Stumbling Corrective Reaction During Fictive Locomotion in the Cat

Abstract

. 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. After 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 seems 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.

A Neural Basis for Motor Primitives in the Spinal Cord

“…Our data are consistent with the modular schemes of Cappellini et al (2006) andd'Avella et al (2006) in man and with Krouchev et al (2006) and McCrea and Rybak and their colleagues in the cat (Quevedo et al, 2005;McCrea and Rybak, 2007). They suppose that locomotor pattern generation comprises separated rhythm generation and pattern shaping with layered connections.…”

Organization of mammalian locomotor rhythm and pattern generation

“…Intracellular recordings from motoneurons show, however, that this mixed synergy is not due to the activation of extensor CPG circuitry during the flexion phase. Rather it results from an excitatory cutaneous reflex directed to ankle extensor motoneurons (Forsberg 1979, Quevedo et al, 2005a. Cutaneous reflex excitation overcomes the swing phase-related inhibition produced by the CPG and activates ankle extensors (Quevedo et al, 2005b).…”

“…Rather it results from an excitatory cutaneous reflex directed to ankle extensor motoneurons (Forsberg 1979, Quevedo et al, 2005a. Cutaneous reflex excitation overcomes the swing phase-related inhibition produced by the CPG and activates ankle extensors (Quevedo et al, 2005b). The point here is that not all motoneuron activity occurring during locomotion necessarily involves the locomotor CPG.…”

Individual Premotor Drive Pulses, Not Time-Varying Synergies, Are the Units of Adjustment for Limb Trajectories Constructed in Spinal Cord

“…In such bursts, muscles must covary proportionally and synchronously. The premotor drive bursts may relate to the pattern shaping layers in spinal pattern generation suggested by McCrea and colleagues (Lafreniere-Roula and McCrea, 2005;Quevedo et al, 2005;McCrea and Rybak, 2007). An alternative perspective is the idea of time-varying synergies.…”