History

The concept of arousal dates well back into the histories of many linguistic groups. According to Webster's Dictionary, the root arouse was used in English as far back as 1593. The first scientific investigation into arousal appeared in the 1860s. Arousal from sleep was measured by determining from how high an object needed to be dropped for the subject to awaken. Kohlshutter reported that sleep depth increased up to 1 hr post-sleep onset and then decreased through the remainder of the night.

Encyclopedia of the Human Brain Volume 1

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Modern work on arousal systems began in the era after World War I, when viral epidemics occurred that caused damage to the brain stem and produced "sleeping sickness,'' which was characterized by a profound hypoarousal. About two decades later, the first evidence for arousal centers in the upper brain stem was obtained from experimental animals. Perhaps the most cited work in this area of medicine is that of Moruzzi and Magoun. The major conclusion of their work was that brain stem influences modulated thalamocortical neurotransmission. Although the details of this conclusion have changed substantially, the core idea published more than 50 years ago was essentially correct.

In the past two decades, progress in understanding arousal systems has been advanced most dramatically through neurochemistry. It is now clear that the arousing structures of the brain stem and hypothalamus have neurochemically distinct profiles, and the neurotransmitters in each may use distinct intracellu-lar pathways to regulate the excitability of cortical neurons. Most, if not all, of these neurotransmitters show increased turnover during the aroused state and, conversely, drugs that stimulate the release of these substances cause increased cortical arousal. In addition, these neuroactive substances regulate cells of other arousing structures as well, making it all but certain that variations in behavioral states result from a change in the patterns of activity between the arousing structures.

The potential complexity of these patterns of activity is impressive. As will be discussed later, the arousing structures release five major neurotransmit-ter substances that bind to at least 33 pre- and postsynaptic receptor types. Although found in very low concentrations within the axon terminals, nine additional neuroactive substances, including neuro-peptides and adenosine triphosphate (ATP), are coreleased and bind to many postsynaptic sites. Both classes of substances are undoubtedly important for the patterns of arousal. Ultimately, understanding how the arousal response occurs depends on knowing which neurotransmitters are active and where they are exerting their effects.

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