Arousal System (Part I)

Arousal is a physiological state of wakefulness and response to environmental and internal stimuli.

Arousal brings us to state of readiness for action. Arousal system is our guard and watchdog. It is designed to respond with increase vigilance to internal (such as hunger or pain) or external (danger or seeing a prey) stimuli. Evolutionary speaking, arousal is essential for survival and appropriately adjusted level of preparedness is necessary for accomplishing the task. Arousal (also called alertness, vigilance, or wakefulness) is not limited to keeping us awake.

In nature, insufficient arousal might turn predator into a pray and constantly high arousal is wasteful and exhausting. In addition, it is also important for arousal to rise quickly in danger and to decrease when the alarm proves to be false or resolved. Anything short of that would be unsafe or unwise.

Arousal originates in Reticular Formation, one of the oldest parts of the brain located in brain stem. It is also known under the new name Reticular Activation System. Damage to this area of the brain can lead to coma. The amygdalae (plural of amygdala – almond shaped structure on each side of the brain) are believed to be main regulators or controllers of arousal, like a relay. Other parts of the brain (e.g., suprachiasmatic nucleus, perifornical area, locus coeruleus, etc.) also play important role in regulation of arousal and sleep.

As you probably guessed, the picture is more complex because other structures, sensory organs, pain pathways, executive functions, and many others contribute to state of arousal. Reticular Activation System provides nonspecific stimulation to cerebral cortex to prepare it for incoming sensory perception (information coming from senses). Anterior cingulate and insula are active during fearful experiences and also during monitoring surroundings and paying attention to important (salient) information. Other regions, including  hippocampus and prefrontal cortex are specialized in supporting memory for emotional experiences and play important role in emotional processing, memory and attention.

It is hard to find neurophysiological function not connected to arousal  one way or another. Our behavior and emotions, sleep, nutrition, safety and fears, romance and disgust include components of arousal system. Arousal fluctuates significantly in course of a day. It is at the lowest when we are asleep (even during the sleep it is not flat but changes with sleep phases).

Vignette
Imagine a real estate agent leaving late evening office meeting. She is hypervigilant (high arousal) as she walks through a dark street in unfamiliar part of town and suspicious of any movements until getting into her car (some decrease of arousal). On a freeway she becomes more calm (deceased arousal ), and familiar sounds of her favorite station are soothing and comforting for the rest of the trip, except for a brief episode of break lights (temporary increase of arousal and vigilance) when passing the site of a recent car accident. Tired, after a glass of wine at home, she reads a novel and is ready fall asleep (low arousal) when, all of a sudden, hears screeching yelp outside (rapid increase of arousal to alarm mode) and rushes to the window to investigate. Apparently, that was only a cat (rapid decrease of arousal) and she goes back to bed to falls asleep in 10 minutes (very low arousal).

Physiological variations of arousal described in the vignette are in concordance with the events and serve ongoing needs. We need to get excited and alerted when circumstances call for it and lower our tension when necessary.

Everyone has different life experiences. Yet, our brains respond to the myriads of scenarios with limited set of available tools. There is no special path for loss in baseball game and a separate one for a missing cat. A lottery winner and a father of a newborn get excited through the same emotions.  Brain does not have specialized organ for every unique stressful situation but responds via existing fixed pathways.

High or low levels of arousal are dictated by sum of multiple influences, internal (glucose level, hormones, muscle tension, etc.) and external (boring or exciting news, relationships, losses, etc.) We feel in ourselves and observe in others only the final tally of these influences. Relevance of the news determines the level of arousal,  the content gives valence (positive or negative emotions associated with the news). Hence, for a mother hearing about her son breaking finger would be more impactful to her than a news story of dozens of people missing after an earthquake in Turkmenistan. And a hypothetical 50 year old might be equally unable to fall asleep after a news of potential job promotion and abnormal blood test.

In addition to maintaining alertness, arousal system is closely involved in sleep-awake cycle. When well regulated, this system promotes healthy sleep and awakening. Functionality of the systems is determined by ease with which it bring itself into fully awaken and sleep state.

Higher than average arousal may not be all that bad and is associated with success in modern society. Slightly anxious student do better on tests when compared with students with too high or too low arousal. Mildly anxious teenagers are less likely to break the rules and experiment with drugs. In adults, mild anxiety is associated with perfectionism, thoroughness, and diligence

Mild anxiety carries some favorable features in modern society. It might not have been as important for the Vikings or Genghis Khan’s Mongolian hoards warriors who valued bravery, cruelty, and impulsivity over cautious perfectionism. The cautious and careful were less likely to put themselves in harm’s way and ignore danger. They did not show disregard for injury and death. They passed these qualities to their children, the anxious genes persisted and ultimately survived. Although the spoils of war went to the remaining victors, the meek ultimately inherited the earth.

Michael Levin, M.D., Medical Director of EBPG, psychiatric group practice serving San Ramon, Danville, Dublin, Pleasanton, Walnut Creek, and other cities in Contra Costa and Alameda Counties, CA.

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