A muscle cell


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


The cytoplasm of the muscle cell

Sarcoplasmic reticulum

The endoplasmic reticulum of the muscle cell. Specialized to store calcium ions.


Tubules containing the contractile proteins. The myofibers of skeletal and cardiac muscle are filled with these tubules.


One segment of a myofibril (like one car of a train)


The membranes that cross the myofibril to divide it into sarcomeres


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.


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


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.


The regulatory protein in smooth muscle. It controls whether the myosin can pull the actin.