The<unk>  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<unk> , 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<unk>  are thinner projects, that run between the terminal cisternae/junctional<unk> , 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.
Sarcoplasmic reticulum
Microvesicles
Nucleus 
Node of Ranvier
Endosome
Synaptonemal complex
Specific granule
Myelomere