Although it is striated, cardiac muscle differs from skeletal muscle in that it is highly branched with cells connected by overlapping projections of the sarcolemma called intercalated discs. These discs contain desmosomes and gap junctions. In addition, cardiac muscle is autorhythmic, generating its own action potential, which spreads rapidly throughout muscle tissue by electrical synapses across the gap junctions.
Structure of Cardiac and Smooth Muscle
A muscle fiber (cell) has special terminology and distinguishing characteristics:
Due to its irregular arrangement of actin and myosin filaments, smooth muscle does not have the striated appearance of skeletal muscle. In addition, the sarcolemma does not form a system of transverse tubules. As a result, contraction is controlled and relatively slow—properties appropriate for smooth muscle function.
In addition to the thick myosin and thin actin filaments, smooth muscles possess noncontracting intermediate filaments. The intermediate fibers attach to dense bodies that are scattered through the sarcoplasm and attached to the sarcolemma. During contraction, the movements of myosin and actin are transferred to intermediate fibers, which pull on the dense bodies; these in turn pull the muscle cells together. In this way, the dense bodies function similarly to the Z discs in striated muscles.