Analyses of fictive scratching motor patterns in the spinal turtle with transverse hemisection provided support for the concept of bilateral shared spinal cord circuitry among neurons responsible for generating left- and right-side rostral, pocket, and caudal fictive scratching. Rhythmic bursts of hip flexor activity, the hip extensor deletion variation of fictive rostral scratching, were elicited by ipsilateral stimulation in the rostral scratch receptive field of a spinal turtle [transection at the segmental border between the second (D2) and third (D3) postcervical spinal segments] with a contralateral transverse hemisection one segment anterior to the hindlimb enlargement (at the D6-D7 segmental border). In addition, other sites were stimulated in this preparation: (1) contralateral sites in a rostral, pocket, or caudal scratch receptive field or (2) ipsilateral sites in a caudal scratch receptive field. A reconstructed fictive rostral scratch motor pattern of rhythmic alternation between hip flexor and hip extensor activation was produced by simultaneous stimulation of one site in the ipsilateral rostral scratch receptive field and another site in one of the other scratch receptive fields. This reconstructed rostral scratch motor pattern resembled the normal rostral scratch motor pattern produced by one-site rostral scratch stimulation of a spinal turtle (D2-D3 transection) with no additional transections. The observation of a reconstructed rostral scratch motor pattern produced by two-site stimulation in the spinal turtle with transverse hemisection supports the concept that hip extensor circuitry activated by stimulation of other scratch receptive fields is shared with circuitry activated by ipsilateral rostral scratch receptive field stimulation.
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).
The usual interlimb coordination pattern during fictive rostral scratching in turtles is 1:1 coordination. We describe an example in a turtle of 2:1 coordination during fictive rostral scratching in which there were two cycles of ipsilateral hip flexor activity during each cycle of contralateral hip extensor activity. During 2:1 coordination, there were fluctuations in the ipsilateral hip flexor cycle period such that a larger ipsilateral hip flexor normalized period, which was associated with the onset of a contralateral hip extensor burst, alternated with a smaller ipsilateral hip flexor normalized period, which was associated with the absence of the onset of a contralateral hip extensor burst. These observations support the concept that contralateral circuitry modulates the timing of ipsilateral motor rhythms and therefore contributes to the production of the ipsilateral motor pattern for rostral scratching.
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