<P> 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 . 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 . These processes are explained in more detail below and are fundamental for the process of excitation - contraction coupling in skeletal, cardiac and smooth muscle . </P> <P> The SR contains ion channel pumps, within its membrane that are responsible for pumping Ca into the SR . As the calcium ion concentration within the SR is higher than in the rest of the cell, the calcium ions won't freely flow into the SR, and therefore pumps are required, that use energy, which they gain from a molecule called adenosine triphosphate (ATP). These calcium pumps are called Sarco (endo) plasmic reticulum ATPases (SERCA). There are a variety of different forms of SERCA, with SERCA 2a being found primarily in cardiac and skeletal muscle . </P> <P> 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 Ca to enter . The cytosolic side of the pump then closes and the sarcoplasmic reticulum side opens, releasing the Ca into the SR . </P> <P> A protein found in cardiac muscle, called phospholamban (PLB) has been shown to prevent SERCA from working . It does this by binding to the SERCA and decreasing its attraction (affinity) to calcium, therefore preventing calcium uptake into the SR . Failure to remove Ca from the cytosol, prevents muscle relaxation and therefore means that there is a decrease in muscle contraction too . However, molecules such as adrenaline and noradrenaline, can prevent PLB from inhibiting SERCA . When these hormones bind to a receptor, called a beta 1 adrenoceptor, located on the cell membrane, they produce a series of reactions (known as a cyclic AMP pathway) that produces an enzyme called protein kinase A (PKA). PKA can add a phosphate to PLB (this is known as phosphorylation), preventing it from inhibiting SERCA and allowing for muscle relaxation . </P>

What is the major function of the sarcoplasmic reticulum in muscle contraction