Static stretch sensitivity of Ia and II afferents in the cat's gastrocnemius.

The static discharge characteristics of deefferented Ia and II spindle afferents in the cat medial gastrocnemius muscle have been compared with respect to incidence and firing rate of spontaneously firing units, length threshold of nonspontaneous units, linearity of the response to stretch, and position sensitivity. Measurements of the stretch response were taken 20s after each step increase in length so as to obtain the nearly fully adapted rate. Under this condition the frequency-length relationship was quite linear, especially for group II units. The mean position sensitivity of group II fibers exceeded that of Ia fibers in the ratio of 1.5: 1.0. The incidence of spontaneously discharging units was about 20% for both types of unit. The relationships between some of these measures and axonal conduction rate were examined. No correlation of position sensitivity with axonal conduction velocity was detected for either afferent type, but the length thresholds of the group II afferents showed a significant trend for slower conducting, that is, smaller fibers to be less sensitive. There was some indication of a direct relationship between conduction velocity and length threshold for Ia fibers. The contradiction to the "size principle" and mechanisms contributing to the static discharge behavior of the spindle are discussed.