The maintenance of membrane potential is explained. The changes during depolarisation and subsequent repolarisation are described. The structure of the peripheral nerve determines its conduction velocity by virtue of its diameter and whether or not it is myelinated. The effect of the high resistance of myelin is usually well understood but that of its low capacitance much less so. This is addressed. The concepts of absolute and relative refractory periods are described, as well as how these govern the firing frequency of the nerve and the direction of nerve impulse propagation.