[The fast and slow components of receptor adaptation in the discharge frequency of the primary muscle spindles in the cat].

The discharge frequency of primary muscle spindle afferents was recorded out of the tibial anterior muscle of the cat under a ramp-and-hold stretch. The rate of the ramp stretch was increased stepwise from 1 to 100 mm/s. Its amplitude was kept constant and was 7 mm. Whereas the initial length of the host muscle was enhanced in steps of 3 mm to reach a maximal augmentation of the muscle length of 12 mm. The decrease of the discharge frequency from the dynamic peak (the discharge frequency at the end of the dynamic phase of stretch) onto the static value (the discharge frequency 3 s after the end of the dynamic phase of stretch) comprises the receptor adaptation which had been analysed. Two components could be defined in the course of the adaptation, the fast and the slow decay (fig. 2). The border between the two components is the discharge frequency of static maximum. The static maximum is the first discharge frequency at the beginning of static phase of stretch. The static maximum increases with the initial length (fig. 3) and is independent of the stretch rate. The fast decay, the decrease of the discharge frequency from the dynamic peak onto the static maximum, lasts for only some 10 ms. The degree of the fast decay turns out to the dependent on the rate of ramp stretch by a power function whose exponent has in the mean a value of 0.5 (fig. 4).(ABSTRACT TRUNCATED AT 250 WORDS)