Conductivity of dorsal column fibers during experimental spinal cord compression and after decompression at various stimulus frequencies.

The effects of spinal cord compression on conduction of dorsal column fibers at various stimulus frequencies were analyzed in pentobarbital anesthetized cats. The responses to L6 dorsal root stimulation at 1 to 500 Hz were recorded from the L2 cord dorsum. The L4 cord segment was compressed gradually until the compound action potential (CAP) at 1 Hz was flat. There was no significant change of CAP at any frequency during the first part of compression, but there was progressive conduction failure, which was more severe with increased stimulus frequency, at a later stage. After decompression, the CAPs at all frequencies recovered progressively for 1 hour but slowly thereafter. However, marked differences were observed in recovery rate at different stimulus frequencies. The recovery rate at 500 Hz was much slower than that at 1 Hz, whereas the recovery rate at 100 Hz exceeded those at 1 Hz. Serial analysis of a train of high frequency impulses revealed the following different response patterns with stimulus frequencies after decompression. At 333-500 Hz the amplitude of CAPs decreased progressively, whereas at 33-125 Hz it increased up to 110-134% of the first CAP and then reached an almost steady level. At 200-250 Hz the amplitude increased transiently and then decreased progressively. The latency increased with decreased amplitude, and decreased with increased amplitude. Conduction failure at a high stimulus frequency (500 Hz) was observed at the compression site. In contrast, augmentation of CAPs at moderately high stimulus frequency (100 Hz) was observed rostral to the compressing site. The conduction failure at high stimulus frequency indicates incomplete impairment of spike generation in axons injured by mechanical compression and that these axons can transmit impulses at a low stimulus frequency. High frequency stimulation may be useful for monitoring of the function of the CNS axons. The mechanism underlying the augmentation of CAPs at moderately high stimulus frequency is briefly discussed.