Skeletal and Smooth MusclesMuscles

Skeletal and Smooth Muscles

Muscles are the active part of the motor apparatus: their contractions are producing various movements, when they are active. Functionally we divide all muscles into two groups: voluntary and involuntary muscles.

Voluntary muscles consist of striated muscle tissue and contract by the will of the man. This group includes all the muscles of the head,

trunk and extremities, i.e., the skeletal muscles, as well as those of some internal organs (tongue, larynx, etc.). The skeletal muscles are the organs of the muscular system. There are more than 400 skeletal muscles in the human organism: in adults they make up about two-fifths of the total body weight. Each skeletal muscle has an arterial, venous, lymphatic and nervous supply. Muscles must always act in groups.

Skeletal muscles are complex in structure. They consist of muscle fibres of different length (up to 12 cm); the fibres are usually parallel to each other and are united (соединены) in bundles. Each muscle contains many such bundles. There are tendons at the ends of muscles by means of which they are bound (связаны) to bones.

Smooth muscles form the muscular coat of internal organs such as esophagus, stomach and intestines, bladder, uterus and so on. They also form the part of the capsule and the trabeculae of the spleen; they are present as single cells or as little cylindrical bundles of cells in the skin. They also form the walls of arteries, veins and some of the larger lymphatics. Smooth muscles are not rich in blood vessels, as are striated muscles. A smooth muscle is capable of spontaneous contraction and can contract in two ways. Firstly, individual cells may contract completely and secondly, a wave of contractions may pass from one end of the muscle to the other. Smooth muscle cells are usually elongated cells. In the skin and intestines they are long and thin, but in the arteries they are short and thick. They vary in length from 12-15 mm in small blood vessels to 0.5 mm in the human uterus but their average length in an organ such as the intestine is about 200 mm. These cells have an oval nucleus that encloses nucleoli, and when the cell is contracting the nucleus may become folded or twisted.

Muscles have both motor and sensory nerve fibres. Impulses (signals) about the state of the muscle reach the brain along the sensory fibres. The nerve impulses which cause the muscle to contract come from the brain along the motor fibres. Injury to the nerves which innervate muscles causes disturbances in voluntary movements (muscular paralysis).