Computer-aided reconstruction of normal, and regenerated individual vestibular primary afferent neurons will be employed in a study of the differential brainstem distribution of individual horizontal semicircular canal afferents in the toadfish, Opsanus tau. This canal nerve contains approximately 350 primary afferents. Each afferent carries a signal related to head movement. Utilizing adequate stimulation, these signals have been divided into three rough groups of: a) low gain velocity sensitive, b) high gain velocity sensitive, and 3) acceleration sensitive afferents. Preliminary studies indicate that each group of afferents terminate within discrete portions of the vestibular nuclei and cerebellum as well as in overlapping, common sites. The output from individual vestibular nuclei, as in all vertebrates, is directed to disparate sites within the neuraxis. There are rostrally projecting neurons innervating, e.g., the extra ocular motor nuclei, ipsi- and contralaterally projecting spinal neurons, projections to the ipsilateral and contralateral brainstem, and projections to other sites. for the present study, individual afferents, physiologically identified by their unique signals during adequate stimulation will be injected intra- axonally with HRP or biocytin to visualize their axons and collaterals, terminal fields, and somatic morphology. Afferent neurons will be computer-reconstructed and morphological information entered into digital memory for subsequent analysis. We will study relevant parameters such as soma size, axon diameter, number and mode of axonal branches, sites of axonal termination, and number of terminal boutons. Hypothesis 1 to be tested is that; in the transfer of information from the VIIIth nerve to its central targets the structure and terminal loci of fibers with diverse physiological signals parallels their diverse functions. Hypothesis 2 is that; functionally different physiological afferent signals will be transferred to appropriate secondary neurons. This will be tested by evaluating the response dynamics during rotation of identified secondary neurons. Intrasomatic electrophysiology following electric pulse stimuli of primary afferents will also be used to evaluate differences in mono- and poly- synaptic post synaptic potentials in different brainstem target neurons. Hypothesis 3 is that; following nerve section, regenerating afferents will find their appropriate brainstem targets and reconnect, transferring the appropriate physiological signals to the correct secondary neurons within the vestibular nuclei. The structure and function of regenerated afferents and their target neurons will be rigorously evaluated with the above mentioned morphological and physiological techniques.
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