T-tubule

  A T-tubule (or transverse tubule), is a deep invagination of the plasma membrane found in skeletal and cardiac muscle cells. These invaginations allow depolarization of the membrane to quickly penetrate to the interior of the cell.

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Structure

Each muscle fiber consists of T-tubules which run perpendicularly (transversely) to the long axis of the fiber. As they are invaginations of the plasma membrane, each T-tubule is composed of a phospholipid bilayer in which are embedded a large number of L-type calcium channels. The T-tubule extends from the surface of the muscle fiber into its interior, effectively bringing the extracellular environment in proximity to the innermost confines of the cell.

In skeletal and cardiac muscle, the T-tubules are adjacent to the terminal sacs (or terminal cisternae) of the fiber's sarcoplasmic reticulum. In skeletal muscle, the T-tubule is surrounded by a pair of terminal cisternae in an arrangement called a triad that is found at the junction of the A and I bands, a.k.a. the "region of overlap." Cardiac muscle has a similar structure, the diad, which is composed of a T-tubule and a single terminal cisterna; it occurs at the Z line. It is physiologically important that the t-tubules are so positioned, as their location allows for close excitation-contraction coupling. That is, the wave of depolarization that spreads down the t-tubles allows for opening of calcium-release channels in the sarcoplasmic reticulum immediately adjacent to the actin thin filaments (which cause contraction via the sliding filament mechanism). There is consequently very little delay between muscle cell depolarization and contraction of the muscle.

Excitation-contraction coupling

T-tubules are the major sites for the coupling of excitation and contraction, which is the process whereby the spreading depolarization is converted into force production by muscle fibers. The L-type calcium channels in T-tubules activate in response to electrical stimulation; their opening allows calcium to flow down its electrochemical gradient and into the cell. Activation of the L-type channel also causes a mechanical interaction between it and calcium-release channels located on the adjacent sarcoplasmic reticulum membrane.

In skeletal muscle, the influx of calcium through the L-type calcium channel on the T-tubule contributes little to excitation-contraction coupling, whereas it is crucial to the proper function of cardiac muscle (see Cardiac action potential). Conversely, the mechanical interaction between the T-tubule's L-type calcium channel and the calcium-release channel is critical to proper skeletal muscle contraction, whereas it contributes little to the contraction of cardiac muscle.

Detubulation

It is possible to physically and functionally uncouple T-tubules from the surface membrane using a technique known as detubulation. This relies on osmotically active chemicals, such as glycerol (for skeletal muscle) or formamide (mainly for cardiac muscle). Addition of these chemicals to the solution surrounding muscle cells causes the cells to shrink; when the chemical is withdrawn the cells rapidly expand before returning to their normal size. The rapid expansion is thought to cause the t-tubules to detach from the surface membrane, which reseals, and to reseal within the cell. This technique can be used to investigate the function of the t-tubules.

There is some evidence that heart failure precipitates the loss of the T-tubule network, again indicating their importance.^{[citation needed]}

See also

• Muscle contraction