[Subdivision of Ia fibers of de-efferented cat muscle spindles into two groups].

The discharge frequency of primary muscle spindle afferents of the tibial anterior muscle of the cat was determined under a ramp-and-hold stretch of the host muscle. The rate of ramp stretch was increased stepwise from 1 to 100 mm/s. Its amplitude was kept constant and was 7 mm. The initial length of the host muscle was enhanced in steps of 3 mm to reach a maximal augmentation of the muscle length by 12 mm. At each discharge pattern obtained under a rate of ramp stretch of 10 mm/s the slow receptor adaptation was established. The slow receptor adaptation is the difference between the discharge frequency occurring at the beginning of static phase of a ramp-and-hold stretch, the static maximum, and at the end of the third second of static phase, the static value (Fig. 1). The amplitude of the slow receptor adaptation was plotted against the amplitude of static maximum (Fig. 2). The diagram showed two groups of Ia fibres. One of the two groups had a slow receptor adaptation of high amplitudes. Its primary afferents will be called bag1 Ia fibres. The other group had a slow receptor adaptation of low amplitudes. Its primary afferents will be named bag2-chain Ia fibres. Furthermore, the dynamic response and the static value were determined at the discharge patterns of the two groups of spindle fibres. The dynamic response of the bag1 Ia fibres was with p < 0.0001 significantly higher in its mean amplitude than that one of the bag2-chain Ia fibres (Fig. 4a).(ABSTRACT TRUNCATED AT 250 WORDS)