Paraesthesiae induced by prolonged high frequency stimulation of human cutaneous afferents.

1. The present study has explored the behaviour of human cutaneous afferents following conduction of prolonged trains of impulses at 200 Hz for 10-20 min, correlating the resultant changes in excitability with the perception of paraesthesiae. 2. Tetanization for 10 min resulted in activity-dependent changes in axonal excitability, with an initial period of hyperexcitability, followed by a long-lasting subexcitability. All subjects experienced paraesthesiae soon after cessation of the tetanic train, and these subsided gradually over 16 min. 3. Longer tetanic trains of 20 min duration resulted in greater changes in axonal excitability, but with paraesthesiae of a similar time course. The post-tetanic increase in excitability was abolished when short tetanic trains were delivered > 30 min before long trains, but all subjects still experienced paraesthesiae. 4. Threshold distributions following tetanic stimulation for both 10 and 20 min established that all axons contributing to the sensory volley underwent a uniform pattern of post-tetanic threshold changes. There was no evidence of a bimodal distribution with some axons hyperpolarized and others depolarized, as occurs with motor axons. However, the excitability changes were graded, with axons of lowest threshold undergoing a proportionately greater increase in excitability than axons of higher threshold. 5. The post-tetanic excitability changes were greater at the site of stimulation than elsewhere along the peripheral nerve. However, DC polarizing currents applied at this site failed to alter the sensation of paraesthesiae in the post-tetanic period. Furthermore, local anaesthetic block of the peripheral nerve proximal to the stimulation site failed to suppress the paraesthesiae. 6. The uniform pattern of post-tetanic threshold changes for cutaneous afferents differs from the bimodal distribution seen with post-ischaemic and post-tetanic motor axons. This difference in behaviour may reflect greater inward rectification and greater expression of a non-inactivating threshold conductance in cutaneous afferents. It is suggested that the ectopic activity responsible for paraesthesiae in the post-tetanic period arises from a more central site than the peripheral nerve.