Somatic sensory transmission to the cortex during movement: Phasic modulation over the locomotor step cycle

Abstract

We determined whether or not information transmitted through sensory pathways to the primary somatosensory (SI) cortex is selectively gated over the step cycle in locomotion. Moment-to-moment changes in the potential for sensory transmission through the afferent pathways to these cells were tested by delivering test stimuli through electrodes chronically implanted in the skin of the palm during different phases of the step cycle. The averaged intensities of these neural responses were calculated and plotted sequentially in a “perifootfall sensory gating histogram.” The results showed that the sensory responsiveness of all cells studied was strongly diminished when averaged for the whole step cycle, but most cells also exhibited short, phasic periods of facilitation (or “disinhibition”) of afferent input. We found that the group of neurons (described in the previous paper) that responded strongly when the forepaw touched the ground at footfall did so because they were phasically facilitated just before the footfall event, but were suppressed during all other phases. Conversely, the sensory inputs to the group of cells which did not respond to footfall were either tonically suppressed throughout the whole step cycle or were facilitated during the early swing phase. For this reason, these latter cells responded to touch of the area of their forepaw receptive fields with a hand-held probe during the swing phase of locomotion but did not respond to footfall. These results suggest that the afferent sensory pathways arising to the rat SI cortex may be subject to a gating process which is temporally specific.