Experiments have been carried out in order to study which ascending spinal pathways are utilized by peripherally-originated impulses and activating callosal fibres. In chloralose-anaesthetized cats stim uli of graded intensities were applied to the superficial (SRN) and deep (DRN) radial nerves. Mass potentials were recorded simultaneously from the cerebral cortex (SI, SII and 3a areas, according to the experiment), from the somesthetic callosal region (SCR) and, on some occasions, from the distal end of a cut dorsal rootlet (C7-C8; electroneurogram). The amplitude curves of cortical and callosal responses (averagings of 50 responses) were plotted as a function of stimulus intensities. In a first series of experiments (preparations with SCT intact) the dorsal columns (DC) were transected. After this section, the cortical and callosal responses elicited upon SRN stimulation were clearly reduced (60-75% in the cortex and 80-85% in the SCR) and their thresholds were slightly raised. The latency of the cortical responses was slightly increased (about 0.5 msec), whereas that of the callosal ones exhibited a greater increase (1.73 msec). In the cortical responses (3a) elicited upon DRN stimulation DC transection provoked amplitude reductions of 80-85% in respect of controls, and an increase of threshold up to about 2 cT (cortical threshold) whereas the callosal responses exhibited amplitude reductions of 55-65% and but a slight increase in their threshold. The latency of the cortical responses increased by 2.83 msec and that of the callosal ones by 2.73 msec. In a second series of experiments (preparations with DC intact) the spino-cervico-thalamic tract (SCT) was transected. After this section, the cortical potentials which were elicited upon SRN stimulation exhibited amplitude reductions of 15-45% when stimulus strengths lower than 2.6-2.8 cT were applied, whereas no amplitude change was observed when stronger stimuli were utilized. The impairment in the amplitude of callosal potentials was observed not only in the responses evoked by weaker stimuli but, usually, in all responses evoked over the entire strength range tested. No significant change was seen in the latency of both cortical and callosal responses. Upon DRN stimulation the cortical mass potentials exhibited a change in amplitude, threshold and latency. On the contrary, the callosal responses showed an amplitude reduction of about 50% and a slight increase in threshold, whereas no change was observed in their latency. The differential effects exerted by the sections on some parameters of the cortical and callosal mass potentials can be discussed with reference to the “filtering action” exerted by the somatosensory cortex on its callosal output.
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Con il contributo del Ministero per i Beni e le Attività Culturali