Afferent effects in wipe reflex corrections

Adult spinal bullfrogs perform rapid hindlimb wipe reflexes in response to noxious stimuli. Both cutaneous and proprioceptive feedback modalities are essential in the planning and execution of this reflex. Although gated cutaneous feedback is known to organize online (mid-wipe) corrections, studies suggest that proprioceptive feedback does not elicit online corrections, at least in the early phase of the wipe. We tested this by applying continuous elastic and pulsed perturbations at the ankle of the frog limb during the wipe. Our results indicate significant EMG variations in response to elastic field that systematically change with the elastic field stiffness. Through multielectrode recordings, we have isolated neural underpinnings of this modulation in the spinal cord. Pulsed perturbations could be ignored or compensated actively depending on their timing in the wipe motor pattern. All our previous work suggests that the reflex organization and corrections follow a primitive based framework that uses a linear combination of muscle synergies to create muscle activations. Our results support this framework and we have isolated associated interneurons that show firing-rate changes proportional to the magnitude of amplitude modulation of certain primitives during the proprioceptive perturbations. This lays a strong foundation and body of evidence for selective online proprioceptive integration in reflex wipes. We used a 3D biomechanical model to study the effects of introducing continuous feedback in a spinal wipe reflex. We performed simulations from various starting limb configurations and under varying environmental conditions including elastic field, inertial loading and pulsed force perturbations. We have previously published the importance of joint space estimation in fine tuning the directionality of a wipe reflex. Here we test the efficacy of initial joint space estimation in conjunction with continuous spindle feedback. Continuous application of spindle feedback shows significant reduction in target error and result in convergent trajectories under unperturbed, elastic and inertial loading. However pulsed perturbations get exaggerated and result in divergent trajectories. From this we conclude that proprioceptive feedback is definitely incorporated during wipe planning and execution. However the nature of its integration during execution may not be continuous and may be subject to gating, filtering and modulation to optimize performance under the constraints of spinal circuitry.