Mesencephalic and bulbar reticular formation influences on somatosensory transmission through the thalamus

The influence of the brain stem reticular formation (RF) on transfer of somatosensory information has been studied in intact cats and in cats with a chronic hemisection of the brain stem at the pretrigeminal level. An air-jet applied to the hairy skin receptive field evoked the discharge of thalamocortical relay cells in nucleus ventralis postero-lateralis, extracellularly recorded. Conditioning stimuli were brief trains of electrical pulses to mesencephalic (MRF) and bulbar (BRF) reticular formation. In intact cats both MRF and BRF induced in a small percentage of cells slight facilitation or inhibition of evoked discharge. In pretrigeminal cats MRF stimulation increased the probability of discharge in response to peripheral stimuli, whereas BRF stimulation induced a striking decrease in evoked firing in a great percentage of neurones. It is suggested that RF activation can decrease the incoming peripheral volley by means of its caudalmost part, while it is able to enhance thalamic response by way of the more rostral structures.