Arousal System (Part II)

It might be useful to compare certain aspects of arousal system with car burglar alarm. The alarm threshold sensitivity is individually set and when triggered blasts to its maximum loudness, in a manner of ‘yes-or-no’ response. A sensitive burglar alarm can be activated by a passing truck or mild earthquake or actual burglar. The alarm is not designed to be so sophisticated as to know the difference between the truck and the burglar. After being triggered, it will ring at a preset loudness and duration; after that, it turns off. Its job is to alert the car owner who then will choose the right course of action.

Our Arousal System is similarly designed but differs from the burglar alarm in the following way:
– The threshold of the brain alarm is continuously changed in response to the environment and internal mechanisms

– We use effective tools (anatomically independent from the alarm) to control intensity and duration of the high arousal state

– We can anticipate and avoid the triggers

– Humans differ from one another by threshold, intensity, and duration of arousal

One might get an impression that human and animal arousal systems are very similar. In fact, they are. What makes us different from the animals is our ability to recognize high or low arousal and adjust it depending on tasks and circumstances. We can also overcome our high state of high anxiety and, for example, call for a dental appointment, even if the though alone makes us scared. Alternatively, we can force ourselves stay alert and vigilant when being tired and driving at night. We can do these with help of well-developed prefrontal lobe (see the section on Executive Function in later posts), other parts of the brain, and also chemicals that became part of our everyday life.

Individual arousal system can be manipulated chemically and mechanically. Practically every culture and civilization has had the tools to either increase or decrease the arousal. In the western culture and in the Far East, alcohol has been widely used to decrease arousal. In the Middle East and parts of India, hashish and other cannabinoids served the same role in addition to various opioids coming from poppy plants. Kava or kava-kava (Piper methysticum) has been used in Polynesia and parts of Micronesia. Valerian root ( a relaxant) was known to ancient Greeks. ‘Calming herbs’ list also includes passion flower, chamomile, ashwagandha (India), fennel, green tea called L-theanine, lavender, and Evening Primrose.

Natural psychoactive stimulants include caffeine “abused” by billions in form of coffee and tea. Khat, an   amphetamine – like plant, banned in several middle eastern countries for its psychoactive effect, grows in the Horn of Africa and the Arabian Peninsula. Ginseng, cocoa, Betel nut, and ephedra (known in Chinese medicine as ma huang) have been promoted as natural stimulants.  And, of course, there are coca leaves.

There is also nicotine with complex mood altering effect which can be both, activating and inhibiting.

Arousal system can be influenced mechanically. Have you watched cats waking up from a sleep? Cats stretch their bodies sending myriads of signals from muscles to the brain.  Afferentation theory states that any action that increases muscle contraction or stretch will stimulate the brain. Muscle stretch receptors (proprioception) stimulates arousal centers inside the brain leading to higher level of wakefulness and alertness.

Neurotransmitters (brain chemicals, messengers) and hormones modify arousal as a sum of all influences.  Generally, very generally indeed, glutamate, dopamine, norepinephrine, histamine receptors in amygdala, when stimulated, usually increase the arousal. Amygdala is also rich in cortisol (stress hormone) receptors  while gamma-aminobutyric acid (GABA) and certain – but not all – serotonin receptors, tend to decrease or modulate arousal. The reason for making so many reservation about serotonin and dopamine effect comes from understanding that their activating or inhibiting properties depend on

a) type of receptors (5HT1A vs. 5HT2A or D1 vs. D2)
b) their location (post-synaptic vs. pre-synaptic)
c) their positions in the brain (reticular formation vs. frontal lobe)
d) and their reciprocal interaction with other neurotransmitters (e.g. increase of serotonin in mesolimbic area decreases dopamine in frontal cortex)

We should not make simplistic statements about neurotransmitters relationship and state of arousal, such as acetylcholine is always activating but serotonin is always inhibiting.

To appreciate complexity take a look at histamine receptor. Histamine, you might recognize by its name from commonly used anti-histamines prescribed for allergies, is a chemical also present in the brain. Stimulating post-synaptic histamine H1 receptors activate cerebral cortex, making us more alert. However, if the same histamine goes to pre-synaptic H3 receptors, then release of histamine from this neuron stops, less histamine will be releases, hence less activation. That means that if we give a medication that blocks H3 receptor, this blockade will release more histamine and the end result would be activation of histamine receptors. By analogy, think about a device that holds back the foot that pushes on the brakes.  The car will move faster.

Arousal system is also influenced mechanically. Have you watched a cat waking up from a sleep? The cat stretches its body sending myriads of signals from cat’s muscles to the brain. Afferentation theory states that any action that increases muscle contraction or stretch will stimulate the brain. Activating muscle stretch receptors (proprioception) stimulates arousal centers inside the brain leading to higher level of wakefulness and alertness.

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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