Question

How does the action potential quickly get from the outside of the muscle fiber to the inside of the cell?

Answer 1

Sarcolemma of the skeletal muscle cell invaginates inside the muscle like a tube. This is known as T-tubule. Since T-tubule is a continuation of sarcolemma, it contains ligand and voltage gated Na+ channels such that, when the Ach is released into the synaptic cleft, it binds with the ligand gated channels causing depol;arisation of membrane and opening of votage gated sodium channels. This depolarisation travels deep into the cell very fast due to presence of T-tubules.

Terminal cisternae of SR also lies close to the T-tubule and causes release of Ca2+ inside the cell such that it causes contraction of the sarcomere.