what triggers the release of calcium from the sarcoplasmic reticulum
Sarcoplasmic reticulum Calcium ion release from the SR, occurs in the junctional SR/terminal cisternae through a ryanodine receptor (RyR) and is known as a calcium spark.[10] There are three types of ryanodine receptor, RyR1 (in skeletal muscle), RyR2 (in cardiac muscle) and RyR3 (in the brain).[11] Calcium release through ryanodine receptors in the SR is triggered differently in different muscles. In cardiac and smooth muscle an electrical impulse (action potential) triggers calcium ions to enter the cell through an L-type calcium channel located in the cell membrane (smooth muscle) or T-tubule membrane (cardiac muscle). These calcium ions bind to and activate the RyR, producing a larger increase in intracellular calcium. In skeletal muscle, however, the L-type calcium channel is bound to the RyR. Therefore activation of the L-type calcium channel, via an action potential, activates the RyR directly, causing calcium release (see calcium sparks for more details).[12] Also, caffeine (found in coffee) can bind to and stimulate RyR. Caffeine works by making the RyR more sensitive to either the action potential (skeletal muscle) or calcium (cardiac or smooth muscle) therefore producing calcium sparks more often (this can result in increased heart rate, which is why we feel more awake after coffee).[13]