Muscle

Muscle is a special kind of soft tissue, and it is one of the four main types of animal tissues. It helps animals move and do many important things in their bodies. There are three main types of muscle in vertebrates: skeletal muscle tissue, cardiac muscle tissue, and smooth muscle tissue. These muscles work together to make hearts beat and help you move your arms and legs.

Skeletal muscles are the ones you use when you decide to move, like lifting your hand or running. They are made of long cells called muscle fibers and are connected to bones by tendons. Other muscles, like those in your heart and digestive system, work without you thinking about them. They are controlled by your nervous system and can also be influenced by hormones.

Inside muscle cells are special proteins called actin and myosin that help muscles contract and relax, creating movement. Muscles form early in development, through a process known as myogenesis. Learning about muscles helps us understand how our bodies move and stay strong.

Etymology

The word muscle comes from the Latin word musculus, a smaller form of mus, which means mouse. This is because when a muscle like the biceps bends, it looks a bit like the back of a mouse. The same idea is also found in Greek, where the word μῦς, mȳs, means both "mouse" and "muscle".

Structure

There are three types of muscle tissue in vertebrates: skeletal, cardiac, and smooth. Skeletal and cardiac muscle both look striped under a microscope. This is called striated muscle. Smooth muscle does not look striped.

Invertebrates also have three types of muscle tissue: transversely striated, obliquely striated, and smooth muscle. Vertebrate skeletal muscle is long and striped. It is attached to bones by tendons. It helps us move and stand up. Cardiac muscle is only found in the heart. It works on its own to pump blood. Smooth muscle is found in organs like the stomach, intestines, and blood vessels. It also works without us thinking about it.

Main article: Skeletal muscle

Main article: Smooth muscle

Main article: Cardiac muscle

Development

Main article: Myogenesis

All muscles start from a special part of the body called paraxial mesoderm. As the body grows, this mesoderm breaks into pieces called somites. These match the segments of the body, like the bones in the back.

Each somite splits into three parts: one helps make bones, one makes skin, and one makes muscle. This muscle part is called the myotome.

Inside the myotome are cells called myoblasts. These cells either stay near the backbone to make back muscles or move out to make muscles in the arms and legs. Before they move, special connective tissues form a guide. The myoblasts follow chemical signals to find their right places. There they join together to make the muscles we use every day.

Function

Further information: Sliding filament theory

The main job of muscle tissue is to contract, or tighten up. There are three types of muscle in our bodies: skeletal, cardiac, and smooth. They all work by moving special things called actin against myosin. This makes the muscle shrink.

Skeletal muscle moves when electrical signals from our nerves tell it to. Cardiac and smooth muscles work on their own. Smooth muscle is found in many places in our body, like our stomach and blood vessels. It helps move things along. Cardiac muscle is the heart’s muscle, and it works all the time to keep our blood flowing.

Invertebrate muscle

Invertebrates, like insects and spiders, have three types of muscle tissue. These types are named by how they look under a microscope: transversely striated, obliquely striated, and smooth muscle. In arthropods, such as insects and crustaceans, smooth muscle is not found. The transversely striated muscle in invertebrates works most like the skeletal muscles in vertebrates, which are the muscles we use to move our bodies.