Die schnelle und langsame Komponente der Rezeptoradaptation in der Entladungsfrequenz primärer Muskelspindelafferenzen der Katze

Abstract

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 cept 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). At the same time the fast decay is independent of the initial muscle length. The slow decay, the decline of the discharge frequency from the static maximum onto the static value, is dependent on the initial muscle length (fig. 5) and negligably dependent on the rate of ramp stretch. Additionally, the time constant x of the slow decay had been determined (fig. 6). x has in the mean a value of 720 ms at a stretch rate of 10 mm/s. And it turns out to decrease weakly with increasing stretch rate. The results are compared with those of other mechanoreceptors. Mechanisms for the rise of two components in the course of the receptor adaptation in cat's muscle spindle response are discussed.