T-tubule

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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.

Contents

1 Structure
2 Excitation-contraction coupling
3 Detubulation
4 See also

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 closely apposed to the terminal sacs (or terminal cisternae) of the fiber's sarcoplasmic reticulum. They are found adjacent to the terminal cisternae (or lateral sacs) of the muscle 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. Cardiac muscle has a similar structure, the dyad, which is composed of a T-tubule and a single terminal cisterna; it occurs at the Z line.

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 the technique known as detubulation. This relies on chemicals, such as dextran, that can initiate osmotic shock in the T-tubules when applied in high concentrations and then washed out.

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

T-tubule

Structure

Excitation-contraction coupling

Detubulation

See also