Discharge from total populations of Ia and II afferent fibers in the cat's deefferented medial gastrocnemius muscle during static stretch

Estimates were made of the changing volumes of discharge arising from the total populations of Ia and II spindle afferent fibers in the cat's deefferented medial gastrocnemius as the muscle was extended stepwise over its excursion range. The estimates were based on values reported elsewhere for (i) the incidence of active units and (ii) their rates of discharge at static muscle lengths normalized against the excursion range of the particular muscle, together with (iii) the numbers of Ia and II afferent fibers in the medial gastrocnemius as derived from a critical review of published information. Curves representing the total discharge of the two afferent types are similar and show three phases: an initial level of spontaneous activity, a somewhat curvilinear rise associated with early recruitment of units during stretch, and rectilinear increase after full recruitment. There was no appearance of saturation. Completion of recruitment of both types of units occurred about halfway in the extension range of the passive muscle. Over most of the excursion range, inflow from the group II population was about one-third greater than that from the Ia units.