The main excitable cell in the nervous system is the neurone. Non-excitable cells or glial cells support neurones and perform various other functions (Figure 18.1). Neurones specialise in processing and transmitting information. The human nervous system contains between 10 and 10 neurones. Functionally, neurones are classified as sensory, motor and interneurone. Structurally, a typical neurone is made up of three parts: a cell body, an axon and terminal buttons (Figure 18.2). The cell body consists of intracellular organelles by which the cell maintains its functional and structural integrity. The axon originates from the cell body and divides into terminal branches; each branch terminates in enlarged endings called terminal buttons. The axon is a long projection surrounded by supporting cells (oligodendrocytes or Schwann cells). When there is a layer of lipid–protein complex deposited within the Schwann cell membrane, the neurone is said to be myelinated, otherwise it is unmyelinated. Myelination allows saltatory conduction with an accompanying increase in speed of propagation of a nerve impulse. Mammalian neurones have varying fibre diameters and speeds of conduction, as summarised in Figure 18.3.