The goal of this research is to clarify the cellular mechanisms of synaptic transmission and impulse generation in the vestibular endorgans through theoretical and morphophysiological studies of the synaptic activity of vestibular afferents and their peripheral innervation patterns. The proposed experiments will examine the relative roles of synaptic input from type I and type II receptor hair cells and the physiology of the afferent nerve terminal--more particularly, its passive membrane properties and postspike recovery mechanisms--in the overall transformation of vestibular stimuli into afferent discharge patterns. Motivating these studies is the knowledge that while the transduction mechanisms of type II hair cells and the discharge characteristics of vestibular afferents are well understood, little is known, particularly in mammals, about the cellular mechanisms of synaptic transmission from hair cells to vestibular afferents or the properties of synaptic input from type I and type II hair cells. Intra-axonal recordings and subsequent dye injections will be made in vitro from vestibular afferents innervating the bullfrog and chinchilla utricular maculae and, ultimately, in vivo from afferents innervating the chinchilla semicircular canal cristae. The discharge regularity and responses of afferents to presynaptic (caloric) and postsynaptic (galvanic) stimulation will be compared and correlated with their synaptic activity. The effects of intracellular current and divalent cations on synaptic activity will be studied. Postspike recovery of the afferent nerve terminal will be measured in vitro before and after the administration of specific potassium channel blockers. To study the properties of synaptic input from type I and type II hair cells, attempts will be made in vitro to record the responses of vestibular, afferents to the direct mechanical stimulation of individual hair cells. The time course of evoked synaptic potentials, in conjunction with compartmental models, will be used to infer the electrotonic distances at which these potentials originate. These estimates will be compared with the actual distances between labelled afferents and stimulated hair cells revealed by dye injection. A similar procedure will be used in vivo to associate spontaneous synaptic potentials with individual synaptic endings or, at the very least, with type I or type II hair cells. It is expected that these experiments will elucidate the functional roles of type I and type II hair cells in the vertebrate labyrinth.
