<P> The sarcoplasmic reticulum (SR) is a membrane - bound structure found within muscle cells that is similar to the endoplasmic reticulum in other cells . The main function of the SR is to store calcium ions (Ca). Calcium ion levels are kept relatively constant, with the concentration of calcium ions within a cell being 100,000 times smaller than the concentration of calcium ions outside the cell . This means that small increases in calcium ions within the cell are easily detected and can bring about important cellular changes (the calcium is said to be a second messenger; see calcium in biology for more details). Calcium is used to make calcium carbonate (found in chalk) and calcium phosphate, two compounds that the body uses to make teeth and bones . This means that too much calcium within the cells can lead to hardening (calcification) of certain intracellular structures, including the mitochondria, leading to cell death . Therefore, it is vital that calcium ion levels are controlled tightly, and can be released into the cell when necessary and then remove from the cell . </P> <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 cardiac muscle or junctional SR 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>

Where are calcium ions stored in a muscle fiber