REGULARITY IN THE GENERATION OF DISCHARGE PATTERNS BY PRIMARY AND SECONDARY MUSCLE-SPINDLE AFFERENTS, AS RECORDED UNDER A RAMP-AND-HOLD STRETCH

The discharge frequency of primary (Ia) and secondary (II) muscle spin dle afferents from the tibial anterior muscle of the cat were recorded under a ramp-and-hold stretch of the host muscle. The rate of ramp st retch and the prestretch of the muscle were varied systematically. The degree of stretch was kept constant. For a discharge pattern recorded at a ramp rate of 10 mm/s, the peak dynamic discharge, the maximum st atic value and the final static value were determined. These three dis charge rate values were plotted against the maximum static value. In t he resulting charts the II afferents presented themselves as a homogen eous group of spindle afferents, whereas the Ia fibers separated into three subgroups. The existence of three subpopulations of Ia fibers wa s verified by the method of Hald. Furthermore, it is shown that each s ubpopulation generated its discharge patterns in its own regularly sys tematic manner. It was concluded that, as one of the three Ia subpopul ations exhibits much the same dynamic and static stretch properties as the II fibers, the encoder of this subpopulation must receive its rec eptor current from the sensory terminals of passive intrafusal chain f ibers. The encoder of a second Ia subpopulation indicates its action p otentials using the receptor current stemming from the bag, sensory te rminals, these Ia fibers eliciting a slow adaptation component of a hi gh magnitude which is assumed to be the consequence of a high level of ''creep'' in the passive intrafusal bag, fiber. The third Ia subpopul ation initiates its action potential sequences by means of the recepto r current stemming from the passive bag, fiber, producing behavior pat terns that lie between those of the other two Ia subpopulations.