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

Abstract

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.

Precontractile optical response during excitation-contraction in human muscle revealed by non-invasive high-speed spatiotemporal NIR measurement

“…The detailed study by Erb et al . 9 concluded that the optical signals in the later regime are related to the motion of blood vessels in the measurement volume during stimulus-induced muscle contraction. All together, while the present study concludes that optical signals in the early regime are related to the excitation–contraction process, the previous studies conclude that optical signals in the later regime are related to the mechanical motion.…”

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

“…The results are summarized in table 1 and detailed in sections 3.2 – 3.7 and elsewhere 5, 6 . In the animal model, optical signals were completely eliminated following both pharmacological and surgical separation of the nerve from the muscles it innervates.…”

“…The timescale of the signals and their wavelength dependence suggested that changes in absorption, rather than scattering, associated with tissue hemodynamic events were likely to underlie those changes. In the present work, we report further studies on the spatial and spectral dependence of these signals, as well as the contribution of stimulus-induced motion and the effect of vascular occlusion, the results of which have been recently published 5, 6 . We also discuss ongoing work aimed at understanding the usefulness of these signals for non-invasively assessing the health of both nerve and muscle.…”

“…A detailed description of the data processing has been described elsewhere 5, 6 . Briefly, the average change in optical irradiance over N nerve stimulations is written as <δ I stim ( t )/ I ( t 0 )>.…”

Introduction: Feature Issue on Optical Imaging and Spectroscopy

“…Chen et al [ 19 ] report on a study of the spatial and spectral dependence of the optical signals induced by stimulation of the human median and sural nerves. In a companion paper by the same group of researchers, Erb et al [ 20 ] focuses on the contribution of non-artifactual vascular motion induced by muscle contraction to the biological origin of the same signal. The influence of muscle fiber motion during exercise on diffuse correlation spectroscopy (DCS) measurements of skeletal muscle blood flow is explored by Shang et al [ 21 ].…”