What structure transmits the electrical message for contraction into the interior of the muscle cell?

Question

Quizplus · Accepted Answer

T-tubules are invaginations of the sarcolemma that penetrate deep into the muscle fiber, allowing the electrical message for contraction to quickly reach the interior of the muscle cell. The motor endplate is the point where a motor neuron meets a muscle fiber and releases the neurotransmitter acetylcholine, but it does not transmit the electrical message for contraction. The sarcolemma is the plasma membrane of a muscle cell, but it is not the structure that directly transmits the electrical message for contraction. The Z-line marks the boundary between sarcomeres and is involved in muscle contraction, but it does not transmit the electrical message for contraction.

What Structure Transmits the Electrical Message for Contraction into the Interior