Every so often, I am asked a question that triggers an idea. That idea percolates into more thoughts, and the next thing I know, I find myself almost compelled to write something. In this case, a blog entry, about why writers feel so tired despite sitting at a computer all day with "minimal" exertion.
|Image Copyright Dyonisos Design, 2012
Used under license from Shutterstock.com
In response, my mental exercise goes "Blog Post!" so here goes:
First, I'd like to say to the Facebook commenters that they are essentially correct - the brain consumes more than 1/4 of the "energy" of the body each day - that energy being in the form of glucose in the blood, ATP (the energy transport molecule) produced in cells, and oxygen. Muscles can store both oxygen (the myoglobin molecule in muscle is what gives red meat its characteristic color) and glucose (as the complex molecule glycogen, and as fatty acids). What physiologists call "fast-twitch" muscle, uses its own stores to quickly respond before the rest of the body can adapt to supplying blood and oxygen. Fast-twitch equates to sheer strength and sprint-like speed, but endurance requires the heart to pump and the lungs to exchange O2 and CO2 more efficiently to deliver oxygen and glucose to the "slow-twitch" (or "white-meat") muscles than can continue exertion for hours.
The brain is effectively "slow-twitch." It does not have the capability of storing either glucose or oxygen, so in the absence of muscle exertion or digesting a very large meal, the brain is the largest "customer" for oxygen and glucose. In fact, Neuroscientists and doctors take advantage of this fact when doing functional brain imaging. MRI is typically used to scan for the hydrogen molecules in water, and are able to develop detailed images of soft tissues of the body because of differences in water content. However, if the MRI is adjusted to scan for oxygen, we can develop pictures of the function of the brain on the basis of the flow and usage of oxygenated blood to the most active brain regions over a period of just a few minutes. We call this type of imaging "functional MRI" and it is just one of the techniques now being used to study detailed brain function in humans, although without the fine detail we can get from electrodes which can record the activity of single brain cells. Another imaging technique - Positron Emission Tomography (PET) - relies on the usage of glucose by using "2-deoxyglucose" (2-DG) which is glucose minus the second -OH group that allows it to be broken down in brain and muscle cells. For imaging, one of the two hydrogens where the -OH was removed is replaced with the briefly radioactive isotope fluorine-18, forming "18fluoro-2-deoxyglucose" or FDG. A very small amount of FDG is injected into the blood, and it is taken up normally by cells that need glucose - such as active brain areas. The FDG can then be detected by a scanner, and provides a map of the most metabolically active areas of the brain (or the body - it is used to map metastatic cancer, as well).
So, the brain really does use up a lot of the energy that the body produces - it has to, since it doesn't really make that much on its own - but what about this idea of "mental exercise?" To answer that, I will use the example of dreams. During the "rapid-eye-movement" phase of sleep (see the Dreams blog: http://teddysratlab.blogspot.com/2011/02/dreams-story-in-brain-so-what-really-is.html), the brain is active, associating information and comparing with new and stored memories.We typically call that "dreaming," and scientists have shown that many parts of the brain are active during dreaming, especially the parts that control muscles. Since we can't see another person's dreams, scientists rely on waking a subject up and questioning them about what they dreamed - leading to the understanding that dreaming of physical activity actually activates the parts of the brain that cause the activity. The signals from the brain never quite reach the muscles thanks to a "switch" in the thalamus that blocks the signals. However, not every signal goes through this switch, and activity dreams result in many small twitches of the muscles that would be moving if the subject was awake and actually performing their dreams. Thus it really is possible for a person to "dream all night and wake up tired" because their muscles really did work during their dreams.
Getting back to Sarah's complaint of being tired just from writing, though - how does writing tie in to dreams? Well, one of the fascinating things about the human brain is that we can imaging movements and motions. Martial arts and many forms of active training require the participant to imagine the movement before actually doing it. Thinking about an action, still involves all of the same brain areas as doing the movement, just as in dreams. This is a critical feature in learning accuracy and effective movements, but it is also the key principle in how advanced neural-based prosthetics operate. "Bionic" limbs, as I will discuss later this month, rely on reading the pattern in the brain that results from a patient thinking about a limb movement, even when that limb is paralyzed or amputated. In fact, patients can't help but think about moving limbs, and scientists can detect those patterns in the brain, and in tiny muscle twitches in the affected limb. The normal human that imagines movement has that same brain activity and same tiny muscle twitches. A writer working on an action scene with any shred of detail or accuracy is imagining the motions and activities of that scene. As their mental image of activity unfolds, the muscles of their body go through those activities, just not enough to consciously move.
It's why we hold our breath during tense moments, why the parent of a teen driver wears out the passenger floorboard with the brake pedal that isn't there, why we are so touched by a good love scene, or why the person playing "Angry Birds" tilts their phone (and whole body) to get that last block to fall even when they know it will have no effect on the final outcome.
Mental exercise. It's not enough to condition muscles or lose weight, but it does involve brain activity, and can produce tiredness despite apparently sitting still. The brain and the body feel that they have been active,and that's where the sense of tiredness comes from.
So, yes Sarah and the other writers out there, you have been exerting yourself as you sit and write. It may just seem like dreams and imagination, but to the brain and the body, it is quite real.