Normal movement involves changes in posture, maintenance of balance, and mechanisms that place the limbs and fingers precisely where they are needed. This and the following chapter are concerned with the pathways that are responsible for the planning, initiation, sequencing and smooth coordination of muscle activities while also maintaining posture and balance.

The corticospinal tract consists of neurones whose cell bodies are in the primary motor cortex (in the pre-central gyrus) and have axons that terminate in the ventral horn of the spinal cord. The majority of these axons cross the midline in the decussation of the pyramids within the medulla; as a consequence the motor cortex of one hemisphere controls the movements of the opposite side of the body. Sometimes the term 'upper motoneurone' is used to describe this function.

The motor cortex (the pre-central gyrus) contains a somatotopic map of the range of movements of the opposite side of the body; each area within the map can induce contractions of relevant muscle groups rather than individual muscles. This is the executive pathway in the control of voluntary movements. For movements where the greatest precision is required, the area of motor cortex that controls that movement is markedly increased. The prehensile thumb of primates has an especially large repertoire of movements, and has a very large area of motor cortex devoted to the control its muscles.

The cells of origin of the corticospinal tract are in the deeper layers of the pre-central gyrus and include pyramidal cells and the very large pyramidal neurones called Betz cells; they project to the ventral horn of the contralateral ventral horn as well as the somatic motor nuclei of the brainstem. The pre-motor cortex, in front of the motor cortex, is concerned with higher aspects of motor control, such the planning, initiation and sequencing of movements that are executed by the corticospinal tract.

Sone corticobulbar axons make contact with the motor nuclei of cranial nerves. In addition, many others terminate in pontine nuclei that pass a copy of the motor commands to the cerebellum, or in the brainstem reticular formation, where they synapse on bulbospinal axons concerned with movements and the tone of the body musculature - the stiffness of muscles generated by tonic activity in alpha and gamma motoneurones.

One of the bulbospinal pathways that receives input from the motor cortex and accompanies the corticospinal tract is the rubro-spinal tract. The pathway originates in the red nucleus of the midbrain, and has a similar role to the cortico-spinal tract, except that there is a synapse in the red nucleus, which also receives an input from the cerebellum. This pathway is said to be small in humans, but appears to provide motoneurones with a modified form of the motor command that takes into account the error signals generated by the cerebellum.

There is a heirarchy of circuits controlling motoneurones: at the simplest level are the spinal reflexes; the development of the brainstem and the flocculonodularlobe of the cerebellum added the sense of balance and the regulation of posture as well as eye movements.

The forebrain became much larger when, during evolution, animals adapted the upright posture; so the evolution of primates and humans was associated with a great increase in the size of the forebrain and cerebellum, and of pathways between the two. These developments are sometime referred to as the neo-cortex and neo-cerebellum.

The development of the forebrain and consciousness gave organisms the power to plan, coordinate and execute voluntary movements. Subcortical forebrain structures such as the basal ganglia are intimately involved in planning and sequencing voluntary movements, while the actual movements are executed by the cortico-spinal and other pathways. During the movements, the cerebellum is intimately involved in the coordination of the musculature, using feedback from systems that track the progress of the limbs as they move towards the desired outcome.