Brief notes on the sarcoplasmic reticulum in a skeletal muscle

In a muscle cell, parts of the sarcoplasmic reticulum run parallel to the length of the fiber (the longitudinal or L-tubules) while others are disposed radically, coursing from the surface towards the center of the fiber (the transverse or T-tubules).

The transverse tubules are transverse invaginations of the sarcolemma into the cell. They typically occur at the A-I junctions in mammals (and at the Z disc in amphibians). Being extensions of the sarcolemma, the transverse tubules conduct the action potentials from the sarcolemma into the interior of the cell along its membrances. The longitudinal tubules are disposed longitudinally along the entire length of A band. They end in dilated sacs called lateral cisterns.

Describe the structure of the contractile filaments. The myosin filaments (680 kDa) are thick and disposed longitudinally at the center of a sarcomere. It has a globular head and a fibrous tail. The fibrous tails are grouped into a bundle from which the globular heads called myosin heads project out. The myosin heads have two special sites on them: (i) a site with ATPase activity (the myosin ATPase), and (ii) an actin-binding site.

The actin filament (F-actin) is a double helical filament. It is made of globular subunits called G-actin (43 kDa). In a sarcomere, actin filaments are attached at one end to the Z disc. During contraction, the actin filaments slide over myosin filament. This is made possible by the presence of active sites on the actin filament that bind to the myosin head.

Tropomyosin is a fibrous molecule that consists of 2 chains – a and (3. It lies in the grove between the two filaments of actin. Each tropomyosin filament spans 7 G-actin subunits. It serves to cover and uncover the active sites of the actin molecules during muscle contraction and relaxation.

Troponin is attached to tropomyosin. It has 3 subunits: (i) Troponin-T (TpT) binds to tropomyosin; (ii) Troponin-C (TpC) is a calcium-binding protein. Each molecule of TpC binds to 4 molecules of Ca²⁺ ions; (iii) Troponin-I (Tpl) prevents the interaction of the myosin heads with the active sites on actin when TpC is bound to Ca²⁺. The inhibition occurs possibly due to the rolling of the tropomyosin molecule into a position that blocks the active sites.