Neuromodulatory systems of thalamus and neocortex

Abstract

Following the original description of an ascending reticular activating system located in the brainstem core, the chemical codes of several cellular aggregates with widespread projections to the thalamus and cerebral cortex have been identified by combining axonally transported tracers with immunocytochemistry, the state-dependent activities of brainstem cholinergic neurons with antidromically identified thalamic projections have been investigated in behaving animals, and the actions of generalized modulatory systems on postsynaptic thalamic and cortical targets have been explored intracellularlyin vivoandin vitro. In this concluding article on brain neuromodulatory systems I place emphasis on(a)fast (20–40 Hz) spontaneous rhythms of electrical activity that are synchronous in intracortical as well as corticothalamic networks and represent a major component of activated electrical patterns during brain arousal; the subthreshold depolarizing oscillations may bias neurons to respond synchronously within this frequency range when relevant signals reach the forebrain;(b)the converging and/or competitive effects of various activating systems; and(c)the similarities between the actions of ascending/descending glutamatergic activating projections and those of other (cholinergic and monoaminergic) modulatory systems.