The Human Brain : From Neurone to Nervous System
- The pre-central gyrus contains the motor cortex, organised as a somatotopic map of the full range of movements possible on the opposite side of the body. The delicate control of fine movements is carried out by larger areas of cortex than those required for the execution of coarse movements.
The pre-central gyrus contains the motor cortex, organised as a somatotopic map of the full range of movements possible on the opposite side of the body. Stimulation of each part of the map induces a movement using the relevant muscle groups rather than individual muscles. This is the executive pathway in the control of voluntary movements.
Very precise movements are executed by small motor units, and large areas of motor cortex are involved in their control. 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 spinal grey matter, and 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 and sequencing of movements that are executed by the corticospinal tract.
Some 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.