what allows the nerve impulses that initiate muscle contraction to travel along the sarcolemma
T-tubule Transverse tubules (T-tubules) are extensions of the sarcolemma (muscle cell membrane) that penetrate into the centre of skeletal and cardiac muscle cells. T-tubules are highly specialised structures, that form within the first few weeks of life,[1] containing large amounts of specific proteins known as ion channels that allow for electrical impulses (action potentials) travelling along the sarcolemma, to enter rapidly into the cell, to initiate muscle contraction.