We examined the rhythmogenic capacity of the midbody D3-D7 spinal 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). TD7 and OD7 innervate transverse- and oblique-abdominus muscles, respectively. D3-end preparations had intact spinal cords caudal to a D2-D3 transection 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 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. D3-D7 preparations 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-D7 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 cord by simultaneously exciting reciprocally inhibitory circuit modules.

We examined interactions between the spinal networks that generate right and left rostral scratch motor patterns in turtle hindlimb motoneurons before and after transecting the spinal cord within the anterior hindlimb enlargement. Our results provide evidence that reciprocal inhibition between hip circuit modules can generate hip rhythmicity during the rostral scratch reflex. "Module" refers here to the group of coactive motoneurons and interneurons that controls either flexion or extension of the hip on one side and coordinates that activity with synergist and antagonist motor pools in the same limb and in the contralateral limb. The "bilateral shared core" hypothesis states that hip flexor and extensor (HF and HE) circuit modules interact via crossed and uncrossed spinal pathways: HF modules make reciprocal inhibitory connections with contralateral HF and ipsilateral HE modules and mutual excitatory connections with contralateral HE modules. It is currently unclear how much reciprocal inhibition between modules contributes to scratch rhythmogenesis. To address this issue, fictive scratch motor patterns were recorded bilaterally as electroneurograms from HF, HE, knee extensor (KE), and respiratory (d.D8) muscle nerves in immobilized animals. D3-end (low-spinal) preparations had intact spinal cords posterior to a complete D2-D3 transection. Unilateral stimulation of rostral scratch in D3-end turtles elicited rhythmic alternation between ipsilateral HF and HE bursts in most cycles; consecutive HF bursts were separated by complete silent (HF-OFF ) periods. D3-D9 and D3-D8 preparations received a second spinal transection at the caudal end of segment D9 or D8, respectively, within the anterior hindlimb enlargement. This second transection disconnected most HE circuitry (located mainly in segments D10-S2 of the posterior enlargement) from the rostral scratch network and thereby reduced the HE-associated inhibition of HF circuitry. Unilateral stimulation of rostral scratch in most D3-D9 and D3-D8 preparations evoked rhythmic or weakly modulated ipsilateral HF discharge without HF-OFF periods between bursts and without ipsilateral HE activity in the majority of cycles. In contrast, bilateral stimulation in D3-D9 and D3-D8 preparations reconstructed the HF-OFF periods, increased HF rhythmicity (assessed by fast Fourier transform power spectra and autocorrelation analyses), and reestablished weak HE-phase motoneuron activity. We suggest that bilateral stimulation produced these effects by simultaneously activating reciprocally inhibitory hip modules on opposite sides (right and left HF) and the same side (HF and residual ipsilateral HE circuitry). Our data support the hypothesis that reciprocal inhibition can contribute to spinal rhythmogenesis during the scratch reflex.

D uring fictive rostral scratching in low-spinal immobilized turtles, cutaneous stimulation of the lateral midbody generates a pattern of rhythmic discharge in ipsilateral hindlimb motor neurons which, in a moving animal, would cause the limb to reach toward and rub the stimulated site. 1 Sensory input from the rostral scratch-receptive field enters the midbody spinal cord segments (D3-D6) and activates rhythmic motor output from hindlimb enlargement (D8-S2: innervating hindlimb muscles) and preenlargement segments (D6-D7: innervating respiratory muscles). "D3-end" preparations are low-spinal turtles with intact spinal cords posterior to a D2-D3 transection site (D2 = the second postcervical segment). Unilateral rostral stimulation in D3-end preparations elicits fictive rostral scratching characterized by alternating discharge in ipsilateral hip flexor (HF) and hip extensor (HE) nerves; monoarticular knee extensor (KE) bursts occur during the late HF phase of each scratch cycle. 1 D3-end turtles also display weak motor output in contralateral hindlimb 2-4 and preenlargement 5 muscle nerves during unilaterally evoked rostral scratching that alternates with the stronger ipsilateral activity. Simultaneous bilateral stimulation in the right and left rostral receptive fields elicits bilateral rostral scratch motor patterns in which mirror-image nerve activity alternates from side to side. [2][3][4][5] These studies demonstrated right-left alternation during unilateral and bilateral fictive rostral scratching and supported the hypothesis that mirror-image hindlimb circuit modules are coupled by reciprocal inhibition.We recently showed that D3-D7 preparations, in which the entire hindlimb enlargement was surgically disconnected from the rostral scratch network, produced only tonic or weakly modulated motor output in D7 muscle nerves (ipsilateral "TD7" and contralateral "OD7") in response to unilateral rostral stimulation. 5 These data seemed to confirm the conclusion of Mortin and Stein 6 that the rhythmogenic capacity of the preenlargement spinal cord is very limited. However, bilateral stimulation of the same D3-D7 preparations produced robust rhythmicity in which coactive right TD7-left OD7 bursts alternated with left TD7-right OD7 bursts. The right-left alternation of homologous preenlargement motor pools (e.g., TD7) implied that the associated interneuronal circuitry is coupled by crossed reciprocal inhibition. In addition, these results provided correlative evidence that right-left reciprocal inhibition (as well as interactions between ipsilateral TD7 and OD7) can generate rhythmicity.In the present experiments, we wished to assess the ability of crossed reciprocal inhibition between right and left HF "modules" 4 to generate rhythmicity in hindlimb motor neurons. According to the "bilateral shared core" model of Stein et al., 4 HF and HE circuit modules, composing the core of the scratch rhythm generator, interact via crossed (interlimb) and uncrossed (intralimb) pathways: HF modules make (1) reciprocal inhibitory