Some of the most dangerous patients a physician will ever encounter are hyper-aroused. That is, the normal mechanisms to sustain and modulate conscious alertness have gone haywire, and have rendered a person confused and often frightened and agitated for all the wrong reasons.
I'll talk about those mechanisms now, and come back to describe some of the problems of arousal in the next few posts.
There are really two distinct arousal systems in the brain. One sets the general, background tone of brain activity and the other alters the level of activity, according to shifting phases of need.
The first kind, termed "tonic arousal," maintains the brain in readiness for all other functions. Tonic arousal arises largely from neurons that originate in deep brain (subcortical) structures and stimulate other neurons throughout the brain with the neurotransmitter acetylcholine.
You can actually see tonic arousal, if you look at a brain wave tracing (electroencephalogram, or EEG). An EEG measures the electrical activity in the brain, via electrodes placed around the scalp. When you are awake and alert, your EEG tracing reveals rapid, synchronized activity across the entire brain, at a pace of about 40 cycles per second (40 Hz). This rapid, rhythmic signal corresponds to the constant cycle of neuronal activity between cortex and the major subcortical structures (like the thalamus). It takes about a 40th of a second for a thalamic neuron to signal cortex and for the cortex to send a return signal.
The rhythm of these oscillations has been tied to the nature of conscious perception. In the sleeping brain, a limited state of tonic arousal defines the rapid eye movement (REM) sleep stage, when dreaming occurs. In deep sleep, the rapid oscillations disappear, and the tracing becomes relatively flat, with occasional slow waves.
The high frequency oscillations during wakefulness are critical to consciousness. They indicate that across the brain, the neurons that are active are firing in unison. Picture an orchestra performing in perfect synchrony, each musician playing a series of designated notes at the same time to produce a powerful, complex tone--like the synchronous cycles in the waking EEG--versus the cacophony of an orchestra in the warm-up before a symphony, when the pitch rises and falls randomly and the notes from one musician become lost in the noise.
The fact that the activities of the conscious brain occur in a regular, staccato pattern may challenge one's personal experience that consciousness feels like a fluid stream of thought. Instead, it appears to be more like a movie or an animated cartoon. Filmed movies consist of long strips of still images projected at a pace of 24 frames per second, which produces the illusion of motion in the characters on screen. But one no more sees these figures on a screen actually move than one sees anything move. Just as we assemble the perception of motion from a series of still movie images, we construct the continuity of consciousness out of a series of 40 Hz cycles of synchronized cortical activity. Our brains automatically impose the quality of movement on objects that shift position in our sensory fields during the brief interludes between thalamocortical cycles.
Now that the movie of your mind is running smoothly, how do you modulate it? The other arousal process, "phasic arousal," alters the intensity of arousal on demand. Intense hunger, thirst, heat, cold, pain, injury, alarm, or threat conveys urgent signals to subcortical structures that, in turn, signal the brain to arouse the body for action. These signals travel to the cortex, by way of neurons that project the neurotransmitters serotonin, norepinephrine, and dopamine (also called the monoaminergic neurotransmitters) across the cerebral cortex.
Phasic arousal varies to a mild degree throughout any moderately stimulating day and tends to increase in short bursts on demand (for example, to sharpen one's alertness while preparing to cross a busy street). Phasic arousal disengages upon satisfaction of the drive that triggered its onset. When the intense need has been satisfied, or the source of environmental threat eliminated, subcortical structures no longer receive urgent signals from the body, the monoaminergic systems no longer receive urgent signals from subcortical structures, and arousal diminishes appropriately. When phasic arousal continues unabated, then you have stress, and all the problems that can go with it (as discussed in an earlier post, The Plastic Brain).
The problems with arousal tend to come from having too much of it, too little of it, or from a lack of regulation over it, as I will illustrate in the next few posts.
