Understanding the biophysical properties of single neurons and how they process information is fundamental to understanding how the brain works. Previous studies of signaling in neuronal processes were limited to electrode recordings from a single site and to high-resolution imaging experiments using intracellular ion-indicators. However, regional electrical properties of neurons are extraordinarily complex and impossible to understand in the absence of detailed, spatially well resolved measurements of electrical signals. Recent developments in this laboratory now permit monitoring membrane potential transients at many sites by an optical imaging technique that employs intracellular voltage-sensitive dyes. The long-term goal is to use the information about individual nerve cells as the basis for understanding of the fundamental question of how networks of interconnected neurons operate to control behavior. The main objective of this proposal is to investigate by direct measurement how one particular neuron is functionally organized and to build a complete biophysical compartmental model. The model would represent a full description of the computational properties of a nerve cell. For instance, the obtained information will be used to test directly the hypothesis that individual neurons can be functionally subdivided, with some of the processes functioning as independent units. If true, this postulate would have important implications for the functional complexity of individual neurons. Individual nerve cells will be selectively stained in situ by intracellular application of a membrane impermeant voltage- sensitive fluorescent dye. Propagation of and interactions between electrical signals evoked by stimulating the cell will be analyzed by optically monitoring voltage transients at multiple sites in the soma and neuro nal processes. Standard electrophysiological techniques for recording and stimulation and advanced modeling software will also be used. One exemplar neuron, the giant metacerebral cell of the land snail Helix, will be characterized in detail. The complete description will include: (1) determination of the number, size, and position of spike trigger zones; (2) determination of the propagation properties of different axonal branches; (3) determination of the spatial distribution of synaptic inputs; (4) determination of spatial and electrical distances between trigger zones and functionally important parts of the neuron (sites of synaptic contacts and axonal branch points). The results of this analysis will bear on the basic neuroscience of signaling.
