Excitation-Contraction Coupling

An action potential in the skeletal muscle cell is what triggers muscle cell contraction. We have seen that calcium ions regulate whether or not contraction can occur. Thus, what is needed is a way to link muscle excitation (the depolarization of the action potential) to Ca++ release from the sarcoplasmic reticulum. This link is known as excitation-contraction coupling.

The T-tubules are essential structures for excitation-contraction coupling. T-tubules are tube-shaped invaginations of the sarcolemma (skeletal muscle plasma membrane) that penetrate throughout the muscle fiber (red in Figure 16-73A). The action potential in the muscle fiber is conducted into the interior of the muscle cell along the T-tubules. The lumen of the T-tubule is continuous with the extracellular fluid, and the membrane depolarization during an action potential occurs across the T-tubule membrane.

On either side of the T-tubule are swellings of the sarcoplasmic reticulum called lateral sacs. The lateral sacs are in close apposition to the T-tubule. In fact, in very high magnification electron micrographs (see Figure 16-73B), one can see regularly-spaced densities, which were dubbed "feet" when they were first discovered by electron microscopy about 40 years ago. Subsequent molecular studies have identified the feet as being part of the calcium channel protein in the membrane of the sarcoplasmic reticulum (SR Ca++ channel).

How does depolarization in the T-tubule membrane open a Ca++ channel in the sarcoplasmic reticulum membrane? The mechanism is summarized in the figure below, which is a cartoon of what is occurring at one of the densities between the lateral sac and T-tubule membranes.  Located in the T-tubule membrane, closely associated with the foot of the SR Ca++ channel, is the T-tubule voltage sensor.  The voltage sensor changes conformation in response to the depolarization of the action potential. This conformational change is transmitted to the foot of the SR Ca++ channel, causing it to open, and allowing Ca++ release. Note that this direct mechanical interaction between the T-tubule voltage sensor and the SR Ca++ channel is specific for excitation-contraction coupling in skeletal muscle. As we shall discuss in class, there are different mechanisms and different sources of Ca++ used in smooth muscle excitation-contraction coupling.

What causes the depolarization required for excitation-contraction coupling? Find out in the next page on muscle fiber excitation.