Right-Left Interactions Between Rostral Scratch Networks Generate Rhythmicity in the Preenlargement Spinal Cord of the Turtle

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Abstract
Currie, Scott N. and Gregory G. Gonsalves. Right-left interactions between rostral scratch networks generate rhythmicity in the preenlargement spinal cord of the turtle. J. Neurophysiol. 78: 3479–3483, 1997. We examined the rhythmogenic capacity of the midbody D3–D7spinal cord during stimulation of the rostral scratch reflex in turtles. Fictive scratching was recorded bilaterally as electroneurograms (ENGs) from prehindlimb enlargement nerves [transverse D7(TD7) and oblique D7(OD7)] and hip flexor nerves (HF). TD7and OD7innervate transverse- and oblique-abdominus muscles, respectively. D3-end preparations had intact spinal cords caudal to a D2–D3transection site. Unilateral stimulation of the rostral receptive field in D3-end preparations evoked rhythmic bursting in the ipsilateral (ipsi) HF nerve and bilateral rhythmic discharge in the TD7and OD7nerves. Right HF bursts were coactive with right TD7and left OD7bursts and alternated with left TD7and right OD7bursts. D3–D7preparations received a second spinal transection at the caudal end of segment D7, thus resulting in activation of strictly preenlargement circuitry in response to rostral scratch stimulation and preventing activation of hindlimb enlargement circuitry in segments D8–S2. D3–D7preparations responded to unilateral stimulation with modulated or tonic discharge in the ipsi TD7and contralateral (contra) OD7nerves. In contrast, bilateral stimulation reestablished robust bursting in which coactive right TD7-left OD7bursts alternated with coactive left TD7-right OD7bursts. These data imply that TD7circuit modules make 1) crossed excitatory connections with contra OD7circuitry, 2) crossed inhibitory connections with contra TD7circuitry, and 3) uncrossed inhibitory connections with ipsi OD7circuitry. Our results also suggest that bilateral stimulation evokes rhythmic alternation in the preenlargment cord by simultaneously exciting reciprocally inhibitory circuit modules.