These are essentially machines for converting chemical energy into mechanical work. These also adjust the body against the environmental changes.
The movement of the body as a whole from one point in space to another or the movement of a limited part of the body in respect to the body itself is also brought about by the muscles.
These also maintain the particular posture of the animal against the effects of gravity. In addition vital processes such as contraction of heart, constriction of blood-vessels, breathing, peristalsis of the digestive tract are also accomplished by muscles. All these functions result from the contraction and relaxation of muscles.
Muscles consist of thousands of elongated fibres or cells organized in a variety of ways and bound together by connective tissue.
These have the power to contract on getting the stimuli of any sort transmitted down the nerve that innervates them.
The particular kind of contraction exhibited by an innervated muscle depends largely upon the pattern of its innervations.
Although some muscles are capable of contracting independently of nervous stimulation, most muscles contract after a neuromuscular transmitter diffuses across a synaptic cleft from a nerve fibre to a muscle fibre.
The excitory or inhibitory effects of a transmitter are coupled to contraction and relaxation respectively.
The contractility of muscle is due uniquely to contractile proteins which are present in all muscle fibres but the organization of these proteins differs in different muscles. Different kinds of contractile units are found accordingly. These produce heat during muscular activity.