A neuron processes information by integrating many synaptic inputs onto its dendritic tree, leading to its spike output. This project will investigate this neural processing by stimulating individual sensory afferents to a neuron in the vertebrate brainstem, using a sensory pathway which has the unique advantage of lacking topography. As a consequence, its afferent cell bodies are widely distributed in the sensory periphery. Specifically, intracellular recordings in the accessory optic system will be performed in a unique in vitro turtle brain preparation in which the eyes remain attached and the brain remains visually responsive. Neurons in the accessory optic system presumably receive their synaptic inputs from a distinct class of retinal cell, the direction sensitive ganglion cell. The interaction between the presynaptic terminals and a postsynaptic cell performs an essential conversion from local retinal directional information into a measure of global image motion, called retinal slip, that has a well defined role in vestibular and oculomotor reflexes. The turtle's accessory optic system is called the basal optic nucleus, a surface brainstem structure whose neurons are easy to locate in vitro. Postsynaptic events of cells in the basal optic nucleus are readily recorded with high fidelity in the whole cell configuration using patch pipettes. Visual and electrical stimulation of individual retinal afferents will be used to study the size and shape of unitary postsynaptic events as well as their direction tuning. Stimulating two afferents will determine how two synaptic potentials interact on the postsynaptic membrane. This interaction is complicated by the finding that synaptic responses to individual afferents are quite variable in amplitude and that the postsynaptic membrane has voltage sensitive channels that can be modulated by synaptic potentials. To further understand this interaction, we will also study the passive membrane properties of each cell, the voltage sensitive channels in its membrane, the ionic and pharmacological nature of the synaptic currents and the cell's morphology. This full analysis will ultimately clarify the neural transformations from the retinal synaptic input to the accessory optic system output. The results of these studies will provide insights into the functioning of neurons that encode the direction of visual field motion in order to maintain our balance and stabilize our gaze. Such visual processing will also provide an excellent model to understand general mechanisms of synaptic integration and the sensory processing of the brainstem.
