During relaxation, the myosin filaments decrease in number, disintegrating into soluble cytoplasmic components.

E. Initiation of Smooth Muscle Contraction: Like cardiac muscle fibers, smooth muscle fibers are capable of spontaneous contraction that may be modified by autonomic innervation. Motor end-plates are not present. Neurotransmitters diffuse from terminal expansions of the nerve endings between smooth muscle cells to the sarcolemma. Both sympathetic (adrenergic) and parasympathetic (cholinergic) endings are present and exert antagonistic (reciprocal) effects. In some organs, contractile activity is enhanced by cholinergic nerves and decreased by adrenergic nerves, whereas in others the opposite occurs