This chapter discusses physiology of nerve fibres, where axons which travel in peripheral nerves are called nerve fibres. A nerve cell or neuron consists of a cell body and a variable number of processes or extensions. The action potential is propagated without decrement along the membrane of the excitable cell. Myelinated nerve fibres are enclosed in a myelin sheath. Myelin is rich in lipids and is an excellent insulator. The nearest point where an action potential can induce depolarization electrotonically is the nearest node of Ranvier. Chronaxie is a better index of sensitivity than rheobase. If an arrangement is made for recording a monophasic potential from a whole nerve, the record consists of a large peak followed by a few smaller peaks. It is generally believed that neurons do not multiply during life, and therefore a neuron once lost is lost for good.