Sarcoplasm is the cytoplasm of a muscle cell. It is comparable to the cytoplasm of other cells, but it contains unusually large amounts of glycogen (a polymer of glucose), myoglobin, a red-colored protein necessary for binding oxygen molecules that diffuse into muscle fibers, and mitochondria. The calcium ion concentration in sarcoplasm is also a special element of the muscle fiber; it is the means by which muscle contractions take place and are regulated. The sarcoplasm plays a critical role in muscle contraction as an increase in concentration in the sarcoplasm begins the process of filament sliding. The decrease in in the sarcoplasm subsequently ceases filament sliding. The sarcoplasm also aids in pH and ion balance within muscle cells.
While sarcoplasm and myoplasm, viewed etymologically, might seem to be synonyms, they are not. Whereas sarcoplasm is a type of cytoplasm, myoplasm is the entire contractile portion of muscle tissue.