SENSORY INTEGRATION WITHIN RETICULOMOTOR NETWORKS

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The neural causes of dysfunctional motor behavior are often addressed in terms of impaired integrative circuitry. Parkinsonian patients, for example, suffer a loss of fine motor control when the basal ganglia are no longer regulated by nigral dopaminergic afferents. Yet, an almost involuntary, ballistic behavior can be elicited by a sudden stimulus: patients may leap from their chair when startled. Thus, an abrupt change in the sensory image of their surrounding environment is immediately integrated with motor circuitry. In particular, sensory information may be integrated with the more primitive reticulospinal motor network to generate an adaptive response. The short latencies of these motor behaviors preclude sensory modification of the response immediately after it is initiated. Thus, a continuously updated sensory image of the environment must be available for use in selecting or biasing the ensuing motor program. Our goal is to elucidate some of the common principles involved in the integration of such neural networks. We hope to determine how a highly specialized sensory system is integrated with a primitive motor system to allow an animal to continuously monitor its environment and produce accurate ballistic behaviors. Electrosensory modulation of the escape system in fish is a relevant and experimentally tractable type of neural integration. The teleostean electrosensory system is known to be used for electrolocation, social communication, control of the Jamming Avoidance Response, and electromotor control during lateral and longitudinal tracking. The Mauthner (M-) cells, a bilateral pair of medullary neurons used to initiate the startle response, are present even in primitive fishes. While past studies have found auditory, visual, and lateral line afferents to the M-cells, we have recently shown that the electrosense can modulate the startle response of electrogenic gymnotiform fish. Physiological recordings from the inhibitory axon cap of the M-cell have demonstrated synaptic potentials in response to amplitude modulations of the fish's electric signal. The specific aims of this project are to 1) examine the influences of electrosensory cues on startle by looking at the effect of cue duration, the roles of ampullary (low frequency) or tuberous (high frequency) stimuli, and jamming stimuli on modulating the response. 2) Acute experiments will determine the lateralization of M- cell inhibition by simultaneously recording from the axon caps of both M-cells while reproducing the most behaviorally relevant stimuli. 3) Chronic recordings in freely behaving fish will verify the integration of the electrosensory surround in modulating the startle response.