Further observations on the depression of group Ia facilitation of motoneurons by vibration in man.

Stimulation of low threshold afferents in the peroneal nerve causes a short latency facilitation of individual tibialis anterior motor units considered to be due to the composite Ia EPSP. This facilitation is depressed by vibration 20 to 128 Hz applied over the tibialis anterior. The depression occurs without any change in the firing rate of the motor unit or in the facilitation from cutaneous afferents and so is unlikely to be due to postsynaptic inhibition. The depression can occur with vibration frequencies as low as 40 Hz and is therefore unlikely to be due to occlusion in Ia afferents. There is no evidence that vibration alters the electrical threshold of large afferents. A similar facilitation of soleus motor units resulting from stimulation of low threshold afferents in the tibial nerve is depressed for up to 75 ms following a 40 ms burst of 50 Hz vibration applied to the tendon of the tibialis anterior. The burst of vibration itself did not facilitate soleus motor units so there is no evidence to suggest that the vibration spread to soleus spindles. Homosynaptic depression is, therefore, unlikely. These findings provide further evidence that presynaptic inhibition of Ia afferents occurs in man.