# Application of near infra-red laser light increases current threshold in optic nerve consistent with increased Na+-dependent transport

## TL;DR

Near infra-red laser light increases the current threshold in optic nerve axons, possibly by affecting sodium transport mechanisms.

## Contribution

The study identifies a novel mechanism involving Na+-dependent transport in laser-induced threshold changes in optic nerve axons.

## Key findings

- Laser light increases current threshold in optic nerve axons, with effects fading over time.
- Threshold increases are partially blocked by Li+ and bumetanide, suggesting involvement of Na+ transport.
- Threshold changes are not due to myelin resistance but may involve temperature-dependent Na+ influx.

## Abstract

Increases in the current threshold occur in optic nerve axons with the application of infra-red laser light, whose mechanism is only partly understood. In isolated rat optic nerve, laser light was applied near the site of electrical stimulation, via a flexible fibre optic. Paired applications of light produced increases in threshold that were reduced on the second application, the response recovering with increasing delays, with a time constant of 24 s. 3-min duration single applications of laser light gave rise to a rapid increase in threshold followed by a fade, whose time-constant was between 40 and 50 s. After-effects were sometimes apparent following the light application, where the resting threshold was reduced. The increase in threshold was partially blocked by 38.6 mM Li+ in combination with 5 \documentclass[12pt]{minimal}
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\begin{document}$$\mu$$\end{document}μM bumetanide, a manoeuvre increasing refractoriness and consistent with axonal depolarization. Assessing the effect of laser light on the nerve input resistance ruled out a previously suggested fall in myelin resistance as contributing to threshold changes. These data appear consistent with an axonal membrane potential that partly relies on temperature-dependent electroneutral Na+ influx, and where fade in the response to the laser may be caused by a gradually diminishing Na+ pump–induced hyperpolarization, in response to falling intracellular [Na+].