Which non metallic element is an essential constituent of nervous and muscular tissue?

Phosphorus

Explanation:

The nonmetallic element, phosphorus, can be found in the tissues of the muscles, bones and nerves, and in the chemical adenosine triphosphate which is the major source of energy for the cells of the body.

Phosphorus is mainly concentrated in the bones, while phosphorus is also required to make the protein molecules that repair the body tissues as well as for growth

Phosphorus is also important for nerve signaling and normal heartbeat, contraction of the muscles and functioning of the kidney.