Functional organization of the brain stem reticular formation and sensory input.

Injections of 20 microg of adrenaline, dissolved in 10, microl of Tyrode solution, directly into the brain stem reticular formation, are followed by a transient increase in the amplitude of the cortical-evoked response obtained from single-shock stimulation of the optic chiasma, while injections of the same amount of acetylcholine produce a transient decrease. Furthermore, injections of procaine in the medial region of the rostral pontine reticular formation are followed by a long-lasting increase in the amplitude of these evoked responses, while the same injections at mesencephalic levels produce a marked decrease. These findings are interpreted in terms of the presence, in the reticular formation, of two antagonistic ascending systems: one adrenergic, the other cholinergic, whose tonic activity originates in the mesencephalic tegmentum in one case and in the caudal regions of the brain stem in the other.