Electrical activation of the pocket scratch central pattern generator in the turtle

1. A low-spinal, immobilized turtle displays a fictive scratch reflex in hindlimb motor neurons in response to tactile stimulation of the shell. Turtles exhibit three forms of the scratch reflex: rostral, pocket, and caudal. Each form is elicited by tactile stimulation of a different receptive field on the body surface. The ventral-posterior pocket (VPP) cutaneous nerve innervates the ventral-posterior portion of the pocket scratch receptive field. Natural stimulation within the VPP nerve's receptive field evoked a pocket scratch reflex. Electrical stimulation of this nerve elicited robust pocket scratch reflexes. 2. A single electrical pulse to the VPP nerve delivered at a voltage (>5 V, 0.1 ms) that activated all the axons in the nerve was termed a 'maximal' pulse. A single maximal pulse did not evoke a scratch motor response. It raised the excitability of the pocket scratch central pattern generator for several seconds, however. We revealed such excitability changes by applying maximal pulses to the VPP nerve at multisecond intervals. When we delivered maximal pulses with interpulse intervals of ≤5 s, the first pulse produced no motor response and the second pulse evoked one or more cycles of pocket scratch. 3. A stimulus pulse applied to the VPP nerve was used as a probe for studying changes in the excitability of the pocket scratch CPG following scratch motor patterns. In a rested preparation, the stimulus pulse did not activate motor output. In contrast, the stimulus pulse evoked one or two cycles of pocket scratch activity if delivered within 2.5 s after the cessation of rhythmic pocket scratch motor activity. These results are consistent with the hypothesis that the pocket scratch CPG has elevated excitability for seconds following the cessation of pocket scratch motor output. A single pulse applied to the VPP nerve evoked no response if delivered after the cessation of rostral scratch motor activity, however. 4. We used a train of maximal pulses to the VPP nerve to probe the form-specificity of the changes in the excitability following a rostral scratch motor pattern. We set the stimulus parameters so that the train evoked one or two cycles of a pocket scratch motor pattern in a preparation that had rested for over 1 min. The train still evoked a pocket scratch motor pattern when delivered 12.5 s or later after the end of a rostral scratch motor pattern. In contrast, the train evoked a cycle of rostral scratch when delivered within 1.5 s after the cessation of the rostral scratch motor pattern. These observations are consistent with the hypothesis that there are central interactions between the pattern generating networks for the pocket scratch and the rostral scratch and/or between the interneurons that provide input to the respective networks. One possible mechanism for such interactions may be 'shared' interneurons that are activated during both rostral scratch production and pocket scratch production. 5. Our results support the hypotheses that there are interneurons in the turtle spinal cord that are excited for several seconds following a brief tactile stimulus in a scratch receptive field and that these interneurons either impinge on or are part of the scratch central pattern generator. An implication of these hypotheses is that summation of tactile information over a time course of several seconds is a critical feature of spinal cord sensorimotor integration.