The major neuroactive substances we discuss are noradrenaline (NA), dopamine (DA), acetylcholine (ACH), serotonin (SE), and histamine (HA). Most of these neuromodulatory transmitters (collectively referred to as NMTs) are located in brain stem or forebrain nuclei and modulate both subcortical and cortical structures. NA (also known as norepinephr-ine) is contained in many cell groups of the pons but the locus coeruleus is the critical structure for cortical arousal. DA-containing cells reside in the pontine tegmentum and the cortex receives dopaminergic fibers from the ventral tegmental region. ACH neurons are found in the pontine pretrigeminal area and the basal forebrain. SE (also called 5HT) neurons are located along the medullary midline in the raphe nuclei. HA is manufactured by cells of the hypothalamus.

Other neuroactive substances also influence brain arousal, but are different from the NMTs in one or more ways. Adenosine is an amino acid that inhibits other NMTs and is found in many locations throughout the brain. Unlike the other NMTs, however, it is neither a classical neurotransmitter, because it is not released synaptically, nor is it contained in cells that reside in the brain stem. Adrenaline (epinephrine), which resides in brain stem neurons and uses the same receptors as NA, does not project to the cortex and is generally thought to contribute little to conscious arousal. Adenosine and adrenaline will not be discussed further. Glutamate is occasionally listed as an NMT. Glutamate is contained in the terminals of the thalamocortical neurons and provides the synaptic inputs that drive cortical cells. We take the view, however, that arousal is a modulation of the thalamocortical inputs, not the action of the inputs themselves, and we do not include glutamate as an NMT.

This article reviews neuroscientific research conducted within the past 20 years on the roles of each NMT in arousal and identifies, where possible, patterns of activity among the arousing structures that are correlated with behavioral states. We begin by defining the terms that will be used and by describing the physiological indices used to assess arousal tone. We then provide an overview of the anatomical components of the arousal systems and their associated NMTs. For each NMT, its anatomy, its effects on target cells, and its role in specific behaviors will be discussed. We then attempt to integrate the properties of these systems and show how they shape and modulate behavior. We will not discuss the connections between arousal and sexual or consummatory behavior.

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