Recovery functions of short and long latency reflexes in hand muscles evoked by paired stimulation of peripheral nerves.

Double-shock bifocal peripheral nerves stimulation with varying interstimulus intervals (ISI) from 1 ms to 5000 ms was applied to healthy human subjects. Short (SLRs) and long latency reflexes (LLRs) from the slightly active first interosseus dorsalis muscle were recorded after an electrical conditioning stimulation of the index finger and test stimulation of the median nerve. It was found that with intervals from 1 to 5 ms the size of the SLR and LLRs after the second shock was unstable and almost oscillated around that evoked by a single shock, possibly due to transmitter depletion at these short ISIs. With intervals of 10 ms and longer, the test responses progressively increased: the SLR to 100 ms ISI, the LLR1 to 500 ms ISI and the LLR2 to 2000 ms ISI, probably due to temporal summation and some release of the transmitter substance after the conditioning cutaneous stimulus. All the recovery curves fully returned to the control value at 2000 ms and 3000 ms ISI, respectively. The main result is that cutaneous conditioning stimulation is not able to reduce or even to block the test SLR and LLRs. Interference phenomena did not occur between different afferents when paired stimulation was applied bifocally as the conditioning input in respect to a test nerve volley, was very limited. On the contrary, the test responses were far larger than the control response, probably due to some temporal summation with double-shock stimulation and because of the enhanced spinal excitability during slight voluntary contraction.