A study has been made of the effect of continuous vibration, at 150 Hz, upon the response of muscle spindle afferents to low frequency sinusoidal stretching (1 and 8 Hz). Using the soleus muscle of the anaesthetized cat, with severed ventral roots, recordings were made of single Ia afferents and of the massed Ia afferent discharges in the main bulk of the cut L7 dorsal root. 2. When the amplitude of vibration was large (50 micrometers, short pulses) and that of the sinusoidal stretching was not too great (50-100 micrometers, peak-to-peak) the discharge of the afferents was largely locked 1:1 to the vibration and the response to the sinusoidal stretching was abolished. 3. When the amplitude of the vibration was reduced to below that eliciting continuous afferent driving, then the response to sinusoidal stretching of any amplitude was often markedly increased. This arose through the vibration having a much more powerful excitatory action during the rising phase of the sinusoidal stretch than it did during the falling phase. 4. Averaged over a full cycle, the phase of the response to the sinusoidal stretching tended to be delayed during the vibration in comparison with the normal. This was largely dependent upon the afferents continuing to respond maximally to the vibration around the peak of the sinusoidal stretch, at which stage their unvibrated response is declining, rather than to a phase lag of the whole pattern of response. 5. The results are discussed in relation to the effects of vibration on tremor and the human stretch reflex, and on the determination of the frequency-response of spindle afferents.
