Question

Explain the function of the sarcoplasmic reticulum and T tubules in skeletal muscle contraction.

Answer 1

The sarcoplasm is the specialized cytoplasm of a muscle cell that contains the usual subcellular elements along with the Golgi apparatus, abundant myofibrils, a modified endoplasmic reticulum known as the sarcoplasmic reticulum (SR), myoglobin and mitochondria. Transverse (T)-tubules invaginate the sarcolemma, allowing impulses to penetrate the cell and activate the SR, the SR forms a network around the myofibrils, storing and providing the Ca2+ that is required for muscle contraction.

The SARCOPLASMIC RETICULUM (SR) is a bit like the endoplasmic reticulum of other cells, e.g., it's hollow. But the primary function of the SARCOPLASMIC RETICULUM is to STORE CALCIUM IONS. Sarcoplasmic reticulum is very abundant in skeletal muscle cells and is closely associated with the MYOFIBRILS (and, therefore, the MYOFILAMENTS). The membrane of the SR is well-equipped to handle calcium: there are "pumps" (active transport) for calcium so that calcium is constantly being "pumped" into the SR from the cytoplasm of the muscle cell (called the SARCOPLASM). As a result, in a relaxed muscle, there is a very high concentration of calcium in the SR and a very low concentration in the sarcoplasm (and, therefore, among the myofibrils & myofilaments). In addition, the membrane has special openings, or "gates", for calcium. In a relaxed muscle, these gates are closed and calcium cannot pass through the membrane. So, the calcium remains in the SR. However, if an impulse travels along the membrane of the SR, the calcium "gates" open &, therefore, calcium diffuses rapidly out of the SR & into the sarcoplasm where the myofibrils & myofilaments are located.