Title: Muscle: The Histology Guide
Headings: Muscle:
Stimulation

Muscle: Stimulation
Content: The T-tubules lie over the junction between the A- and I-bands (see diagram). The two terminal cistemae of the SR together with their associated T tubule are known as a triad. Inside the muscle fibre, the T-tubules lie next to the terminal cisternae of an internal membrane system derived from the endoplasmic reticulum, called the sarcoplasmic reticulum (SR), which is a store of calcium ions. Stimulation of the muscle fibre, causes a wave of depolarisation to pass down the t-tubule, and the SR to release calcium ions into the sarcoplasm. Calcium is pumped back up into the SR to lower calcium ion concentration in the sarcoplasm, to relax the muscle (turn off contraction). Cardiac Muscle also has T-tubules, and SR. However the T-tubules lie over the Z-line in cardiac muscle, are less numerous and wider. The SR is smaller and less elaborate, and stores less calcium ions. Cardiac muscle cells also depend on extracellular calcium ions, that enter through the T-tubules and triggers release of calcium ions from the SR. Cardiac muscle cells are electrically connected through gap junctions, so that waves of electrical stimuli pass around the heart from cell to cell, and all the cells are stimulated to contract.