Muscle

Muscle is part of the muscular system or musculoskeletal system that is composed of specialized cells called muscle fibers.

Muscles of the hands

Function

Their predominant function is contractibility. Muscles, attached to bones or internal organs and blood vessels, are responsible for movement.

Movement and muscle action

• Nearly all movement in the body is the result of muscle contraction.
• Exceptions to this are the action of cilia, the flagellum on sperm cells, and amoeboid movement of some white blood cells.
• The integrated action of joints, bones, and skeletal muscles produces obvious movements such as walking and running.

Muscle fiber

Facial expressions

Skeletal muscles also produce more subtle movements that result in various facial expressions, eye movements, and respiration.

Posture and stability

• In addition to movement, muscle contraction also fulfills some other important functions in the body, such as posture, joint stability, and heat production.
• Posture, such as sitting and standing, is maintained as a result of muscle contraction.
• The skeletal muscles are continually making fine adjustments that hold the body in stationary positions.
• The tendons of many muscles extend over joints and in this way contribute to joint stability.
• This is particularly evident in the knee and shoulder joints, where muscle tendons are a major factor in stabilizing the joint.

Muscle fiber cells

Thermal actions of muscles

• Heat production, to maintain body temperature, is an important by-product of muscle metabolism.
• Nearly 85 percent of the heat produced in the body is the result of muscle contraction.
• The brown adipose tissue in the skin helps regulate body heat at times of cold climate.

Structure of skeletal muscles

• A whole skeletal muscle is considered an organ of the muscular system.
• Each organ or muscle consists of skeletal muscle tissue, connective tissue, nerve tissue, and blood or vascular tissue.
• Skeletal muscles vary considerably in size, shape, and arrangement of fibers.
• They range from extremely tiny strands such as the stapedium muscle of the middle ear to large masses such as the muscles of the thigh.
• Some skeletal muscles are broad in shape and some narrow.
• In some muscles the fibers are parallel to the long axis of the muscle; in some they converge to a narrow attachment; and in some they are oblique.

Types of muscles

There are three types of muscle: skeletal (striated), smooth, and cardiac.

Skeletal Muscle

Muscles of the lower extremity

• Skeletal muscle, attached to bones, is responsible for skeletal movements.
• The peripheral portion of the central nervous system (CNS) controls the skeletal muscles.
• Thus, these muscles are 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, found in the walls of the hollow internal organs such as blood vessels, the gastrointestinal tract, bladder, and uterus, is under control of the autonomic nervous system.
• Smooth muscle cannot be controlled consciously and thus acts involuntarily.
• The non-striated (smooth) muscle cell is spindle-shaped and has one central nucleus. Smooth muscle contracts slowly and rhythmically.

Cardiac Muscle

• Cardiac muscle, found in the walls of the heart, 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.

Muscle groups

• There are more than 600 muscles in the body, which together account for about 40 percent of a person's weight.

External links

• University of Dundee article on performing neurological examinations (Quadriceps "strongest")
• Muscle efficiency in rowing
• Muscle Physiology and Modeling Scholarpedia Tsianos and Loeb (2013)
• Human Muscle Tutorial (clear pictures of main human muscles and their Latin names, good for orientation)
• Microscopic stains of skeletal and cardiac muscular fibers to show striations. Note the differences in myofibrilar arrangements.

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