T. Manzoni

Patterns of the somesthetic messages transferred through the corpus callosum

Summary1.In the rostral part of the corpus callosum (somesthetic callosal region, SCR) fibres were identified, through which the callosally-projecting cells of the somatosensory areas transmit to the other hemisphere signals originated in the body surface.2.With seriate macroelectrode penetrations it was found that, to some extent, the body surface is represented somatotopically in the rostrocaudal extent of the SCR. The strongest mass potentials to trigeminal, fore- and hindlimb stimulation were recorded from the rostral, middle and caudal portions of the SCR. The whisker region and the forelimb (both paws and proximal segments) appeared to have the widest callosal representation.3.Ablation experiments showed that callosal somesthetic fibres originate in both SI and SII areas and that only impulses set up in the contralateral hemibody are relayed in these areas. Direct stimulation of the latter evoked within the SCR mass potentials whose rostrocaudal distribution parallels that of the peripherally evoked responses.4.Exploring the SCR with microelectrodes, 43 spontaneously active fibres were isolated, all reactive to electrical and physiological stimulation of the related peripheral receptive fields. These were located in trigeminal (31 fibres), segmental (10 fibres) or both in trigeminal and segmental regions (2 fibres). The extent of the receptive fields and the reactivity characteristics of almost all the fibres sampled were lemniscal in type, and similar to those of the somatotopic neurones of cortical somatosensory areas.

Sychronizing and desynchronizing fastigial influences on the electrocortical activity of the cat, in acute experiments ☆

Abstract 1. 1. The EEG effects following electrical stimulation of the roof nucleus and following selective ablations of parts of it have been systematically studied in acute cat preparations. 2. 2. In addition to the well known EEG activating effect, fastigial stimulation can also bring about definite EEG synchronization. This effect is generally observed with low frequency stimuli and is not dependent on haemodynamic changes or on those changes in muscular tone that fastigial stimulation is known to bring about in noncurarized preparations. The synchronizing response always occurs bilaterally and generally involves the whole extent of the hemispheral cortex. The response appears at the beginning of stimulation or after some delay and often outlasts the stimulation period for seconds or even minutes; it can still be evoked after bilateral interruption of the superior cerebellar peduncles but it disappears after severance of the inferior ones. 3. 3. Selective bilateral electrocoagulation of localized portions of the roof nuclei are followed by different EEG changes. To some extent the effects can be related to the site of the lesion. In particular, localized lesions of the rostro-medial portions are followed by clear-cut and long-lasting EEG activation, whereas, after lesions confined to the rostro-lateral portions, the opposite changes are seen, though of shorter duration. 4. 4. Experiments on selective stimulation of rostro-medial or rostro-lateral parts of the fastigial nucleus were made possible by destroying, some days before stimulation, the part whose effects were not to be tested. Under these conditions it was observed that synchronizing responses can easily be obtained to low frequency rostro-medial stimulation after destruction of the rostro-lateral portion, whereas once the rostro-medial portion has been destroyed the only effects elicitable by rostro-lateral stimulation are desynchronizing. 5. 5. To some extent synchronizing structures might be electively distributed within the rostro-medial portions of the roof nuclei. The close functional and hodological connections between the roof nuclei and the brain-stem systems of diffuse ascending projection might account for the facts observed.