After-effects of fusimotor stimulation on the response of muscle spindle primary afferent endings.

1. The experiments were performed on the soleus muscle of the anaesthetized cat in which the ventral roots had been cut.2. A short period of repetitive stimulation of a single fusimotor fibre which influenced a particular spindle primary ending invariably caused a characteristic alteration in the response of the same ending to a subsequently applied ramp stretch of the muscle. The change consisted in the appearance of a burst of impulses at the beginning of the stretch where none had been present before. Occasionally, such an ;initial burst' was spontaneously present; it was then enhanced following fusimotor stimulation.3. This after-effect of fusimotor stimulation was abolished by a subsequent stretch of the muscle, but otherwise persisted for over a minute.4. When the muscle was released to below the length at which the spindle had been facilitated and a testing stretch applied from the new initial length there was no burst of impulses at the beginning of stretch. There was, however, a burst as the muscle was stretched through the length at which the fusimotor fibre had been stimulated.5. These effects are suggested to be due to the persistence of stable bonds between the actin and myosin filaments of the intrafusal fibres, so that their previously activated regions were ;stuck' at the length they were when the fusimotor stimulation was applied.6. Such effects were produced both by static and by dynamic fusimotor fibres. The effects of the two kinds of fusimotor fibre, however, appeared to be mediated by different intrafusal muscle fibres. This was shown by stimulating one kind of fibre with the muscle slightly stretched, then releasing the muscle a few mm and stimulating the other kind of fibre to the same spindle. A subsequent testing stretch then elicited two bursts, one at the beginning and one in the middle of the stretch.