The command to contract is distributed throughout a muscle fiber by the

The correct answer will be - Transverse tubules

Explanation:

Transverse tubules are the tubular structure formed by the extension of the plasma membrane which innervates the cardiac and skeletal muscle cells. The T-tubules structure is structured between 20-40 nm and located at the junction of the I and A band.

The signals from the motor neurons once raches the motor endplate in muscles are carried into the muscles by these T-tubules which transmits the signals to muscle cells at very high speed. The T-tubules cause the release of the Calcium ions from the sarcoplasmic reticulum that cause contraction of the skeletal muscle.

Thus, Transverse tubules are the correct answer.