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the cytoplasm of smooth-muscle cells and striated and myocardial muscle fibers. The matrix, or basic material, of sarcoplasm contains glycolytic enzymes and other globular proteins, for example, myoglobin; it also contains salts, polyphosphates, and glycogen, which is depleted during muscle contraction. Sarcoplasm surrounds nuclei and fills the space between myofibrils. It contains ribosomes, mitochondria (sarcosomes), and a complex system of membrane-bound vesicles, tubules, and cisterns, all of which are combined into a continuous sarcoplasmic reticulum.
The sarcoplasmic reticulum is divided into two parts. One part is oriented along the myofibrils and is equivalent to the endoplasmic reticulum found in other types of cells. The other part is oriented across a muscle fiber and forms the T system, which is a system that conducts impulses from the surface to deep within a muscle fiber and, in some places, changes into sarcolemma. The sarcoplasmic reticulum probably transmits excitation within a fiber. It also contains the Marsh factor, which inhibits the activity of the enzyme adenosine triphosphatase. The amount of sarcoplasm varies in different striated fibers: collagenous fibers contain little sarcoplasm, whereas red fibers contain a great deal.
L. V. DANILOVA