Somatotopic analysis of fibre and terminal distribution in the primate corticospinal pathway

This is an anatomical study of the precision of fibre and terminal orderliness in the direct corticospinal projection. It was conducted to assess the degree of somatotopy in this projection in primates and to deduce the mechanism(s) possibly responsible for guiding fibres to their segmental destinations in development. As fibres leave the cortex they are grouped in an orderly way so as to be placed (within the pathway cross-section) according to their points of origin. Systematic neighbourhood relations are rather abruptly broken down as the descending pathway traverses the pons. Fibres are randomly distributed within each medullary pyramid and this disorderliness is maintained throughout the spinal pathway in the dorsolateral columns. Nonetheless, fibres exciting thumb movements end selectively at cervical levels. Fibres exciting foot movements end preferentially at lumbar levels but also show a secondary maximum of terminations in cervical segments, with relatively few terminations at intervening thoracic levels. These two sets of fibres ('thumb', 'foot') are derived from distinct, separate subareas of motor cortex and pass through exclusive, separate zones of the internal capsule. Thus, developmental contact guidance between somatotopically originating neighbours is inadequate to account for the observed specificity of corticospinal fibre destinations. Fibres must be observing distinctive local segmental cues (perhaps of cytochemical nature) and/or utilising somatotopically distinctive arrival sequencing (i.e. securing specific destinations without neuronal recognition).