Question

What is a T tubules?

Answer 1

They conduct Action Potentials down into a skeletal muscle cell.

Explanation:

Skeletal muscle fibers, or cells, have a long surface membrane. This is called sarcolemma.
This sarcolemma will have invaginations along it and into itself.

This promotes electrical impulses, called Action Potentials, to move from the cell surface membrane into the inside of the muscle cell.

That way, the electrical signal is 'converted' into ions being released: specifically, calcium (Ca+2) ions.

These calcium ions are stored in the Sarcoplasmic Reticulum.
The end result is that the stored Ca+2 ions are released from the Sarcoplasmic reticulum, enabling muscle cell contraction.

All of this from an electrical impulse at the neuromuscular junction.