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 perform many important functions 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 allow everything from beating hearts to the movement of 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 types of muscle, like those in your heart and digestive system, work without you even 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. Understanding muscles helps us learn 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 have a striped appearance under a microscope, called striated muscle. Smooth muscle does not have this striped look.

Invertebrates also have three types of muscle tissue based on their appearance: transversely striated, obliquely striated, and smooth muscle. Vertebrate skeletal muscle is long and striped, attached to bones by tendons and helps us move and stand upright. Cardiac muscle is found only in the heart and works automatically to pump blood. Smooth muscle is found in organs like the stomach, intestines, and blood vessels, and 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 come from a special part of the body called paraxial mesoderm. As the body grows, this mesoderm splits into pieces called somites, which match the segments of the body, like the bones in the back. Each somite then divides into three parts: one that helps form the bones, one that forms the skin, and one that forms muscle. This muscle part is called the myotome.

Inside the myotome, there are cells called myoblasts. These cells either stay close to the backbone to form back muscles or move out to form muscles in the arms and legs. Before they move, special connective tissues form a framework, and the myoblasts follow chemical signals to find their right places, where 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. Even though they are different, they all work by moving special things called actin against myosin. This movement makes the muscle shrink.

Skeletal muscle moves when electrical signals from our nerves tell it to. Cardiac and smooth muscles work on their own, with special cells that beat regularly. Smooth muscle is found in many places in our body, like our stomach and blood vessels. It helps move things along and can hold things closed. 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, also have three types of muscle tissue. These types are named based on 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 in a way that is most similar to the skeletal muscles in vertebrates, which are the muscles we use to move our bodies.