Sarcoplasmic Reticulum and Transverse Tubules

Sarcoplasmic Reticulum and Transverse Tubules

Inside the sarcoplasm of a muscle fiber, a network of channels surrounds each myofibril. These membranous channels form the SR, which has interconnected tubules surrounding each myofibril (FIGURE 9-6). Most of these tubules run longitudinally and communicate at the H zone.

Sarcoplasmic Reticulum

Each skeletal muscle fiber contains transverse tubules as well as a SR, which is a complex smooth endoplas-mic reticulum. It has interconnecting tubules that surround each myofibril. Most of these tubules run lengthwise along the myofibrils and communicate with them at the H zone. However, the terminal cisterns form large cross-channels that are perpen-dicular, located at the A band- I band junctions. These always occur in pairs. Large numbers of glycogen granules and mitochondria are involved in produc-ing energy needed for muscle contractions. The SR regulates intracellular ionic calcium levels. It stores calcium and also releases it as needed, when muscle fibers are stimulated to contract. Calcium provides the final “start” signal for muscle contraction. Inte-gral proteins protrude into the intermembrane spaces from the SR and T-tubules. These proteins act to sense voltages. The SR proteins form gated channels through which calcium ions are released by the ter-minal cisterns.

Transverse Tubules

Transverse tubules (T-tubules) are other mem-branous channels extending inward and passing through the fiber. Action potentials are conducted into a skeletal muscle fiber by T-tubules. These tubules open to the outside of the muscle fiber and contain extracellular fluid. Each tubule lies between enlarged structures called terminal cisternae, near the point where actin and myosin filaments over-lap. Together, the SR and T-tubules activate muscle contraction when stimulated. The functional role of the T-tubules is to enhance cellular communica-tion during muscle contraction. The lumen of each T-tubule is continuous with the extracellular space. The T-tubules form triads , which are groupings consisting of themselves in between two terminal cisterns. Ultimate control of muscle contraction is via electrical impulses initiated by nerves that travel through the sarcolemma. The continuous T-tubules conduct impulses deeply into muscle cells and all sarcomeres. This signals the release of calcium from nearby terminal cisterns. At the triads, organelles are in close contact and integral proteins protrude into intermembrane spaces to act as voltage sensors. The proteins in this location that derive from the SR form gated channels. Through these channels, ter-minal cisterns release calcium ions.

1. Describe action potential generation and propagation.

2. Identify the functions of the transverse tubules and SR.