A cartoon section of skeletal muscle, showing T-tubules running deep into the centre of the cell between two terminal cisternae/junctional<unk> . The thinner projections, running horizontally between two terminal cisternae are the longitudinal sections of the<unk> . The<unk>  <unk>  is a membrane-bound structure found within muscle cells that is similar to the smooth endoplasmic<unk>  in other cells. The main function of the<unk>  is to store calcium ions (Ca2+). Calcium ion levels are kept relatively constant, with the concentration of calcium ions within a cell being 10,000 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 removed from the cell.
Sarcoplasmic reticulum
Microvesicles
Nucleus 
Node of Ranvier
Endosome
Synaptonemal complex
Specific granule
Myelomere