Answer
Sarcolemma is the cell membrane of a muscle cell. Muscle cells (myocytes/myofibers) are unusual in that they are multinucleated; this is the case because in embryogenesis, several myoblasts fuse to form a single myocytes/myofiber.
The sarcoplasm is the cytoplasm of the muscle cell/myofiber. Myofiber sarcoplasm is occupied by myofibrils which are long cylindrical tubes or cords that run parallel to one another in the myocyte cytoplasm. Myofibrils are about 1 um in diameter. Mitochondria of these cells are located between the myofibrils of the myofiber, and the sarcoplasmic reticulum (SR) -- the smooth endoplasmic reticulum of myocytes. Each segment of SR forms an enclosing network around each myofibril.
Work Step by Step
Terminal cisternae (TC) are dilated ends (sacs) of sarcoplasmic reticulum tubes(SR) that cros myofibers from side to side. Another important organelle is the transverse tubule (T-tubule) which are tubular foldings of the sarcolemma which also penetrate the cell from side to side. . Each transverse tubule is closely associated with two TCs , one running along each side of a T-tubule. Triads are structures which represent one T-tubule with a TC running on each side.
The SR is a reservoirs of calcium ions ( Ca++). There are gated ion channels in the SR, and at the right moment when the ion channels are opened Ca++ ions pour out into the cytosol, and this triggers muscle contraction.
The sarcoplasm stores glycogen granules that supply glucose for energy; it also stores myoglobin which stores oxygen for use by muscles. Like hemoglobin, myoglobin binds oxygen . But a hemoglobin molecule comprises several polypeptide chains, while the myoglobin molecule is a single polypeptide chain. Hemoglobin transports oxygen in the blood, but the finding of myoglobin in the blood is a diagnostic sign that usually indicates muscle injury (crush).
