Spinal motor patterns in the turtle.

Rhythmic alternation between ipsilateral hip flexors and extensors occurs during the normal pattern of fictive rostral scratching in response to unilateral midbody stimulation in D3-end turtles (complete spinal transection posterior to the forelimb enlargement). Unilateral midbody stimulation evokes rhythmic bursts of ipsilateral hip flexor activity with no hip extensor activity in D3-end turtles with D6-D7 contralateral hemisection (transverse hemisection anterior to the hindlimb enlargement). Bilateral midbody stimulation in these turtles evokes reconstruction of rhythmic alternation between intact side hip flexors and extensors. These normal motor patterns in response to two-site stimulation are reconstructed because one-site stimulation in this preparation activates only hip flexor rhythms (J. Neurosci. 18: 467). Hip flexor rhythms can therefore occur without hip extensor activation. This supports the concept that reciprocal inhibition between flexor and extensor interneurons is not required for flexor motor rhythm generation. Reciprocal inhibition, when present, also contributes to rhythmicity (J. Neurophysiol. 78: 3479; see also Currie and Gonsalves, this volume). Both mechanisms for rhythmicity are included in the Grillner unit burst generator model: hip flexor unit burst generators may be rhythmogenic in the absence of hip extensor activity and reciprocal inhibition contributes to rhythmogenesis. Contralateral midbody stimulation assisted in the activation of ipsilateral hip extensor rhythmicity during reconstructed rostral scratching. This result provides additional support for the hypothesis that a bilateral shared core of hip interneuronal circuitry plays a critical role in the generation of the normal pattern of fictive rostral scratching (J. Neurosci. 15: 4343).