Junctional Sarcoplasmic Reticulum in Excitation-Contraction-Coupling

The junctional sarcoplasmic reticulum (JSR, syn. terminal cisterna) is implicated in Ca++storage and release for muscle contraction. Its discrete ultrastructure permits distinction from the rest of the SR (free SR) even when it occurs without plasmalemmal contact, e.g. as extended JSR (EJSR) in bird, and corbular SR (CSR) in mammalian cardiac cells. The close apposition of JSR to plasmalemma via junctional processes is central to proposed mechanisms of translating voltage-dependent charge transfers at the plasmalemma during the action potential into Ca++release from the JSR. These hypotheses are put into question by the existence of EJSR (and CSR) which in birds constitutes 70-80% of the total JSR. An alternate hypothesis proposes, at least for cardiac cells, that Ca++entering the cell during excitation causes additional Ca++to be freed intracellularly. The notion of a chemical transmitter acting by diffusion is attractive because it will allow for the anomalous topography of EJSR, especially since bird cardiac cells have only about half the diameter of their mammalian relatives and have no transverse tubules.