Comparative analysis of the role of the second and first somatosensory cortical areas in somatic responses of the medullary reticular formation in rats

Evoked potentials and unit activity in the medullary reticular formation were investigated in unanesthetized, curarized rats during cold blocking or after extirpation of the cortical representation of one of the stimulated limbs. Local cooling or extirpation in area SII, unlike blocking of area SI, leads to a small (up to 30%) decrease in amplitude and a very small change (up to 10 msec) in the temporal parameters of evoked potentials arising in the reticular formation in response to electrodermal stimulation of the contralateral limb, whose representation in the cortex was blocked. Predominance of corticofugal influences from SI over those from SII was discovered both in experiments with evoked potentials and during analysis of somatic spike responses of reticular formation neurons. Corticofugal control over activity of the medullary reticular formation in rats exerted by neuronal mechanisms of somatosensory areas SII and SI thus differs both qualitatively and quantitatively.