Enhancement of subthreshold sensory nerve action potentials
during muscle tension mediated noise
Faye Chiou-Tan, M.D.,
Kevin Magee, Ph.D.,
Lawrence Robinson, M.D.,
Maureen Nelson, M.D.,
Stephen Tuel, M.D.,
Thomas Krouskop, Ph.D.,
Frank Moss, Ph.D.
Dept. of Physical Medicine and Rehabilitation,
Baylor College of Medicine, Houston, TX
Dept. of Rehabilitation Medicine,
University of Washington, Seattle, WA
Dept. of Pediatrics, Baylor College of Medicine, Houston, TX
Dept's. of Physics and Astronomy and Biology,
University of Missouri, St. Louis, MO
It was noted in our laboratory that subthreshold sensory nerve action
potentials (SNAP's) appeared during and after isometric muscle
contractions in human subjects. The hypothesis was that muscle
contractions augmented sensory nerve action potentials. A second
hypothesis was that this event may be in part due to stochastic
resonance noise enhanced function. Ten normal subjects were recruited
in a prospective, intrinsically controlled study. During baseline, 25%,
50%, 75%, and 100% maximal abductor pollicis brevis contractions
against a force gauge, 10 subthreshold stimuli were delivered to the
median nerve at the elbow at 0.2 Hz. Responses were signal averaged.
Results showed an increase from baseline of 0.5 to 6.7 
V
(p=0.0001)
in SNAP amplitude and 0.05 to 0.5 mV (p=0.078) in compound motor action
potential (CMAP) amplitude during maximal contraction. There was no
significant change in the latency or duration of the sensory potentials,
or temperature and impedance recorded from the skin. Statistical
analysis was performed with ANOVA for repeated measures. In conclusion,
it appears SNAP's increase in amplitude during motor contraction.
Results
thus far are consistent with the hypothesis that noise can enhance the
signal transmission efficiency in this modality. Research is underway to
further examine causality.