Single neuron response to pulse-periodic laser microirradiation. Action spectra and two-photon effect

Abstract

Cytoplasm regions of crayfish stretch receptor neurons were irradiated by the microbeam of a pulse-periodic dye laser (diameter 5 μm, pulse duration 15 nsec, pulse frequency 400 Hz). Irradiation wavelengths varied from 434 to 600 nm, and beam intensity (at 441.6 nm) from 0.25 to 5 × 10 3 W cm −2 (mean), or from 0.4 to 8 × 10 8 W cm −2 (peak). Under irradiation neuron firing typically accelerated and then inhibited. In some cases it accelerated again and then abruptly and irreversibly ceased. Action spectra of parameters describing different response phases had the flavin-like maximum at 460 nm. This suggests flavins as a primary intracellular photoreceptor for blue laser light. It was also found that firing acceleration rate quadratically depended on peak beam intensity. This finding indicates the participation of two-photon processes in neuronal membrane alteration leading to firing acceleration. Parameters describing inhibition phase depended on beam intensity linearly. That is characteristic for usual single-photon process.