SENSORY AND MOTOR INTEGRATION IN THE SPINAL CORD

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Neuronal networks generate the scratch reflex in the turtle spinal cord posterior to a complete spinal transection. These networks process cutaneous information from specific sites on the body surface and generate appropriate motor rhythms, e.g., rostral scratching in response to stimulation of midbody sites and pocket scratching in response to stimulation of sites just anterior to the hip. We test the concept that some spinal neurons that participate in the generation of rostral scratching for the left hindlimb also participate for the right hindlimb. In addition, we test the additional concept that some of these neurons participate in the generation of both rostral and pocket scratching for both the left and the right hindlimbs. These "bilateral shared core" hypotheses that we test are an extension of the classical "half-center" hypothesis. "Bilateral shared core" hypotheses will be tested using two different experimental strategies. First, the spinal turtle with the left hindlimb enlargement removed will be used to examine rostral scratch generation. This preparation generates fictive motor rhythms in right hip flexors in response to stimulation in the right rostral scratch receptive field and in right hip extensors in response to stimulation in the left rostral scratch receptive field. This control over specific rhythms is a powerful experimental tool to examine predictions of the bilateral shared core hypothesis for rostral scratching. We will record axonal activities of single commissural interneurons during fictive scratching activated by left as well as right rostral scratch receptive field stimulation; we predict that some commissural interneurons are activated by left as well as right rostral scratch receptive field stimulation. We will characterize synaptic mechanisms within the bilateral shared core. Second, a spinal turtle with an intact hindlimb enlargement will be used to examine left and right hindlimb motor rhythms. Interlimb phase will be measured during symmetric same-form behaviors such as bilateral rostral scratching and bilateral pocket scratching, as well as during mixed-form behaviors such as combined ipsilateral rostral scratching and contralateral pocket scratching. Each of these cases displays out-of-phase coordination of left and right hip motor output during scratching movements and during fictive scratching. These results support the concept that interlimb phase control during both rostral and pocket scratching is mediated by bilateral shared core commissural interneurons related to hip control. Our experiments provide information concerning the neuronal mechanisms responsible for cutaneous sensorimotor integration in the spinal cord. In particular, we will reveal important characteristics of spinal control centers involved in the generation of coordinated rhythmic motor output. The turtle spinal cord is similar to that of other vertebrates, including humans. The mechanisms we reveal serve as working hypotheses for studies of spinal cord sensorimotor integration in other vertebrates.