<Tr> <Th> Code </Th> <Td> TH H2. 00.05. 2.01018 TH H2. 00.05. 2.02013 </Td> </Tr> <Tr> <Td_colspan="2"> Anatomical terminology (edit on Wikidata) </Td> </Tr> <P> 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, 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 . </P> <P> Each muscle cell is surrounded by a sarcolemma . As T - tubules are simply continuations of the sarcolemma, their membranes are very similar to that of the cell membrane, consisting of two layers of lipid (fat) molecules (known as a lipid bilayer) with proteins, including: L - type calcium channels, sodium - calcium exchangers, calcium ATPases and Beta adrenoceptors, (see below) embedded within . </P>

What allows the nerve impulses that initiate muscle contraction to travel along the sarcolemma