Electrical wires are generally protected by a tube of plastic. a similar insulating function is performed in the nervous system by the:

The myelin sheath is the protective layer, which is present in the outer layer of the neural cells. This myelin sheath prevents the loss of electrical impulses, which are being conducted in the neurons. This also act as an insulator between neuron and its external environment.

This layer is mainly made up of the protein and fatty substance. The degeneration of the myelin sheath result in many neurological disorders.

Hence, the correct answer is myelin sheath.