Locomotion of intact and spinal cats on a tied-belt treadmill.

A and B. Step cycle, stance and swing phase durations for the right hindlimb during tied-belt treadmill locomotion of intact (A, from Frigon et al., 2015; Latash et al., 2020) and spinal (B, from Frigon et al., 2017; Latash et al., 2020) cats with an increasing treadmill speed. Data were obtained from 6-15 cycles in seven intact and six spinal cats (one cat was studied in both states). Modified from Fig. 3C, D of Latash et al., 2020, under the license CC-BY-4. C. Superimposed curves from A and B to highlight differences.

Locomotion of intact and spinal cats on a split-belt treadmill.

A. Step cycle, stance and swing phase durations for the left (slow) and right (fast) hindlimbs during split-belt treadmill locomotion of intact cats (from Frigon et al., 2015; Latash et al., 2020). B. Changes in the same characteristics for the left (slow) and right (fast) hindlimbs during split-belt treadmill locomotion in spinal cats (from Frigon et al., 2017; Latash et al., 2020). In both series of experiments the left (slow) hindlimb was stepping at 0.4 m/s while the right (fast) hindlimb stepped with speeds from 0.5 to 1.0 m/s with 0.1 m/s increments. Data were obtained from 6–15 cycles in seven intact and six spinal cats (one cat was studied in both states). Modified from Fig. 6A,B of Latash et al., 2020, under the license CC-BY-4.

Modeling a single conditional burster and a half-center rhythm-generator.

A. The behavior of a single INaP-dependent conditional burster. B. Changes in the burster’s output when the excitatory input (Drive) progressively increases from 0 to 1.2. With increasing Drive, the initial silence state (zero output) at low Drive values changes to an intrinsic bursting regime with burst frequency increasing with the Drive value (seen in two left insets), and then to a tonic activity (seen in right inset). C. Model of a simple half-center network (RG) consisting of two conditional bursters/half-centers inhibiting each other through additional inhibitory neurons, InF and InE. The flexor half-center (F) receives progressively increasing Drive-F, whereas the extensor half-center (E) receives a constant Drive-E keeping it in the regime of tonic activity if uncoupled. D. Model performance. At low Drive-F values, there are no oscillations in the system. This is a state-machine regime in which the RG maintains the state of extension, until an external (strong enough) signal arrives to activate the F half-center or to inhibit the E half-center (see green arrows) to release the F half-center from E inhibition allowing it to generate an intrinsic burst. Further increasing the Drive-F releases the F half-center from E inhibition and switches the RG to the bursting regime (see two insets in the middle). In this regime, the E half-center also exhibits bursting activity (alternating with F bursts) due to rhythmic inhibition from the F half-center. This is a flexor-driven regime. In this regime, with an increase in Drive-F, the bursting frequency of the RG is increasing (and the oscillation period is decreasing) due to shortening of the extensor bursts with much less reduction in the duration of flexor bursts (see bottom curves and two left insets). Further increasing the excitatory Drive-F leads to a transition of RG operation to a classical half-center oscillatory regime, in which none of the half-centers can generate oscillations if uncoupled, and the RG oscillations occur due to mutual inhibition between the half-centers and adaptive properties of their responses. Also in this regime, with an increase of Drive-F, the period of oscillations remains almost unchanged, and the duration of flexor bursts is increasing partly to compensate for the shortening of extensor bursts, which is opposite to the flexor-driven regime (see bottom curves and right inset).

Model of spinal circuits controlling treadmill locomotion.

A. Model of the intact system (“intact model”). The model includes two bilaterally located (left and right) RGs (each is similar to that shown in Fig. 3B) coupled by (interacting via) several commissural pathways mediated by genetically identified commissural (V0D, V0V, and V3) and ipsilaterally projecting excitatory (V2a) and inhibitory neurons (see text for details). Left and right excitatory supraspinal drives (αLand αR) provide activation for the flexor half-centers (F) of the RGs (ipsi- and contralaterally) and some interneuron populations in the model, as well as for the extensor half-centers (E) (γL and γR ipsilaterally). Two types of feedback (SF-E1 and SF-E2) operating during ipsilateral extension affect (excite) respectively the ipsilateral F and E half-centers, and through V3-E neurons affect contralateral RGs. The SF-E1 feedback depends on the speed of the ipsilateral “belt” (βL or βR) and contributes to extension-to-flexion transition on the ipsilateral side. The SF-E2 feedback activates ipsilateral E half-center and contributes to “weight support” on the ipsilateral side. The ipsilateral excitatory drives (αL and αR) suppress (reduce) the effects of all ipsilateral feedback inputs by presynaptic inhibition. B. Model of spinal-transected system. All supraspinal drives (and their suppression of sensory feedback) are eliminated from the schematic shown in A.

Simulation of locomotion on a tied-belt treadmill using intact and transected models.

A and B. Changes in the durations of locomotor period and flexor/stance and extensor/swing phases during simulated tied-belt locomotion using the intact (Fig. 4A) and transected (Fig. 4B) models with an increasing simulated treadmill speed. C. Superimposed curves from panels A and B to highlight differences.

Simulation of locomotion on a split-belt treadmill using intact and transected models.

A. Changes in the durations of locomotor period and flexor/stance and extensor/swing phases for the left (slow) and right (fast) sides during split-belt treadmill locomotion using the intact model (Fig. 4A). B. Changes in the same characteristics for the left (slow) and right (fast) sides during simulation of split-belt treadmill locomotion using the transected model (Fig . 4B). In both cases, the speed of the simulated left (slow) belt was constant (βL=0.4) while the speed of the simulated right belt (βR) changed from 0.5 to 1.0 with 0.1 increments.

Simulation of the effect of removing SF-E2 feedback in the transected model during simulated tied-belt locomotion.

Changes in the durations of locomotor period and flexor/stance and extensor/swing phases during simulated tied-belt locomotion using the transected model (Fig. 4B) after removal of SF-E2 feedback.

Simulation of the effect of removing SF-E1, or SF-E2, or both feedback types in the intact model during simulated tied-belt locomotion.

Changes in the durations of locomotor period and flexor/stance and extensor/swing phases during simulated tied-belt locomotion using intact model (Fig. 4A) with an increasing simulated treadmill speed after removal of only SF-E1 feedback (A), only SF-E2 feedback (B), and both feedback types (C).

Connection weights.