The Florey lecture, 1987. Corticomotoneuronal projections: synaptic events related to skilled movement.

During infancy, children develop an expanding repertoire of movement skills in parallel with the maturation in their brains of direct nerve-fibre connections between the cerebral cortex and motoneurons in the spinal cord. These corticomotoneuronal connections are characteristic of primates and can be studied in monkeys; in these animals, refinement in the control of movements of the hand is also associated with increasing development of corticomotoneuronal connections. In monkeys, motoneurons innervating distally acting muscles are preferentially excited by convergent activities in corticomotoneuronal fibres. This excitation has been demonstrated to be effective in natural functional states when a conscious monkey is performing learned movement tasks. Extensive intraspinal arborizations of individual corticomotoneuronal fibres could permit the engagement of large numbers of local motoneurons and related interneurons by each of these fibres. Abolition of corticomotoneuronal influences, after section of the pyramidal tracts, causes a permanent deficit in fractionation of use of muscles of the forelimb and an inability to carry out independent movements of the fingers.