Oscillations in the discharge frequency of cat primary muscle spindle afferents during the dynamic phase of a ramp-and-hold stretch.

Under a ramp-and-hold stretch the Ia afferent responds to the beginning of the ramp phase with an initial frequency peak, which is followed during the ongoing dynamic phase of stretching by further frequency peaks. In this investigation the behavior of the initial peak is compared with the behavior of the subsequent peaks under four experimental conditions: (i) increasing prestretch of the muscle, (ii) increasing stretch rate, (iii) change in the waiting time before a ramp stretch, (iv) variations in the degree of prestretch preceding a ramp stretch, which is then started from a medium degree of prestretch. Under three of these four experimental conditions the initial peak behaves qualitatively differently from the subsequent peaks. This result gives rise to the interpretation that the causes of the initial peak and the subsequent peaks are different. The initial peak is caused by a largely synchronous opening of acto-myosin bonds in the polar parts, whereas the subsequent peaks may be caused by intrinsic oscillatory properties of the receptor potential.