circulation, digestion, and even such functions as speaking and seeing.
At one end of some muscles are long white TENDONS that attach the muscles to bone. The point of fixed attachment of a muscle to bone is called the ORIGIN. The more flexible attachments, especially to a movable bone, is termed the INSERTION.
Muscle tissue has a highly developed ability to contract. CONTRACTIBILITY enables a muscle to become shorter or thicker, and this ability, along with interaction with other muscles, produces movement in internal and external body parts. Muscle contraction in a tissue or organ produces motion and provides power and speed for body activity. A contracting muscle is referred to as a PRIME MOVER. A muscle that is relaxing while a prime mover is contracting is called the ANTAGONIST. Muscular tone, or TONICITY, is a continual state of partial contraction that gives muscles a certain firmness. ISOMETRIC muscle contraction occurs when the muscle is stimulated and shortens, but no movement occurs, as when a person tenses his or her muscles against an immovable object. Muscles are also capable of stretching when force is applied (EXTENSIBILITY) and regaining their original form when that force is removed (ELASTICITY).
All types of muscles respond to stimulus. This property is called EXCITABILITY or IRRITABILITY. The mechanical muscular action of shortening or thickening is activated by a stimulus sent through a motor nerve. All muscles are linked to nerve fibers that carry messages from the central nervous system.
The chemical action of muscle fibers consists of two stages, CONTRACTION and RECOVERY. In the contraction stage, two protein substances (actin and myosin) react to provide energy through the breakdown of glycogen into lactic acid. In the recovery stage, oxygen reacts with lactic acid to release carbon dioxide and water.
When a muscle contracts, it produces chemical waste products (carbon dioxide, lactic acid, and acid phosphate), which make the muscle more irritable. If contraction is continued, the muscle will finally cramp up and refuse to move. This condition is known as fatigue. If it is carried too far, the muscle cells will not recover and permanent damage will result. Muscles, therefore, need rest to allow the blood to carry away the waste materials and bring in fresh glucose, oxygen, and protein to restore the muscle protoplasm and the energy that was used.
The importance of exercise for normal muscle activity is clear, but excessive muscle strain is damaging. For example, if a gasoline motor stands idle, it eventually becomes rusty and useless. Similarly, a muscle cell that does not work becomes weak and flabby. On the other hand, a motor that is never allowed to stop and is forced to run too fast or to do too much heavy work soon wears out so that it cannot be repaired. In the same way, a muscle cell that is forced to work too hard without proper rest will be damaged beyond repair. Violent exercise is never good. Exercise should be adapted to the individual and should never be carried to the point of extreme fatigue.
During exercise, massage, or ordinary activities, the blood supply of muscles is increased. This brings in fresh nutritional material, carries away waste products more rapidly, and enables the muscles to build up and restore their efficiency and tone.
When a muscle dies, it becomes solid and rigid and no longer reacts. This stiffening, which occurs from 10 minutes to several hours after death, is called
Muscles seldom act alone; they usually work in muscle groups held together by sheets of a white fibrous tissue call FASCIA. There are three types of muscle tissue: skeletal, smooth, and cardiac. Each is designed to perform a specific function.
SKELETAL MUSCLES are attached to the bones and give shape to the body. They are responsible for allowing body movement. This type of muscle is sometimes referred to as STRIATED because of the striped appearance of the muscle fibers under a microscope (fig. 3-24). They are also called VOLUNTARY muscles because they are under the control of our conscious will. These muscles can develop great power.
SMOOTH, or NONSTRIATED, muscle tissues are found in the walls of the stomach, intestines, urinary bladder, and blood vessels, as Figure 3-24.Striated muscle fibers.