STRUCTURE AND FUNCTION OF LOCAL INTERNEURONS

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Local interneurons comprise the majority of neurons in the mammalian brain, but the cellular analysis of the local circuits they form is unavoidably limited by the large numbers of neurons, their small size,and their inaccessibility. Local circuit neurons will be studied in the simpler and more accessible nervous system of the grasshopper Schistocerca. Here, spiking and nonspiking local interneurons act in concert to integrate sensory information and control motor activity. The first aim is to discover how spiking local interneurons recruit motor neurons in local tactile avoidance reflexes. The goals are: A) To survey the populations of spiking local interneurons that synapse on motor neurons of the hind legs. B) To determine how spiking local interneurons converge to generate the receptive fields of individual motor neurons. C) To determine how the interneurons diverge to recruit groups of motor neurons in local reflexes. The second aim is to discover what part nonspiking local interneurons play in tactile avoidance reflexes. The goals are: A) To characterize the tactile responses of nonspiking local interneurons. B) To determine how nonspiking local interneurons contribute to the tactile receptive fields of motor neurons. C) To determine how the receptive fields of nonspiking interneurons arise from the inputs of spiking local interneurons. Experiments will be performed on minimally dissected animals, so that the activity of individual neurons can be related to defined sensory and motor functions. Experiments will rely on well-established techniques of cellular neurophysiology and neuroanatomy. Intracellular microelectrode recordings will be made in pairs from local interneurons and postsynaptic leg motor neurons. The receptive fields of the interneurons will be mapped, then the interneurons stained intracellularly, so that their morphology can be related to their physiology. The long term objectives are to discover the different functions that spiking and nonspiking interneurons have in local circuits, and to relate these to their morphology and cellular physiology. Ultimately, this may provide a rationale for the two different modes of signaling within local circuits, and give a firm basis for understanding the unique contributions the two interneuronal types make to neuronal integration.