Muscle cell

A muscle cell, also known as a myocyte, is a mature contractile cell in the muscle of an animal. In humans and other vertebrates there are three types: skeletal, smooth, and cardiac (cardiomyocytes). A skeletal muscle cell is long and threadlike with many nuclei and is called a muscle fiber. Muscle cells develop from embryonic precursor cells called myoblasts.

Skeletal muscle cells form by fusion of myoblasts to produce multinucleated cells (syncytia) in a process known as myogenesis. Skeletal muscle cells and cardiac muscle cells both contain myofibrils and sarcomeres and form a striated muscle tissue.

Cardiac muscle cells form the cardiac muscle in the walls of the heart chambers, and have a single central nucleus. Cardiac muscle cells are joined to neighboring cells by intercalated discs, and when joined in a visible unit they are described as a cardiac muscle fiber.

Smooth muscle cells control involuntary movements such as the peristalsis contractions in the esophagus and stomach. Smooth muscle has no myofibrils or sarcomeres and is therefore non-striated. Smooth muscle cells have a single nucleus.

The unusual microscopic anatomy of a muscle cell gave rise to its terminology. The cytoplasm in a muscle cell is termed the sarcoplasm; the smooth endoplasmic reticulum of a muscle cell is termed the sarcoplasmic reticulum; and the cell membrane in a muscle cell is termed the sarcolemma. The sarcolemma receives and conducts stimuli.

Skeletal muscle cells are the individual contractile cells within a muscle and are more usually known as muscle fibers because of their longer, threadlike appearance. Broadly there are two types of muscle fiber performing in muscle contraction, either as slow twitch (type I) or fast twitch (type II).

A single muscle, such as the biceps brachii in a young adult human male, contains around 253,000 muscle fibers. Skeletal muscle fibers are the only muscle cells that are multinucleated with the nuclei usually referred to as myonuclei. This occurs during myogenesis with the fusion of myoblasts, each contributing a nucleus to the newly formed muscle cell or myotube. Fusion depends on muscle-specific proteins known as fusogens called myomaker and myomerger.

A striated muscle fiber contains myofibrils consisting of long protein chains of myofilaments. There are three types of myofilaments: thin, thick, and elastic, that work together to produce a muscle contraction. The thin myofilaments are filaments of mostly actin and the thick filaments are of mostly myosin, and they slide over each other to shorten the fiber length in a muscle contraction. The third type of myofilament is an elastic filament composed of titin, a very large protein.