To assess neuro-physiologic function in clinical patients, electrical stimulation and recording of neural membrane potential are commonly used in veterinary medicine. Optical means of neural stimulation and recording have many potential advantages over traditional electrophysiologic assays. Optical stimulation does not elicit an electrical artifact, which despite removal methods mask early neural events associated with the stimulation. Using focused optics, it is possible to direct focal stimulation anywhere in a two, or three-dimensional field of view, or even provide patterned stimulation. Since a contact tissue interface is not needed, optical stimulation is possibly less invasive, with the potential to transmit through other tissues. Optical recording can measure both direct neural activation and hemodynamically coupled changes, affording the benefit of excellent temporal and spatial resolution respectively.
We extracted walking leg nerves from lobsters and placed them in a nerve chamber. Electrodes within the chamber were used to record the electrical responses to stimulation. We illuminated the nerve with an LED through an optical window in the chamber which allowed for detection of changes in the polarization of light through crossed polarizers (birefringence). The nerve was stimulated using both electrical and optical methods. Electrical stimulus was applied with varying pulse widths via electrodes within the chamber. For optical stimulation, an infrared diode laser was situated close to the nerve surface, and perpendicular to its longitudinal axis.
For our in-vivo studies, we optically stimulated the vagus nerve of Sprague-Dawley rats. Previous studies have shown success in stimulating the sciatic nerve inducing a motor unit action potential by similar means. A lateral approach to the vago-sympathetic trunk allowed for placement of the diode laser adjacent to the nerve. Electrocardiography was concurrently performed along with neural electrical recording distal to the stimulation site.
We tested several different optical power levels, pulse widths, and distances from the nerve to determine the optimal conditions for optical stimulation. As many as 500 triggered action potentials were electrically recorded following optical stimulation. We also observed birefringent signal change to electrical and optical stimulation concomitant with the electrical response. In the rat model, precipitous, synchronous decreases in heart rate were induced with both optical and electrical stimulation.
Data from these experiments suggest that the infrared laser is capable of depolarizing the axons and generating action potentials within a lobster nerve and the rat vagus nerve. Since we could simultaneously stimulate and record neural activation with optical signals, this study paves the way for neurophysiology without the need for wires or electrical conduction in veterinary patients.