Muscle is a soft tissue that is present in the human body and is responsible for various functions. There are three types of muscles in the human body: skeletal, cardiac, and smooth muscle. Each muscle type has unique cellular components, physiology, specific functions, and pathology.

• Skeletal Muscle: Skeletal muscle is attached to bones and is responsible for skeletal movements. It is under conscious, or voluntary, control. The basic unit is the muscle fiber with many nuclei. These muscle fibers are striated (having transverse streaks) and each acts independently of neighboring muscle fibers.

• Smooth Muscle: Smooth muscle is found in the walls of the hollow internal organs such as blood vessels, the gastrointestinal tract, bladder, and uterus. It is under control of the autonomic nervous system and cannot be controlled consciously. The non-striated (smooth) muscle cell is spindle-shaped and has one central nucleus. Smooth muscle contracts slowly and rhythmically.

• Cardiac Muscle: Cardiac muscle is found in the walls of the heart and is also under control of the autonomic nervous system. The cardiac muscle cell has one central nucleus, like smooth muscle, but it also is striated, like skeletal muscle. The cardiac muscle cell is rectangular in shape. The contraction of cardiac muscle is involuntary, strong, and rhythmical.

Skeletal muscle constitutes approximately 40% of the total human body weight. Its composition is many individual fibers bundled together into a muscle spindle; this gives the skeletal muscle a striated appearance. A single muscle fiber is composed mostly of actin and myosin fibers covered by a cell membrane (sarcolemma). These fibers are the functional unit of the organ, leading to contraction and relaxation.

Muscles play a role in nearly every system and function of the body. Different kinds of muscles help with:

• Breathing, speaking, and swallowing.
• Digesting food and getting rid of waste.
• Moving, sitting still, and standing up straight.
• Pumping blood through the heart and blood vessels.
• Pushing a baby through the birth canal as muscles in the uterus contract and relax.
• Seeing and hearing.

Injuries, disorders, and diseases can affect how muscles work. These conditions can cause muscle pain, muscle spasms, or muscle weakness. More severe disorders can lead to paralysis.