What are the regularly spaced infoldings of the sarcolemma called?

The regularly spaced infoldings of the sarcolemma are known as T-tubules, which stand for transverse tubules. These structures play a crucial role in muscle contraction by facilitating the rapid transmission of electrical impulses from the surface of the muscle fiber deep into its interior. This ensures that the entire muscle fiber can contract simultaneously rather than in a delayed fashion.

When an action potential travels along the sarcolemma, it invaginates into the muscle cell as T-tubules, which are positioned in close association with the terminal cisternae of the sarcoplasmic reticulum. This arrangement helps in the efficient release of calcium ions from the sarcoplasmic reticulum, which is essential for muscle contraction. The coordinated action of T-tubules and the sarcoplasmic reticulum is fundamental to the excitation-contraction coupling process in skeletal muscle fibers.

Cisternae refer to the enlarged regions of the sarcoplasmic reticulum that store calcium, but they are not infoldings of the sarcolemma. Myofibrils and sarcomeres are components of muscle fibers that are involved in contraction but do not serve the function of conducting impulses like T-tubules. Thus, T-tubules are specifically identified