NB: Include Necessary Diagrams from text book.
The sarcolemma (cell membrane of muscle cell) along with the sarcoplasmic reticulum (the endoplasmic re- ticulum of muscle cell) forms a highly specialized system called sarcotubular system. This plays an important role in the internal conduction of depolarization within the muscle fibre. The sarcotubular system is primarily formed by a transverse tubular system (T-system) and a longitudinal sarcoplasmic reticulum(L-tubules)
Transverse tubular system (T-system)
- The T-system of transverse tubules is formed by the through and through invagination of sarcolemma into the muscle fibre in the region of junction of A and I bands.
- They are about 0.03 microns in diameter.
- As the T-tubules are formed by the invagination of sarcolemma, their lumen contains extracellular fluid (ECF) which surrounds the muscle cell.
- The membrane of T-tubules contains voltage-gated Ca2+ channels called dihydropyridine (DHP) receptors (as they get blocked by the drug DHP) through which they activate the longitudinal sarcoplasmic reticulum.
- Once initiated, an action potential is rapidly conducted along the sarcolemma over the surface of the muscle fiber, as well as into the interior of the muscle cell by the way of the T tubules, so that deep lying myofibrils are fast activated.
Longitudinal sarcoplasmic reticulum (L-tubules)
- The longitudinal sarcoplasmic reticulum is the name given to the sarcoplasmic tubules of the sarcoplasmic reticulum which run in long axis of the muscle fibre forming a closed, elaborate, membranous, tubular system around each myofibril.
- These L-tubules do not open to the exterior like T-tubules.
- The longitudinal sarcoplasmic tubules, about 0.04 microns in diameter on either side of the T-tubule are dilated to form the so-called terminal cisterns.
- A T-tubule with the two terminal cisterns lying in proximity (contiguity) constitutes a triad which is found at the junction of A and I bands. Thus, there are two triads in each sarcomere.
- A cardiac muscle forms a diad due to presence of one cistern.
- The longitudinal tubules store a large quantity of calcium ions.
- Triad plays a vital role in linking membrane action potential to muscle contraction.
Receptors and Channel Proteins in STS
The T-tubules are separated from the terminal cisternae by a narrow space of about 12-14 nm that contains areas of densities known as “feet”. The junctional feet involve two integral membrane proteins, one in the T-tubule membrane, and the other in the membrane of the terminal cistern.
- The T-tubule protein is a modified voltage-gated Ca2+ channels known as the dihydropyridine (DHP) receptor.
- The DHP receptors are L-type calcium channels and they’re clustered in groups of four called “tetrads”.
- Primarily, the receptor acts as a voltage sensor rather than a calcium channel.
- The portion of the terminal cisternae membrane that faces the T-tubules contains proteins known as the ryanodine receptors. They are so called because they bind to the plant alkaloid ryanodine. They mainly function as calcium release channels.
- There is also calcium ATPases present on the membrane of SR that pumps calcium back into the SR.
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