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Arousal

State of being awoken

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Through Physiology, Psychology and Sense

Arousal is mediated by several neural systems. Wakefulness is regulated by the ARAS, which is composed of projections from five major neurotransmitter systems that originate in the brainstem and form connections extending throughout the cortex; activity within the ARAS is regulated by neurons that release the neurotransmitters norepinephrine, acetylcholine, dopamine, serotonin, and histamine.

Activation of these neurons produces an increase in cortical activity and subsequently alertness.

Arousal is important in regulating consciousness, attention, alertness, and information processing. It is crucial for motivating certain behaviours, such as mobility, the pursuit of nutrition, the fight-or-flight response, and sexual activity. It also holds significance within emotion, having been included in theories such as the James–Lange theory of emotion and being represented as one of the axes in most dimensional models for classifying emotions. According to Hans Eysenck, differences in baseline arousal level lead people to be extraverts or introverts.

The Yerkes–Dodson law states that an optimal level of arousal for performance exists, and too little or too much arousal can adversely affect task performance. One interpretation of the Yerkes–Dodson Law is the "Easterbrook cue-utilisation hypothesis". Easterbrook's hypothesis suggests that under high-stress conditions, individuals tend to focus on a narrower set of cues and may overlook relevant information, leading to a decrease in decision-making effectiveness.

Neurophysiology

Wakefulness is regulated by the ascending reticular activating system, which is composed of five major neurotransmitter systems – the norepinephrine, acetylcholine, dopamine, serotonin and histamine, systems (each considered in the paragraphs below) – that originate in the brainstem and form connections which extend throughout the cerebral cortex. When stimulated, these systems produce cortical activity and alertness.

The noradrenergic system is a bundle of axons that originate in the locus coeruleus and ascends up into the neocortex, limbic system, and basal forebrain. Most of the neurons are projected to the posterior cortex which is important with sensory information, and alertness. The activation of the locus coeruleus and release of norepinephrine causes wakefulness and increases vigilance. The neurons that project into the basal forebrain impact cholinergic neurons that results in a flood of acetylcholine into the cerebral cortex.

The acetylcholinergic system has its neurons located in the pons and in the basal forebrain. Stimulation of these neurons result in cortical activity, shown from EEG records, and alertness. All of the other four neurotransmitters play a role in activating the acetylcholine neurons.

Another arousal system, the dopaminergic system, releases dopamine produced by the substantia nigra. The neurons arise in the ventral tegmental area in the midbrain, and projects to the nucleus accumbens, the striatum forebrain, limbic system, and prefrontal cortex. The limbic system is important for control of mood, and the nucleus accumbens signal excitement and arousal. The path terminating in the prefrontal cortex is important in regulating motor movements, especially reward oriented movements.

The serotonergic system has almost all of its serotonergic neurons originating in the raphe nuclei. This system projects to the limbic system and the prefrontal cortex. Stimulation of these axons and release of serotonin causes cortical arousal and impacts locomotion and mood.

The neurons of the histaminergic system are in the tuberomammillary nucleus of the hypothalamus. These neurons send pathways to the cerebral cortex, thalamus, and the basal forebrain, where they stimulate the release of acetylcholine into the cerebral cortex.

All of these five systems are linked and show similar redundancy. The pathways described are ascending pathways, but there also arousal pathways that descend. One example is the ventrolateral preoptic area, which release GABA reuptake inhibitors, which interrupt wakefulness and arousal. Neurotransmitters of the arousal system, such as acetylcholine and norepinephrine, work to inhibit the ventrolateral preoptic area.

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