Ascending Cholinergic and Monoaminergic Systems in the Brainstem: Do they Constitute a Reticular Activating System?
The concept of the “ascending reticular activating system” (ARAS), promulgated by Moruzzi and Magoun (1949) as the essential element in the neural mechanisms underlying wakefulness, suffers significantly from a lack of specificity as to the neurons which might actually comprise the ARAS. With the new techniques of neuroanatomy several clearly defined structures with ascending projections have been discerned in or near the putative ARAS: the noradrenergic projection from the locus coeruleus (LC), serotonergic projection from the dorsal raphe (DR), and the cholinergic projection originating in the laterodorsal and pedunculopontine tegmental nuclei (LDT and PPT). These projection systems all have anatomical features appropriate for the ARAS such that they send network-like axons with many varicosities directly to diffuse areas of the forebrain, except that the cholinergic influence on most cortical areas is indirect via thalamus, hypothalamus or basal forebrain nuclei (Fig. 1). The obvious question is whether one or more of these newly specified entities is, or constitute a major component of, the ARAS.
KeywordsLocus Coeruleus Dorsal Raphe Repetitive Stimulation Locus Coeruleus Neuron Dorsal Lateral Geniculate Nucleus
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