Organization of human motor and somatosensory areas in the control of movement

Competing principles for the functional-anatomical organization of human primary motor cortex include first, whether muscles or movements are represented in the cortex; and second, whether the large-scale somatotopic principle also applies within the representation of limbs or whether it is replaced by mosaical network arrangement. We review evidence supporting a composite arrangement, wherein both pairs of the seemingly opposite mapping principles can coexist. Presence of somatotopic gradients within motor cortex allows for occasional clinical observations of relatively focal motor pareses. In contrast, there is much less controversy regarding somatotopic maps in the primary somatosensory cortex. Motor and somatosensory cortices show certain similarities, but also significant differences in their functional topography. While the distributed character dominates in M1 and S1, a somatotopic arrangement exists for both M1 and S1 hand representations, with the S1 somatotopy being more discrete and segregated in contrast to the more integrated and overlapping somatotopy in M1. Overall, the different topographic organization may reflect an organization optimized for three-dimensional control of a diverse repertoire of movements (M1) or for predominantly two-dimensional localization of somatosensory input (S1).