Optical stimulation of neural tissue

A novel method of stimulation of the neural tissue using pulsed, low-energy laser light is presented. Optical stimulation elicits compound nerve and muscle action potentials, with resultant muscle contraction, similar to responses obtained with conventional bipolar, electrical stimulation of the nerve. IR pulsed laser in the range 3-7 μm was explored for nerve stimulation in vivo using both frog and rat models. Our observations of conduction velocity, blocking of neuromuscular transmission and cessation of nerve conduction by severing the nerve show that optical stimulation 1) produces action potentials conducted in A-type nerve fibers, 2) results in neuromuscular transmission, and 3) is conducted within the nerve. Within the studied range, wavelengths around 4 μm yielded the least energy required for threshold stimulation which also had higher threshold for ablation. These wavelengths correspond to a minimum in the water absorption spectrum. Threshold stimulation to produce action potentials was 0.7 J/cm² at 4.6 μm. Histological study of the irradiated nerves reveal undetectable damage to the irradiated nerve. Several possible mechanisms exist by which low-energy, pulsed laser light stimulation generate action potentials. These can include, but are not limited to, microscopic photothermal effects, photomechanical micro-stress waves, or photochemical modulation of membrane channels. Regardless of the mechanisms involved, optical stimulation has the potential advantage as a precise tool for basic science studies as well as clinical diagnostics and therapeutics.