Low-threshold afferent signalling of viscous loads during voluntary movements of the human digits
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Humans can discriminate changes in load viscosity during voluntary contractions. The afferent signal origin is unknown. Microneurographic recordings from 83 single low-threshold afferents were made while participants performed triangular ramps either unloaded or with a viscous load. The neural discharges for each cycle were compared across load and velocity. Fifty-eight afferents did not respond. Afferents with sufficient activity were classified as ambiguous--discharges correlated to velocity and load (n=4), infinite viscosity--strong load and weak velocity signal (n=6), no viscosity--strong velocity and weak load signal (n=10) and those with neither (n=5). No single class of afferent provides a coherent signal of viscosity. These data suggest that the central nervous system compares the population response of different inputs to discriminate viscosity.
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