Arousal refers to responsiveness to stimuli. The reticular activating system consists of a portion of the brainstem reticular formation and parts of the thalami that diffusely project glutamate to the cerebral cortex. This system “wakes” the cerebral cortex from sleep in response to stimuli, or based on circadian rhythm (the internal daily “clock”). The reticular activating system, along with the cortex of at least one cerebral hemisphere, is required for consciousness. The reticular activating system could be thought of as producing an empty consciousness that is filled with content by the cerebral cortex.
A patient with normal arousal is called alert, and one with no purposeful responses to stimuli (beyond reflexes) is called comatose. There are many terms for intermediate states of arousal, but they are not used consistently, so it is better to describe what responses occur to stimuli of increasing intensities, from normal voice, to loud voice, to light touch, and then to painful stimuli. Nailbed pressure is the most useful painful stimulus for assessing diminished arousal because, when done properly, it provides a strong stimulus without tissue damage, and it can provide information about motor and somatosensory functions for each limb as well.
Arousal and other cognitive functions, particularly attention, appear to also require diffuse projections of certain neurotransmitters to widespread areas of the cerebral cortex: acetylcholine from the basalis and septal nuclei in the inferior frontal lobe, and histamine from the hypothalamus. Synchronizing the internal circadian rhythm to the external light-dark cycle is called circadian rhythm entrainment. This involves retinal ganglion cell axons that leave the optic tracts to synapse in the hypothalamus, which then regulates activities such as the release of the hormone melatonin from the pineal gland. The pineal gland is a midline structure of the brain posterior to the thalamus. Several groups of brainstem nuclei also play a role in arousal and other cognitive and emotional functions by diffusely projecting certain neurotransmitters to widespread areas of the cerebral cortex, including: norepinephrine from the locus coeruleus; dopamine from the ventral tegmental area; and serotonin from the raphe nuclei.
Decreased arousal is usually either from focal dysfunction of the reticular activating system in the brainstem or thalamus, or from diffuse dysfunction of the cerebral cortex bilaterally. Diffuse cerebral cortical dysfunction may cause states of empty or limited consciousness called either the vegetative or the minimally conscious state. The vegetative state refers to wakefulness without evidence of awareness, and is called persistent after one month. The minimally conscious state refers to observable, but minimal, purposeful behaviors or responses to stimuli. Sleep is a normal state of unconsciousness that is poorly understood, but multiple brain areas, including the hypothalamus, appear to be important for normal sleep-wake transitions and sleep maintenance.