The interaction of somatosensory evoked potentials between mixed-sensory nerves and sensory-sensory nerves

The interactions between two different nerves occur by occlusion or inhibition when two nerves share the synaptic connections. In our previous study, we have demonstrated that posterior tibial nerve and peroneal nerve sensory inputs interact with each other, i.e., preceding stimulus to one nerve suppresses the somatosensory evoked potential (SEP) of the other nerve when two stimuli are delivered in close sequence. The course of suppression follows two phases; the first one occurring at short interstimulus intervals (ISIs) of the two nerves less than 10 msec, and the second one being at around 30 msec ISI after partial recovery following the first suppression phase. In that study, we have postulated that the second phase suppression was equivalent for the movement induced "gating" mechanism. In this study, the interactions of mixed nerve (posterior tibial) and sensory nerve (sural), and also sensory (sural) and sensory (saphenous) nerves were examined. We found that the mixed nerve (posterior tibial) exerted similar dual phases of suppression (as was seen in posterior tibial--peroneal nerve study) on to the sural nerve SEP, but the reverse was not true. Also the sensory and sensory nerve interactions were not mutually equal; the sural nerve stimulation caused two phases suppression but the reverse condition did not show significant suppression. The above findings suggest (1) interference input from the sensory nerve to the mixed nerve is much weaker than the reverse condition, and (2) sensory and sensory nerves interactions occur but two nerves' interference inputs are not necessarily equal and one could dominant the other.