Effects of near-infrared low-level laser stimulation on neuronal excitability

Abstract Effects of near-infrared low-level laser (NIR LLL) on brain excitability have been seldom investigated. A few studies carried out so far showed that following several minutes of transcranial NIR LLL illumination, the output of the primary motor cortex area in humans, measured by transcranial magnetic stimulation (TMS), is inhibited for at least half an hour. The effect seems to be dose dependent. Given that NIR LLL is known to boost metabolic activity in the illuminated neurons, the results may suggest that this allows for more efficient functioning of the cell membrane potential maintenance mechanisms which are then able to resist more effectively external perturbations (such as TMS). Alternatively, it may be that, due to the morphological complexity of the cerebral cortex coupled with the size of the area illuminated by a laser beam, transcranial LLL induces activation in a large population of the cortical cells of various functional features. Because of their abundance in the most superficial cortical layers, inhibitory interneurons may be the principal recipients of the NIR LLL effects thus causing an inhibition of the output from more deeply-seated, large pyramidal cells. Further investigations are needed to better understand these effects.