‘Signalling in the brain’ outlines the mechanisms of nervous signalling, which needs to be accurate, reliable, and quick. It is brought about by the movement of ‘action potentials’ across neuronal membranes. Both excitatory and inhibitory signals act on neurons, and it is the summation of these that determine whether they themselves generate an action potential. The signal is amplified by the control of ionic concentration gradients across the neuronal membrane. Speed of transmission can be increased either by increasing neuronal diameter or insulating the neuronal membrane. Neurons communicate directly at synapses, but other signals can also affect them. The interactions between these cells give rise to almost unimaginable complexity.