Reversed Somatotopical Representations in the Subthalamic Nucleus as Revealed by Cortical Inputs from the Primary Motor Cortex and the Supplementary Motor Area

Abstract

Since discrete lesions in the subthalamic nucleus (STN) result in a violent form of dyskinesia, termed “hemiballism”, which is characterized by involuntary movements occurring in the contralateral limbs (Hamada and DeLong, 1992; Carpenter and Carpenter, 1951; Carpenter et al., 1950; Whittier and Mettler, 1949; Whittier, 1947; Martin, 1927), the STN has long been recognized to play crucial roles in somatic motor control (DeLong, 1990; Albin et al., 1989; Kitai and Kita, 1987). It seems a consensus that the STN, a driving force of the basal ganglia, is composed of a single set of body part representations which is restricted to the lateral STN (Wichmann et al., 1994; Alexander and Crutcher, 1990; DeLong et al., 1985; Hartmann-von Monakow et al., 1978). However, such a somatotopical map of the STN does not provide a scenario that explains the somatotopical specificity of ballistic movements when the nucleus is destroyed in varying degree (Hamada and DeLong, 1992; Carpenter and Carpenter, 1951; Carpenter et al., 1950; Whittier and Mettler, 1949; Whittier, 1947; Martin, 1927). Here, we report that dual sets of body part representations underlie the somatotopical organization in the STN. The first set of body part representations is transformed to the medial STN from the supplementary motor area (SMA) that is the secondary motor cortex involved in various aspects of motor behavior (Tanji, 1994; Wiesendanger, 1986), whereas the second set is transformed to the lateral STN from the primary motor cortex (MI). The somatotopical arrangements of these two distinct cortical inputs are reversed mediolaterally in the STN. This could reconcile with the occurrence of somatotopically-specific dyskinesias in hemiballism.