the na/k pump helps a muscle cell maintain a state of
Na+/K+-ATPase The Na+/K+-ATPase helps maintain resting potential, effect transport, and regulate cellular volume.[2] It also functions as a signal transducer/integrator to regulate MAPK pathway, ROS, as well as intracellular calcium. In most animal cells, the Na+/K+-ATPase is responsible for about 1/5 of the cell's energy expenditure.[citation needed] For neurons, the Na+/K+-ATPase can be responsible for up to 2/3 of the cell's energy expenditure.[3]
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.