Myofiber	A muscle cell
Sarcolemma	The membrane of the muscle cell
Acetylcholine receptors	Proteins on the surface of the muscle cell, to which Acetylcholine can attach
Motor end plate	The region of the membrane right under the motor nerve axon terminals, where the acetylcholine receptors are concentrated.
Motor unit	A motor nerve and all the myofibers it attaches to
Sarcoplasm	The cytoplasm of the muscle cell
Sarcoplasmic reticulum	The endoplasmic reticulum of the muscle cell. Specialized to store calcium ions.
Myofibrils	Tubules containing the contractile proteins. The myofibers of skeletal and cardiac muscle are filled with these tubules.
Sarcomere	One segment of a myofibril (like one car of a train)
Z-disks	The membranes that cross the myofibril to divide it into sarcomeres
actin	A protein that forms filaments attached to the Z-disks. When these filaments are pulled toward each other by myosin, the Z-disks move together and the sarcomere becomes shorter.
myosin	The protein in the center of the sarcomere, which pulls actin filaments toward each other
Troponin and tropomyosin	The regulatory proteins wound around the actin. They control whether the myosin can attach to it and pull it.
Skeletal muscle/striated muscle	The muscle attached to bones. Called ‘striated’ because its actin and myosin are lined up in sarcomeres, making stripes of protein. It has a large sarcoplasmic reticulum which stores calcium.
Cardiac muscle	Also striated, but has a less extensive sarcoplasmic reticulum. It has to get some of its calcium from the blood through calcium channels. Cells fire automatically as calcium leaks in.
Calcium channels	Proteins in the cell membrane that let calcium enter cells.
Calcium leak	Calcium enters the cardiac muscle cell and makes it more positive
plateau	When the cardiac muscle cell has depolarized but cannot repolarize because calcium is still leaking in.
Intercalated disk	The tight connections between cardiac muscle cells, so that if one cell fires it will make the next one fire also.
Smooth muscle	The type of muscle found in tubes (GI tract, arterioles, etc.). It is not striated and has very little sarcoplasmic reticulum.
Calmodulin	The regulatory protein in smooth muscle. It controls whether the myosin can pull the actin.