Low-threshold afferent signalling of viscous loads during voluntary movements of the human digits

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.