The ultimate function of the nervous system is the control of behaviour. Because there is no single cause of behaviour, we cannot find a single starting point in our search for the neural mechanisms that control movement. The brain and spinal cord include several different motor systems, each of which can simultaneously control particular kinds of movements. The primary motor cortex lies on the precentral gyrus, rostral to the central sulcus. The primary motor cortex shows somatotopic organization, which can be illustrated by penfield"s motor homunculus: a disproportionate amount of cortical area is devoted to the movements of fingers and the muscles used for speech. It is important to recognize that the primary motor cortex is organized in terms of particular movements of particular parts of the body: each movement may be accomplished by the contraction of several muscles. The principle cortical input to the primary motor cortex is the frontal association area, which is rostral to it.