The optical spike. Structure of the olfactory nerve of pike and rapid birefringence changes during excitation

1. Electron microscope studies on the olfactory nerve of the pike revealed a population of 4.2 million, densely packed unmyelinated nerve fibres; 95% are small fibres (average diameter 0.19 mum, narrow modal class), 5% are larger (average diameter 0.6 mum). Each fibre is bounded by an axonal membrane with a bilayer structure (80 A thickness). 2. The olfactory nerve is birefringent (negative with respect to fibre axis) and shows at 20 degrees C an average retardation R = 23 mn. The birefringence becomes more negative on lowering the temperature. 3. With the passage of an action potential a rapid, transient increase of retardation--the optical spike-- occurs; deltaR = 0.04 nm. The optical spike corresponds to the time course of structural changes in the axon membrane during excitation; it begins later, peaks earlier and decays more quickly than the voltage changes as recorded externally in the present study.