Fig. 5.

Recordings of an ON-unit descending propriospinal interneuron during 6 cycles of rostral scratch in the D₃–D₁₀ preparation. Top trace, descending propriospinal interneurons; vertical lines in 2nd trace represent the timing of action potentials in the ON-unit, the largest unit in top trace. The first, second, fourth, and sixth cycles of this episode are normal rostral scratch cycles. In each of the normal rostral scratch cycles, the start phase of the ON-unit was near the start phases of knee-extensor motor activities. The third and fifth cycles end with a hip-extensor deletion variation of rostral scratch with no hip-flexor quiescence between 2 successive hip-flexor motor bursts. In each of the cycles with a hip-extensor deletion, there is also a knee-extensor deletion in which there is no activity in each of the knee-extensor nerves. The ON-unit is quiet (q) during each of the knee-extensor deletions. These recordings support the concept that the ON-unit is a knee-extensor interneuron and a member of the knee-extensor module.