Timing of knee-related spinal neurons during fictive rostral scratching in the turtle.

Knee-flexor motor activity rhythmically alternated with knee-extensor motor activity during fictive rostral scratching in the spinal turtle. A critical transition from knee-flexor motor activity to knee-extensor motor activity occurred during hip-flexor motor activity. A key feature of this transition was that the end-phases of knee-flexor motor activity were positively correlated with the start-phases of knee-extensor motor activity. We studied spinal interneurons with activities related to this transition. We previously used single-unit recording techniques to characterize a data set of descending propriospinal interneurons during rostral scratching. We focused here on a group of interneurons from this data set with start-phases (on-units) or with end-phases (off-units) near the start of knee-extensor motor activity. We showed that, for a subset of these units, the start-phases of on-units and the end-phases of off-units were positively correlated with the start-phases of knee-extensor motor activity. We present the hypothesis that some of these knee-related on- and off-units may play a role in timing knee motor activity during rostral scratching.