Right-left interactions between rostral scratch networks generate rhythmicity in the preenlargement spinal cord of the turtle

We examined the rhythmogenic capacity of the midbody D-3-D-7 spinal cord during stimulation of the rostral scratch reflex in turtles. Fictive scratching was recorded bilaterally as electroneurograms (ENGs) from prehindlimb enlargement nerves [transverse D-7 (TD7) and oblique D-7 (OD7)] and hip flexor nerves (HF). TD7 and OD-(7) innervate transverse-and oblique abdominus muscles, respectively. D-3-end preparations had intact spinal cords caudal to a D-2-D-3 transection site. Unilateral stimulation of the rostral receptive field in D-3-end preparations evoked rhythmic bursting in the ipsilateral (ipsi) HF nerve and bilateral rhythmic discharge in the TD7 and OD7 nerves. Right HF bursts were coactive with right TD7 and left OD7 bursts and alternated with left TD7 and right OD7 bursts. D-3-D-7 preparations received a second spinal transection at the caudal end of segment D-7, thus resulting in activation of strictly preenlargement circuitry in response to rostral scratch stimulation and preventing activation of hindlimb enlargement circuitry in segments D-8-S-2. D-3-D-7 preparations responded to unilateral stimulation with modulated or tonic discharge in the ipsi TD7 and contralateral (contra) OD7 nerves. In contrast, bilateral stimulation reestablished robust bursting in which coactive right TD7-left OD7 bursts alternated with coactive left TD7-right OD7 bursts. These data imply that TD7 circuit modules make 1) crossed excitatory connections with contra OD7 circuitry, 2) crossed inhibitory connections with contra TD7 circuitry, and 3) uncrossed inhibitory connections with ipsi OD7 circuitry. Our results also suggest that bilateral stimulation evokes rhythmic alternation in the preenlargment card by simultaneously exciting reciprocally inhibitory circuit modules.