The synaptic events, induced by transcallosal afferents in somesthetic cortical neurones were studied with intracellular technique in 21 units (14 belonging to SI and 7 to SII), out of a larger sample whose characteristics of transcallosal as well as peripheral reactivity were determined extracellularly. The recordings were obtained from chloralose-anaesthetized, curarized cats by means of glass micropipettes (2M K-citrate or 2.5 M KCl; 10-50 M$\Omega$ in resistance) and DC-, AC-amplifying conventional apparatus. Upon peripheral stimulation each neurone was classified as reactive to small, contralateral (Group I; 5 units) or to wide, ipsi-and contralateral peripheral receptive fields (Group II; 16 units). All neurones were further tested with electrical stimulation of contralateral SI and SII cortical areas. Transcallosal excitatory and inhibitory effects were obtained. The excitatory effects were either small, slowly-rising EPSPs, variable both in time-course and amplitude (11 units), or strong and fast-rising EPSPs (5 units). The former triggered inconsistent and unsteady spike discharges, whereas the latter easily precipitated firing activity, followed in turn by clear-cut secondary IPSPs, just as did peripheral stimulation in both groups of units. Pure inhibitory effects were observed in 5 more neurones, yielding primary IPSPs capable of blocking both spontaneous and evoked discharges. Occasionally, units showing interneuronal-like discharges were found, temporally related to the development of primary IPSPs appearing in the former cells. The latencies of both excitatory and inhibitory slow potentials were consistent with their generation by transcallosal impulses. According to the evidence obtained, it is suggested that most transcallosal excitatory effects might be generated by remote synaptic contacts located at apical dendrite spines, thus modulating the neuronal activity induced by peripheral afferents. The inhibitory effects induced over the callosal route are post-synaptic in nature and might be generated in the cortex itself through interneuronal mediation.
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Con il contributo del Ministero per i Beni e le Attività Culturali