[New concepts in relation to generating and maintaining arousal].

INTRODUCTION: First conceived in 1949 by the research conducted by Moruzzi and Magoun, the concept of the ascending reticular activating system (ARAS) played a vital role in understanding the physiology of sleep and arousal, as well as in explaining the pathophysiological bases of diseases characterised by insomnia, hypersomnia or coma. Sixty years after this discovery, advances in our knowledge of the anatomy, electrophysiology and neurochemistry of the pathways involved in the generation and maintenance of arousal have made it necessary to reassess the original concept of ARAS. Nevertheless, in spite of the fact that the pathologies which, in some way or another, affect the state of arousal are common in the daily practice of different medical disciplines, the new physiological concepts in relation to the activating systems (generators of arousal) are not dealt with by a large number of medical practitioners. DEVELOPMENT: This work is a brief update on the activating systems, with special attention given to the concepts that can be applied most readily in order to gain an understanding of the pathophysiology of arousal.
CONCLUSIONS: The new concepts about the activating systems are as follows: a) the activating systems are not only to be found in the reticular formation of the brain stem, but also include specific regions of the posterior hypothalamus and the anterior basal brain; b) the activating systems are made up of different neuronal groups that act by means of specific neurotransmitters or neuromodulators; and c) the activating systems generate arousal by direct modification of thalamic and cortical activity.