Muscles contract or relax when they receive signals from

Answer: motor neuron or the nervous system

The contraction and relaxation of our skeletal muscles which move our body are caused by the messages or signals from our nervous system.

The whole process called the mechanism of muscle contraction involves three steps:

(1) the nervous system sends a message to the muscular system (resulting in chemical reactions).

(2) muscle fibers contracts

(3) muscle relaxes, when there is no more signal from nervous system.

Muscle contraction or relaxation is caused by signals from the nervous system. The signal travels from the nervous system to the muscular system. A chemical reaction is synthesized once, the signal reach the muscular system. The chemical reaction enables the muscle fibers to shorten, thus contraction occurs. And when the chemical reaction is done the muscle goes back to its original shape, or relaxes.