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The effects of induced hypothermia in the unanesthetized immobilized cat on somatic afferent evoked potentials from the posteroventral lateral nucleus of the thalamus, periaqueductal midbrain reticular formation and dorsal column of the spinal cord were studied in 28 cats. At a rectal temperature of 23°C, marked depression of the responses recorded from all loci was demonstrable. The afferent midbrain reticular formation was demonstrated to be more sensitive to hypothermia than the thalamic relay nucleus. Augmentation of the spinal cord dorsal column evoked potentials was noted between 33°C and 25°C. Conduction time measurements indicated that between 37°C and 27°C the velocity of transmission along the nerve fibers is more affected than transmission across a synapse.

Effects of i.v. infusion of urea were studied in 26 cats by recording spontaneous cortical activity and somatic afferent evoked potentials from the posteroventral lateral nucleus of the thalamus and periaqueductal midbrain reticular formation. Subsequent to the administration of a 20% solution of urea, spindling and seizure discharges appeared in the electrocorticogram. The evoked potentials from the posteroventral lateral nucleus of the thalamus and reticular formation remained unchanged. Control studies indicated that the operative procedure, including the insertion of subcortical electrodes, hydration incident to fluid administration, possible deterioration of the preparation and alteration of the osmolarity of the serum did not produce the observed results. It was demonstrated that urea administered intravenously, in amounts producing blood NPN values commensurate with those observed in patients with uremia, alters cortical activity and excitability.