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.
We report our studies on the optical signals measured non-invasively on electrically stimulated peripheral nerves. The
stimulation consists of the delivery of 0.1 ms current pulses, below the threshold for triggering any visible motion, to a
peripheral nerve in human subjects (we have studied the sural nerve and the median nerve). In response to electrical
stimulation, we observe an optical signal that peaks at about 100 ms post-stimulus, on a much longer time scale than the
few milliseconds duration of the electrical response, or sensory nerve action potential (SNAP). While the 100 ms optical
signal we measured is not a direct optical signature of neural activation, it is nevertheless indicative of a mediated
response to neural activation. We argue that this may provide information useful for understanding the origin of the fast
optical signal (also on a 100 ms time scale) that has been measured non-invasively in the brain in response to cerebral
activation. Furthermore, the optical response to peripheral nerve activation may be developed into a diagnostic tool for
peripheral neuropathies, as suggested by the delayed optical signals (average peak time: 230 ms) measured in patients
with diabetic neuropathy with respect to normal subjects (average peak time: 160 ms).
Complex neuronal structures and interactions make studying fast optical signals associated with brain activation
difficult, especially in non-invasive measurements that are further complicated by the filtering effect of the scalp and
skull. We have chosen to study fast optical signals in the peripheral nervous system to look at a more simplified
biological neuronal structure and a system that is more accessible to non-invasive optical studies. In this study, we
recorded spatially resolved electrical and optical responses of the human sural nerve to electrical stimulation. A 0.1 ms
electrical stimulation was used to activate the sural nerve. Electrical signals were collected by an electromyogram
machine and results showed an electrical response spanning a distance of 8 mm across the nerve. Optical signals were
collected by a two-wavelength (690 and 830 nm) near-infrared spectrometer and displayed a characteristic decrease in
intensity at both wavelengths. Data were taken at multiple positions and then reproduced five times. The average optical
data over the five trials showed an optical signal that was spatially consistent with the electrical response to sural nerve
stimulation.
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