Near-infrared signals associated with electrical stimulation of peripheral nerves

Electrical stimulation of peripheral nerves induces optical responses via skeletal muscle kinematics

We have previously reported an optical response in human subjects occurring at 100 ms following electrical stimulation of peripheral nerves. In the present study, an animal model has been created to directly investigate the myogenic components of the signal. In addition, experiments have been performed in human subjects to investigate the signal's neuroanatomical specificity, sensitivity to muscle motion, and spatial and spectral features. The results of this work suggest that the observed optical signal derives from stimulus-induced motion associated with muscle contraction and likely contains myological information of clinical value.

Diffuse optical signals in response to peripheral nerve stimulation reflect skeletal muscle kinematics

Previously we have reported a near-infrared optical response in the region occupied by a peripheral nerve that is distal to the site of electrical stimulation of that peripheral nerve. This "intermediate" signal is vascular in nature but its biological origin not been elucidated. In the present study, an animal model of the signal has been created and our human studies expanded to directly investigate the contribution of non-artifactual vascular motion induced by muscle contraction to the biological origin of this signal. Under non-invasive conditions during stimulation of the exposed sciatic nerve of the Sprague-Dawley rat, optical responses are robust. These signals can be abolished both pharmacologically and surgically using methods that eliminate muscle motion while leaving the electrophysiological health of the nerve intact. In human studies, signals that are elicited on stimulation of nerves containing motor axons, both within and outside the predicted imaging volume of the spectrometer, have similar temporal characteristics of those previously observed. Moreover, stimulation of sensory nerves alone does not elicit an optical response. These results strongly suggest that the intermediate signals are derived from stimulus-induced muscle contraction (whether via an innervating nerve or by direct stimulation) causing translational vascular motion within the optically interrogated region.

Spectral and spatial dependence of diffuse optical signals in response to peripheral nerve stimulation

Using non-invasive, near-infrared spectroscopy we have previously reported optical signals measured at or around peripheral nerves in response to their stimulation. Such optical signals featured amplitudes on the order of 0.1% and peaked about 100 ms after peripheral nerve stimulation in human subjects. Here, we report a study of the spatial and spectral dependence of the optical signals induced by stimulation of the human median and sural nerves, and observe that these optical signals are: (1) unlikely due to either dilation or constriction of blood vessels, (2) not associated with capillary bed hemoglobin, (3) likely due to blood vessel(s) displacement, and (4) unlikely due to fiber-skin optical coupling effects. We conclude that the most probable origin of the optical response to peripheral nerve stimulation is from displacement of blood vessels within the optically probed volume, as a result of muscle twitch in adjacent areas.