[Reconsidering somatotopy with respect to functional recovery from spinal cord injury]

Somatotopy, which is the representation of various body parts on the topographic map in the sensory and motor cortices, is one of the most well-known concepts in neuroscience. However, defining motor cortex areas, for instance, the hand region, the terms of somatotopic organization is a complex process. The "hand region" can be defined either by (i) low-threshold induction of twitch in hand muscles on repetitive microstimulation, (ii) detection of sensory inputs from the hand, (iii) presence of cortico-motoneuronal (CM) cells that communicate directly with the motoneurons of hand muscles, or (iv) presence of neurons that exhibit increase in firing activity before the onset of hand movements. The relationship between these criteria is not clear. Further the somatotopic map is known changes in response to environmental conditions and lesions of the central nervous system. However, the nature of changes depends on the manner in which a given region is defined. To define cortical regions on the basis of somatotopic organization, it is essential to carefully inspect the stimulus threshold for induction of movements and correlate the obtained response and the presence of CM cells. It is also necessary to identity the communicating pathways between the CM cells of a given region and the respective motoneurons. It will lead to understanding the relation between the structural change in the somatotopic representation map and its functional roles in the motor control.