Reciprocal Interactions in the Turtle Hindlimb Enlargement Contribute to Scratch Rhythmogenesis

Abstract

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.

Rostral spinal cord segments are sufficient to generate a rhythm for both locomotion and scratching but affect their hip extensor phases differently

“…The reliability of continuing HF activity as the indicator of HE-phase deletions was confirmed by the recording of the OA nerve, which generated bursts in phase with HE during nondeletion swimming and scratching. We observed that the likelihood of HE-phase deletions increased with caudal spinal cord transections during scratching, as previously found (Currie and Gonsalves 1999;Mortin and Stein 1989). During swimming, however, we rarely observed HE-phase deletions even in the D3-D8 preparation.…”

“…However, the likelihood of HEphase deletions, defined by the omission of an HE burst and the corresponding HF quiescence, increases during turtle rostral scratching in such preparations (Currie and Gonsalves 1999;Mortin and Stein 1989;Stein and Daniels-McQueen 2004;Stein and Grossman 1980). It is unknown whether the caudal segments of the hindlimb enlargement are necessary for turtle swimming motor patterns or whether each spinal cord segment contributes equally to locomotion and scratching.…”

Stereological Estimate of the Total Number of Neurons in Spinal Segment D9 of the Red-Eared Turtle

“…In this respect, scratching in limbed vertebrates is an exception because the entire sensorimotor transduction is executed by a network of neurons confined to a few spinal segments without the need of other parts of the CNS (Gelfand et al, 1988;Stein, 2005). Scratching has been studied in detail in the cat (Orlovsky et al, 1999) and the turtle in vivo (Robertson and Stein, 1988;Currie and Gonsalves, 1999;Berkowitz, 2001Berkowitz, , 2002Berkowitz, , 2005Stein, 2005;Samara and Currie, 2008) but crucially, the spinal cord of the adult turtle is also uniquely amenable to experimentation in vitro (Keifer and Stein, 1983;Hounsgaard and Nicholson, 1990;Currie and Lee, 1996;Alaburda and Hounsgaard, 2003). In vitro experiments provide data of sufficient detail to make mathematical analysis of cell properties and synaptic network activity meaningful (Booth et al, 1997;Svirskis et al, 2000Svirskis et al, , 2001Svirskis and Hounsgaard, 2003;Berg et al, 2008).…”

Premotor Spinal Network with Balanced Excitation and Inhibition during Motor Patterns Has High Resilience to Structural Division

“…They demonstrated that the posterior 40 -80% of the hindlimb enlargement is not essential for both rostral and pocket scratch generation. Currie and Gonsalves (1999) performed similar transection experiments, indicating that at least some of D10 is required to have on/off phase separation in the unilateral rhythm. Since our ENG recordings have clear on/off separation (Fig.…”

“…Since our ENG recordings have clear on/off separation (Fig. 3C), the transection must have been more caudal than those performed by Currie and Gonsalves (1999). Nevertheless, the essential signals for motor pattern activation are carried by descending fibers (Berkowitz and Stein, 1994).…”