Muscular System : Muscles are the .moving force behind our movements. They along with the bones enable us to exercise the body in various ways. Muscles are attached to the bones of the skeleton. They give a rounded shape to the body and also help in the protection of some of the organs along with the bones. Muscles provide the energy for movement by converting the stores of potential energy found in the muscle cells into mechanical energy in the form of work and heat.
Muscles are composed of muscle cells. Like all living cells, muscle cells are excited or aroused by a stimulus. As a result of this effect, they respond to the stimulus by the contraction of its fibres. The ability to contract is the most important character of muscles. The fibres of a muscle contract in the direction of their length and, therefore, shorten the muscle during contraction.
The muscles of the muscular system form about 45% of the body weight and are of three varieties :
1. Voluntary or striped muscles which are under the control of our will and produce the movements of the skeleton.
2. Involuntary or unstriped muscles which are not under the control of our, will and form the muscular coats of intestines, bladder, arteries, etc.
3. Cardiac muscles which are striped but still not under the control of our will. This type of muscle is found only in the heart.
Structure of Muscles :
Because we are mainly concerned with the voluntary muscles of our body, it would be appropriate to have knowledge of its structure. Voluntary muscles are made up of a large number of fibres. Each fibre is a cell enclosed in an elastic sheath or covering called Sarcolemma. Every muscular cell or fibre has a nucleus, protoplasm and long filaments called Myofibrils. Fibres are attached to other fibres with the help of connective tissues but are insulated from one due to the sheath. These fibres are arranged in parallel bundles. An another muscle is formed by the union of many such bundles. Long filaments or threads named myofibrils are the main contracting parts of a muscle cell. The muscles are attached to the bones with the help of tendons which are strong and elastic.
Functioning of Muscles:
The stimulus for muscular action or contraction is received by a muscle fibre through a separate nerve branch. These branches pass the impulses for contraction to the contracting threads through the end plates of nerve fibres found in the mass-of the protoplasm of the cell.
The energy for the contraction of a muscle fibre is received from the nutrients in the blood which are sent to the connective tissue surrounding the muscle fibres by the capillaries of the circulatory system.