The long-term goal of this work is to understand how vestibular organs work. The proper function of these organs is crucial to a healthy existence; damage can lead to debilitating vertigo, dizziness and inability to maintain study gaze. Mamammal, birds and reptiles have similar vestibular organs, with two classes of sensory receptor cell, the type I and type II hair cells. These cells transduce head movements into electrochemical signals that are transmitted across synapses to the terminals of afferent nerve fibers, which convey the signals to the brain in the form of electrical discharges. Efferent nerve fibers from the brain make synapses on h air cells and afferent nerve terminals, through which they influence afferent signals by unknown mechanisms. The specific aims are to characterize: 1) afferent synaptic transmission from the hair cells to the neurons; 2) the cellular mechanisms responsible for discharge regularity and maximum evoked discharge rates of afferent neurons; 3) efferent actions. In vitro preparations of the posterior semicircular canal organ of the turtle will be used. This organ lends itself to comparison of type I and type II hair cells, shows richly diverse efferent actions on afferent nerve fiber discharges, and is robust in vitro. Depending on the specific experiment, stimuli will be mechanical (displacement of the canal fluid), manipulations of membrane voltage or current in hair cells or afferent neurons, or electrical stimulation of efferent nerve fibers. The membrane voltage or current responses of hair cells and afferent neurons to these stimuli will be recorded intra cellularly with sharp micropipettes or patch pipettes. Both conventional (vesicular, orthograde) and unusual forms of transmission between the type I hair cell and afferent neuron will be characterized. Whether afferent discharge regularity is due to presynaptic (hair cell) or postsynaptic (afferent neuron) mechanisms will be tested. whether stages following mechanoelectrical traduction determine saturation of afferent discharge rates will be investigated. Efferent-evoked synaptic potentials and the neurotransmitter receptors responsible will be characterized.