MANUAL DEXTERITY - HOW DOES THE CEREBRAL-CORTEX CONTRIBUTE

1. Manual dexterity, of great evolutionary significance to the primate s, ranges in complexity from the precise opposition of finger and thum b to Brendal playing Mozart. All dexterity depends on a sustained and rapid transfer of sensorimotor information between the cerebral cortex and the cervical spinal cord. 2. Multiple separate corticospinal neur on populations originate from cortical areas four, the supplementary m otor area, anterior cingulate, postarcuate, parietal and insular corte x. Each corticospinal neuron population projects in parallel to all sp inal segments, and has a distinctive pattern of terminations. 3. Each corticospinal neuron population has a unique thalamic input which can relay particular sensorimotor information from the sense organs, cereb ellum and basal ganglia. The overall structural framework of these sen sorimotor pathways, with many parallel corticospinal channels, with in terconnections in the cerebral cortex and spinal cord to enable crosst alk between the channels, is that needed for parallel distributed proc essing, which would enable the very rapid transfer of information betw een the cerebral cortex and spinal cord needed for any sophisticated u se of the hand. 4. Hemisection of the cervical spinal cord in the maca que results in an immediate hemiplegia, with subsequent remarkable alt hough incomplete recovery of hand and finger movements. The only direc t corticospinal input to the hemicord caudal to the hemisection, even after 3 years, is the similar to 10% of flbres which cross the midline caudal to the lesion: the fibres 'spared' by the hemisection, A match ing 'sparing' of somatosensory input from the paresed limb also occurs . No regeneration of the interrupted pathways has been visualized usin g modern tracer techniques. 5. Cervical hemisection permanently reduce s the number of parallel channels which transmit information between c ortex acid spinal cord, but does not reduce their cortical origins nor the neuron populations targeted in the spinal cord. We infer that the content of the information that can be transmitted between the cortex and spinal cord is not greatly changed, but the rate of transmission of this information is sharply reduced, and is the 'bottleneck' that l imits the complete recovery of dexterity following hemisection. The re markable recovery that does occur presumably reflects more economic tr ansmission of information by the few spared channels. We guess that th is involves substantial synaptic reorganization not visualized by the procedures we have used.