Long-lasting depression of central synaptic transmission following prolonged high-frequency stimulation of cutaneous afferents: a mechanism for post-vibratory hypaesthesia.

High-frequency vibration or electrical stimulation of cutaneous afferents may produce long-lasting hypaesthesia. Such stimulation alters the excitability of axons in the peripheral nerve but there is evidence that this does not completely explain the hypaesthesia. The present study was undertaken to determine whether a prolonged afferent barrage results in depression of synaptic transmission at a central site. Changes in central excitability to cutaneous inputs were examined in normal subjects by measuring the cerebral evoked potential at different stages after high-frequency conditioning stimulation of the digital nerves. Changes in peripheral excitability were eliminated by adjusting the stimulus intensity so that a constant afferent volley entered the central nervous system. Following the conditioning stimulation (4-5 T, 200 Hz, 10 min), the cortical potential evoked by constant submaximal test volleys was depressed by up to 50% for 25 min. The attenuation was less profound (10-20%) but more prolonged (greater than 45 min) when maximal test volleys were used, and occurred regardless of whether the high-frequency stimulation was applied to the test digit or to adjacent digits. It is concluded that prolonged activation of cutaneous afferents causes a depression in central excitability independent of and additional to peripheral changes, and it is suggested that this mechanism contributes to the associated perceptual disturbances. By analogy it is suggested that the hypaesthesia associated with prolonged vibration may be of central rather than peripheral origin.