Muscular System

Muscles are organs composed mainly of muscle cells, which are also called muscle fibers (mainly in skeletal and cardiac muscle) or myocytes (mainly in smooth muscle).

Muscles are organs composed mainly of muscle cells, which are also called muscle fibers (mainly in skeletal and cardiac muscle) or myocytes (mainly in smooth muscle).

In the organs of the digestive system, smooth muscles squeeze food through the gastrointestinal tract by contracting in sequence to form a wave of muscle contractions called peristalsis.

Muscles can grow larger, or hypertrophy.

Muscles can also grow smaller, or atrophy, which can occur through lack of physical activity or from starvation.

As people grow older, there is a gradual decrease in the ability to maintain skeletal muscle mass, known as sarcopenia.

The point where a motor neuron attaches to a muscle is called a neuromuscular junction.

The two systems together are often referred to as the musculoskeletal system. Skeletal muscles are attached to the skeleton by tough connective tissues called tendons.

Skeletal muscle is muscle tissue that is attached to bones by tendons, which are bundles of collagen fibers.

Slow-twitch muscle fibers are dense with capillaries and rich in mitochondria and myoglobin, which is a protein that stores oxygen until needed for muscle activity. Relative to fast-twitch fibers, slow-twitch fibers can carry more oxygen and sustain aerobic (oxygen-using) activity.

Fast-twitch muscle fibers contain fewer capillaries and mitochondria and less myoglobin. This type of muscle fiber can contract rapidly and powerfully, but it fatigues very quickly.

Smooth muscle is muscle tissue in the walls of internal organs and other internal structures such as blood vessels.

Cardiac muscle is found only in the wall of the heart. It is also called myocardium.