Somatosensory input onto pyramidal tract neurons in rodent motor cortex.

This study was performed to determine whether pyramidal tract neurons (PTNs), the output cells of the motor cortex, are targets of direct input from the somatosensory cortex (SI). PTNs were identified by light microscope (LM) and electron microscope (EM) retrograde tract tracing techniques. For LM analysis, they were intracellularly filled to exhibit their full dendritic arbor. Axons originating from the electrophysiologically identified forelimb representation of SI were labeled by LM and EM anterograde tract tracing techniques. LM analysis of dual-labeled tissue showed that PTNs were situated to receive direct input from SI. The distribution and density of this input was predicted from the findings. EM analysis confirmed that PTNs receive direct input from SI. The input is sparse and not all PTNs appear to receive input. SI terminals do not appear to target specific segments of PTN dendrites. This corticocortical pathway may provide MI with the short latency sensory feedback which is needed to produce coordinated voluntary movements.