Muscle

Muscle is a specialized soft tissue found in animals. It is one of the four basic types of animal tissue.

This tissue is essential because it allows for movement and the maintenance of bodily functions. Muscles function by contracting and relaxing to create force. This ability is what enables everything from running to the beating of a heart.

At the microscopic level, muscle movement is a mechanical process. It relies on the interaction of contractile proteins called actin and myosin. These proteins slide against each other to cause the tissue to shorten. Two regulatory proteins, troponin and tropomyosin, help control this interaction.

This movement happens within organized units called sarcomeres. In striated muscles, these sarcomeres are arranged in parallel bundles of myofibrils. This specific arrangement creates the striped appearance seen under a microscope.

Vertebrates possess three distinct types of muscle tissue. The first is skeletal muscle, which is striated and attached to bones by tendons or aponeuroses. This type is responsible for locomotion and maintaining posture.

The second is cardiac muscle, which is also striated but found only in the heart walls, known as the myocardium. It connects at branching angles called intercalated discs. The third type is smooth muscle, which is non-striated and found in the walls of internal organs.

Smooth muscle can be single-unit, acting as a connected mass, or multiunit, allowing for finer control.

Skeletal muscle is classified by how it produces energy and speed. Type I fibers are known as slow-twitch or red muscles. They are rich in myoglobin and mitochondria, which allows them to sustain aerobic activity.

Type II fibers are fast-twitch muscles. Type IIa fibers are aerobic and appear red when deoxygenated. Type IIx is the fastest muscle type in humans, capable of intense bursts of activity. Finally, Type IIb is an anaerobic, white muscle that is less dense in myoglobin.

Muscle tissue forms during embryonic development through a process called myogenesis. All muscles derive from the paraxial mesoderm. This tissue divides into segments called somites. Each somite contains a division called the myotome, which produces muscle.

Special cells called myoblasts migrate to specific locations to fuse into long muscle cells. This complex migration is guided by chemical signals and connective tissue frameworks.

There are significant differences in how these muscles are controlled. Skeletal muscle is voluntary, meaning it is controlled by the central nervous system. However, reflexes allow for non-conscious activation of skeletal muscles through the nervous system.

In contrast, cardiac and smooth muscles are involuntary. They can be activated by the autonomic nervous system, hormones, or internal pacemaker cells. For example, cardiac muscle relies on a constant blood and electrical supply to function.

Muscle mass and density vary between individuals and species. In an average adult man, skeletal muscle makes up 42% of body mass. In an average adult woman, it accounts for 36%.

Muscle tissue is also denser than fat. Muscle has a density of approximately 1.06 kg/liter, while adipose tissue is about 0.9196 kg/liter. This makes muscle about 15% denser than fat. The word "muscle" itself comes from the Latin "musculus," meaning "little mouse," because a flexing muscle resembles a mouse moving under the skin.