Stumbling correction reflexes during fictive locomotion

Running title: Stumbling correction reflexes during fictive locomotion. SUMMARY An obstacle contacting the dorsal surface of a cat's hind foot during the swing phase of locomotion evokes a reflex (the stumbling corrective reaction) that lifts the foot and extends the ankle to avoid falling. We show that the same sequence of ipsilateral hindlimb motoneuron activity can be evoked in decerebrate cats during fictive locomotion. As recorded in the peripheral nerves, twice threshold intensity stimulation of the cutaneous superficial peroneal (SP) nerve during the flexion phase produced a very brief excitation of ankle flexors (e.g. tibialis anterior and peroneus longus) that was followed by an inhibition for the duration of the stimulus train (10-25 shocks, 200 Hz). Extensor digitorum longus was always, and hip flexor (sartorius) activity was sometimes inhibited during SP stimulation. At the same time knee flexor, and the normally quiescent ankle extensor motoneurons, were recruited (mean latencies 4 and 16 ms) with SP stimulation during fictive stumbling correction. Following the stimulus train, ankle extensor activity fell silent and there was an excitation of hip, knee and ankle flexors. The ongoing flexion phase was often prolonged. Hip extensors were also recruited in some fictive stumbling trials. Only the SP nerve was effective in evoking stumbling correction. Delivered during extension, SP stimulus trains increased ongoing extensor motoneuron activity as well as increasing ipsilateral hip, knee and ankle hindlimb flexor activity in the subsequent step cycle. The fictive stumbling corrective reflex appears functionally similar to that evoked in intact, awake animals and involves a fixed pattern of short-latency reflexes as well as actions evoked through the lumbar circuitry responsible for the generation of rhythmic alternating locomotion.