Everyone now and then feels tired or fatigued. Actually, next to a headache it is one of the most heard complaints when visiting a doctor. But what is fatigue really?
Two kinds of fatigue can roughly be distinguished: physical or mental. I am not saying here that we have a mental state apart from a physical state. In my opinion we only have a physical state, however, the distinction is well-known and refers to doing things with your body (e.g. walking, gardening, cycling) and things doing with your mind without moving much around.
The Physical form is quite simple to detect. Whenever you do things using your body, be it gardening, cycling, playing soccer, you'll get tired after a while. Simply because you burn your energy reserves (mostly glucose and oxygen) in your body cells. That's where the heat (your warmth) comes from when you move around. Your body temperature rises and can be as high as 40 degrees celsius when jogging. After burning this energy you'll get signals your body has to refuel, normally by drinking and/or eating something. Furthermore, your muscles have to rest to become strong again. You have to rest as well, to build up enough energy reserves for another workout. This kind of being tired everyone knows and can feel quite easily. It is a nice kind of being exhausted because normally it feels good to have done something. Actually, special chemicals are made when doing something physical: endorphines which can give you a special 'high', a sort of a rush that feels quite good. It is made in your brain, actually to get hooked on this kind of activities. That's why running or jogging in a certain frequency can become so addictive.
The Mental form is feeling tired without having done much physical work. For example, when listening to the television or radio for a couple of hours, you can get tired in a way that you have to turn off the television or radio not to get too much irritated. It is as if too much noise or images are then too much to handle. It is the most heard complaint of patients with brain injury but also the least understood. In the following text I would like to explain it a bit more.
A nice and useful definition of fatigue that I could find in the scientific literature (Aaronson et al. 1999) is the following: "The awareness of a decreased capacity for physical and/or mental activity due to an imbalance in the availability, utilization, and/or restoration of resources needed to perform activity". So being tired is related to doing things and to the resources that are needed for it. It represents a problem in the resources you have for doing activities.
A computer metaphor
'Mental' or cognitive fatigue is the problem one has with resources that are needed to do mental activities like thinking straight, problem solving, arithmetic, interpreting images, texts or behavior of other people. Every cognitive activity, that is, activity that deals with processing information in your brain. The metaphor of a modern computer is quite useful here. Right now I am typing on my Windows7 laptop, having 3 Gb RAM memory and 500 Gb hard disk memory. My processor is a 2.4 GHZ dual core Intel Pentium, so it can do a lot of processing in a very short time. This however, costs some energy in the form of electricity that is provided by my battery (which will last for another one hour). But typing this text drains my battery so in about an hour there are no more resources left to do this job: my computer gets exhausted and needs new energy. In a human brain much the same is going on when it is processing information. It takes a lot of energy and more whenever the processing is harder to do, for example in a more complex task. This energy is electricity as well because brain cells communicate with each other via electrical signals. This electricity is of very low voltage (-70 mV) and basically is generated due to the constant movement of specific molecules in the nerve cells (Na+, K+, Ca2+, Cl-). This creates a so-called action potential that results in a release of chemicals (neurotransmitters) into a synapse (nerve ending) so that another linked nerve cell can be activated as well. In this way electrical signals are transported via a lot of nerve cells.
Now, the whole point is that this kind of release and uptake of chemicals costs cells a lot of energy. Cells need a certain kind of power to be able to generate this electricity. They get this power/energy from the intake of oxygen and glucose, traveling around in red blood cells. Because we do breathe every minute, oxygen uptake can last for (almost) eternity. The problem is glucose. Glucose - a simple word for it is blood sugar - is formed in the liver via transformation of glycogen (form of glucose that can be made and stored in the liver and muscles, to put it simply). The basis of glycogen is our food, carbohydrates. Normally, we have a storage of glycogen in our liver and muscles that can be converted to glucose, which is needed in our cells as fuel.When our information processing is increasing, our fuel burning is increasing as well. This can be imaged by a PET-scan (scanning the regional glucose uptake in brain cells). There comes a point when there is too little glucose and your body starts to burn lipids (fats). That's the simple reason why you can indeed loose weight when studying a (whole) lot and not eating properly. However, when your glucose storage is almost depleted your brain cells will not be functioning sufficiently. This has probably a lot to do with our experiencing mental fatigue, but no one scientist really knows exactly how this works.
What we do know however, is that brain injury results in an increase of working brain cells. Instead of working efficiently and staying on a low budget, nerve cells and nerve networks seem to panic and use many more connections in doing mental operations (information processing) than before a brain injury. This fact can most likely explain why brain injured patients get more easily exhausted than normal people when doing mental activities. Simply because they use much more brain cells and networks, they need much more glucose. They energy levels are more quickly depleted.
If this is really true, and I have not found other serious other explanations yet, then how come taking just some extra glucose or dextrose will not help brain injured patients very much? Well, actually it DOES help, but...only in the short term. In the longer run they get even more exhausted! So, most likely blood glucose is not the whole story of mental fatigue. The increased amount of brain cells doing their best after a brain injury, is probably related to an increased mental exhaustion. But no one really knows how exactly. One thing is certain, the brain does not like to be stimulated in such a way that all brain regions are active. Just remember how you react to a sudden increase of traffic when driving and then it starts to rain heavily. After having been in such a situation for half an hour you return home pretty exhausted. Overloading your brain is actually stimulating your attentional network for a longer period of time: concentrating and dividing your attention. We do know that in such situations tension is building up, more specifically, certain basic emotions are building up. Probably, anxiety. Too much build up of anxiety can lead to a defensive reaction like flight or fight (either panic and getting away or an aggressive explosion and anger). No one really knows why we can't take this and react so emotionally but I can start to speculate a bit.
It is reasonable to assume that the brain's main purpose in life is to help us (our body) to stay alive. As Antonio Damasio has proposed in his book 'The feeling of what happens', is that our brain tries to maintain an equilibrium, a homeostasis for the main body functions like body temperature, blood pressure, heart rate. It does so by scanning our environment via our senses (touch, smell, seeing, hearing, tasting) and constantly calculating the state of our bodily functions and predicting our environment. This last element is essential because then our brain can predict what kind of actions we have to do. A very recent model of the human brain, a mathematical one, states in fact the same thing: our brain tries to predict our environment in order to synchronize as optimally as possible our actions with this same environment. In this way it stays out of conflict with this environment and the chances of survival are then optimal. A brain overload is evolutionary a bad thing, not being able to predict or control your environment any longer. Logically, anxiety as an emotional response, is created to do something about this overload situation. Either by fleeing from it or fighting against it (to stop it).
Emotional outbursts explained
Actually, emotional outbursts can be explained simply as a self-defensive response to an information overload. They are meant to stop the information overload. Most of the time, brain injured patients get to hear that they can not control their emotions sufficiently. Perhaps, this is true as well (but difficult to prove and measure). But, another explanation is that their information overload is much quicker reached than in normal brains. Therefore, the emotional defense response like anger or anxiety has to kick in sooner as well. No one is well trained in emotional control whenever emotions like anxiety do build up so quickly. Some evidence for this model of the relation between emotional outbursts and a much quicker information overload can be found in clinical settings and from stories of families with brain injured patients. Most of the time, when information overload is recognized much faster by a family member, and thereby measures are taken immediately to reduce it, the brain injured patient is less likely to have emotional outbursts. Of course, it also still stands that brain injured patients do indeed have a tendency to react more emotionally than usual. So something in the regulation of their emotional intensities is not quite well.
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