Muscle cell

A muscle cell, also called a myocyte, is a special 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 body parts. 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 special microscopic anatomy of a muscle cell gives it its name. The cytoplasm in a muscle cell is called the sarcoplasm. The smooth endoplasmic reticulum is called the sarcoplasmic reticulum. The cell membrane is called the sarcolemma. The sarcolemma helps the cell respond to signals.

Skeletal muscle cells

Main article: Skeletal muscle fibers

Skeletal muscle cells, also called muscle fibers, are the tiny parts that make up muscles. They are long and thin, with many nuclei, and are the only muscle cells that are multinucleated. A muscle, like the biceps brachii, has many muscle fibers. These fibers have protein chains called myofibrils, made of thin actin and thick myosin strands. These strands move over each other to shorten the muscle when it contracts. The sarcoplasm holds glycogen for energy and myoglobin to store oxygen.

Cardiac muscle cells

Main article: Cardiac muscle

Cardiac muscle cells have special parts in their cell membrane, such as the intercalated disc and transverse tubules. Like skeletal muscle, cardiac muscle looks striped and has myofibrils, myofilaments, and sarcomeres. The cell membrane connects to the cell's cytoskeleton, which helps keep the cell stable and in shape.

Smooth muscle cells

Main article: Smooth muscle

Further information: Basal electrical rhythm

Smooth muscle cells are in the walls of hollow organs, such as the stomach, intestines, bladder, and uterus, as well as in blood vessels. They are shorter and wider than skeletal muscle fibers, with one nucleus. Even though they do not have sarcomeres or myofibrils, they have actin and myosin proteins that let them contract.

Development

Main article: Myogenesis

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

These muscle fibers can fix themselves with the help of special cells called satellite cells. Scientists can also make 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 moving thin and thick parts inside them. This makes the muscle get shorter, which is called contraction. This starts when a tiny electrical signal, called an action potential, moves across the cell. This signal helps a chemical called calcium enter the cell. The calcium makes the parts inside the cell move, and the muscle shortens. After this, the calcium goes away, and the muscle gets back to its normal size.

Muscles can contract in different ways. Some moves the muscle, and others hold it still. The heart has special muscle cells that help control the heartbeat. These cells get help from the nervous system to keep the heart beating properly.

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. Others believe they evolved more than once. Researchers continue to study different animals and their muscle structures.

Some studies suggest that muscle cells may have developed 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. This points to multiple origins of muscle cells in different animal groups.