FIG. 4.

Characteristics of pelvic nerve muscular fiber responses to stretch. A: low-threshold muscular afferents responded at low stimulus intensity and encoded increasing stimulus intensity throughout the range of loads tested. The estimated response threshold extrapolates to about 1 g. High-threshold muscular afferents did not respond at low intensities of load and first responded at ≥10 g load. The estimated response threshold extrapolates to about 10 g. The difference between low- and high-threshold muscular afferent responses to stretch is significant (F = 7.39, P = 0.007). B: distribution of low- and high-threshold receptive fields. C: both low- and high-threshold muscular afferents gave an initial dynamic response to stretch (20 g) and similarly adapted to relatively stable, but significantly different rates of discharge for the duration of stretch (20 s) (F = 23.36, P < 0.0001). Response magnitudes of low-threshold fibers were on average significantly greater than response magnitudes for high-threshold fibers. D: low-threshold fibers were further divided into encoding and nonencoding types; 16 of the 25 low-threshold fibers encoded stimulus intensity up to about 10 g load and did not further increase response magnitude as load was increased. The remaining 9 low-threshold muscular afferents exhibited load-dependent responses throughout the range of intensities applied.