Organization of Motor Cortical Inputs to the Subthalamic Nucleus in the Monkey

Abstract

To investigate the motor cortical inputs to the subthalamic nucleus (STN), we examined the responses of STN neurons to electrical stimulation of the primary motor cortex (MI) and the supplementary motor area (SMA). Stimulating electrodes were chronically implanted in the orofacial, forelimb, and hindlimb regions of the MI, and the forelimb and hindlimb regions of the SMA in a macaque monkey. Cortical stimulation induced a neuronal response composed of an early excitation and a late excitation in the STN. Of the STN neurons with cortical inputs, approximately 70% responded exclusively to either MI- or SMA-stimulation and the remaining 30% responded to both MI- and SMA-stimulation. Neurons responding mainly to MI-stimulation were distributed in the dorsolateral part of the STN, while neurons responding mainly to SMA-stimulation were mainly present in the ventromedial region. Approximately 60% of STN neurons received inputs from a single somatotopical region, while the others received convergent inputs from multiple somatotopical regions. In the MI domain of the STN, neurons responding to the stimulation of the orofacial, forelimb, and hindlimb regions were represented in its lateral-most, central and medial parts, respectively. In the SMA domain, neurons receiving inputs from the forelimb region were located more medially than those receiving inputs from the hindlimb region. The present study has clearly demonstrated that the STN is somatotopically organized based on the cortical inputs from the MI and SMA despite considerable convergence.