what is the role of the sarcoplasmic reticulum in muscle contraction
Sarcoplasmic reticulum The sarcoplasmic reticulum is a network of tubules that extend throughout muscle cells, wrapping around (but not in direct contact with) the myofibrils (contractile units of the cell). Cardiac and skeletal muscle cells contain structures called transverse tubules (T-tubules), which are extensions of the cell membrane that travel into the centre of the cell. T-tubules are closely associated with a specific region of the SR, known as the terminal cisternae in skeletal muscle, with a distance of roughly 12 nanometers, separating them. This is the primary site of calcium release.[3] The longitudinal SR are thinner projects, that run between the terminal cisternae/junctional SR, and are the location where ion channels necessary for calcium ion absorption are most abundant.[4] These processes are explained in more detail below and are fundamental for the process of excitation-contraction coupling in skeletal, cardiac and smooth muscle.
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]
Sarcoplasmic reticulum SERCA consists of 13 subunits (labelled M1-M10, N, P and A). Calcium ions bind to the M1-M10 subunits (which are located within the membrane), whereas ATP binds to the N,P and A subunits (which are located outside the SR). When 2 calcium ions, along with a molecule of ATP, bind to the cytosolic side of the pump (i.e the region of the pump outside the SR), the pump opens. This occurs because ATP (which contains three phosphate groups) releases a single phosphate group (becoming adenosine diphosphate). The released phosphate group then binds to the pump, causing the pump to change shape. This shape change causes the cytosolic side of the pump to open, allowing the two Ca2+ to enter. The cytosolic side of the pump then closes and the sarcoplasmic reticulum side opens, releasing the Ca2+ into the SR.[6]
Small intestine Food from the stomach is allowed into the duodenum through the pylorus by a muscle called the pyloric sphincter.