triad
As the membrane depolarizes, another set of ion channels called voltage-gated sodium channels are triggered to open. Sodium ions enter the muscle fiber, and an action potential rapidly spreads or fires along the entire membrane to initiate excitation-contraction coupling. Things happen very quickly in the world of excitable membranes just think about how quickly you can snap your fingers as soon as you decide to do it . Immediately following depolarization of the membrane, it repolarizes, re-establishing the negative membrane potential. Meanwhile, the ACh in the synaptic cleft is degraded by the enzyme acetylcholinesterase AChE so that the ACh cannot rebind to a receptor and reopen its channel, which would cause unwanted extended muscle excitation and contraction. Propagation of an action potential along the sarcolemma is the excitation portion of excitation-contraction coupling. Recall that this excitation actually triggers the release of calcium ions Ca++ from its storage in the cells SR. For the action potential to reach the membrane of the SR, there are periodic invaginations in the sarcolemma, called T-tubules T stands for transverse . You will recall that the diameter of a muscle fiber can be up to 100 m, so these T-tubules ensure that the membrane can get close to the SR in the sarcoplasm. The arrangement of a T-tubule with the membranes of SR on either side is called a triad Figure 10.7 . The triad surrounds the cylindrical structure called a myofibril, which contains actin and myosin.