Muscle cell

A muscle cell, also known as a myocyte, is a special kind of cell that helps animals move. In humans and other vertebrates, there are three main types of muscle cells: skeletal, smooth, and cardiac. Each type has its own job and looks a little different.

Skeletal muscle cells are long and threadlike and have many nuclei. They are called muscle fibers and help us move our arms, legs, and other parts of our body. These cells grow from tiny precursor cells called myoblasts through a process called fusion.

Cardiac muscle cells make up the muscle in the walls of the heart. They have one central nucleus and are connected to each other by special structures called intercalated discs. These cells work together to help the heart beat.

Smooth muscle cells are found in places like the esophagus and stomach. They help with involuntary movements, like pushing food along the digestive system, through contractions known as peristalsis. Unlike the other two types, smooth muscle cells have only one nucleus and do not have certain structures called myofibrils or sarcomeres.

Structure

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

Main article: Skeletal muscle fibers

Skeletal muscle cells, also known as muscle fibers, are the individual contractile cells within a muscle. They are long and threadlike, with many nuclei, and are the only muscle cells that are multinucleated. A single muscle, such as the biceps brachii, contains around 253,000 muscle fibers. These fibers contain protein chains called myofibrils, which are made of thin filaments of actin and thick filaments of myosin. These filaments slide over each other to shorten the muscle fiber during contraction. The sarcoplasm contains glycogen for energy and myoglobin, which stores oxygen.

Cardiac muscle cells

Main article: Cardiac muscle

Cardiac muscle cells have a specialized cell membrane that includes regions such as the intercalated disc and transverse tubules. Like skeletal muscle, cardiac muscle is striated and contains myofibrils, myofilaments, and sarcomeres. The cell membrane is anchored to the cell's cytoskeleton, which helps stabilize the cell and control its shape and size.

Smooth muscle cells

Main article: Smooth muscle

Further information: Basal electrical rhythm

Smooth muscle cells are found in the walls of hollow organs, including the stomach, intestines, bladder, and uterus, as well as in blood vessels. They are spindle-shaped, with a single nucleus, and are much shorter than skeletal muscle fibers. Though they lack sarcomeres and myofibrils, they contain actin and myosin proteins that allow them to contract.

Development

Main article: Myogenesis

A myoblast is a special cell that turns into different types of muscle cells. This change is guided by important proteins like MyoD, Myf5, myogenin, and MRF4. When myoblasts join together, they form long cells called skeletal muscle fibers, which have many nuclei.

These muscle fibers can repair themselves with the help of special cells called satellite cells. Scientists can also create these muscle cells in a lab using stem cells.

Function

Muscle contraction in striated muscle

Main article: Muscle contraction

Muscle cells, also called myocytes, work by sliding thin and thick filaments past each other. This process, called the sliding filament mechanism, pulls certain parts of the muscle closer together, making the muscle contract. This contraction starts when an electrical signal, called an action potential, travels across the cell's surface. The action potential reaches special channels that release a chemical called calcium into the muscle fiber. This calcium helps the filaments slide, causing the muscle to shorten. After the contraction, calcium is taken back up, and the muscle relaxes.

There are different ways muscles can contract. Some contractions move the muscle, while others keep it in place. The heart has special muscle cells that create electrical signals to control the heartbeat. These signals are helped or slowed by the nervous system to keep the heart beating at the right pace.

Evolution

Further information: Evolution and Adaptation

The evolution of muscle cells in animals is still being studied. Some scientists think all muscle cells come from one common ancestor, while others believe they evolved more than once. This debate continues as researchers look at different animals and their muscle structures.

Some studies suggest that muscle cells may have developed along with digestive and nervous systems in early animals. Other research shows that certain proteins and structures used in muscle cells existed before true muscle cells appeared, pointing to multiple origins of muscle cells in different animal groups.