Mental Fatigue

[ronthepon]

Ever found your brain refusing to respond to the environment? Ever found yourself completely unable to concentrate on the words in the book you've been reading for hours at a strech? Try listening to new, never before heard music (that you don't like all that much) for hours. Your brain wierdly ceases to function properly. You lose the ability to think straight, and though your body's not tired, you desparately crave for some sleep.

 

Ever wondered what the hell goes on?

 

Well, this is the thread for finding it out.

 

First, let's just find the reason for 'mental fatigue'. Rep for the first person to come up with the reason behind mental fatigue. Extra points if you can do it in the terms of the layman's vocabulary.:rant: (That's what this is all about)

[InfiniteNow]

All mental action is the result of neurochemical flow between neurons. Basically, the wiring in our brain is like a series of buckets and water droplets. Enough drops of water go into the bucket, it spills over and causes water to flow into another bucket until that one falls over, ad infinitum.

 

The buckets are the neurons, and the spilling over is them firing. The water is a combination of the neurotransmitters (like dopamine, seratonin, acetocholine...) and Sodium/Potassium (which as a result of the NTs cause the neuron to open/close).

 

The water (NTs) are produced by the body, and if they are flowing quickly between the buckets (neurons), they start to dwindle in supply. Let's call it a drought. The water must be reabsorbed (reuptake) to the cells before they can being the next process of bucket overflow (neural cascade).

 

When you are mentally fatigued, you have probably gone into a state where a great majority of your NTs are being reabsorbed (going back into the well to allow for later irrigation), and the ability to start another cascade is reduced until this happens. I believe also the ionic changes of the sodium/potassium transport play a role, and that the balance of the cell has to essentially reequilize before another cascade may begin.

 

 

NOTE to the reader: I just made all that up. Don't rely on it without verifying first. :rant:

[ronthepon]

Yeah, that's pretty much it, although I'll be mose specific about how exactly a nerve works...

 

 

Most of us have come to consider nerves as similar to electrical connecting wires. Yeah, sure, both transmit electrical impulses. So they've got to be similar, right?

 

Nah, nerves are way more technologically advanced.

 

Consider a flexible pipe with all kinds of motors, tiny engines and pumps attached to it's wall.

 

Lets have a look at a special type of these pumps.(Describeable as Sodium, Potassium ion transmembrane, electrogenic pumps:hihi: ) Their job's to keep the nerve ready to fire. How exactly do they work? Simple, really. All they do is transfer sodium and potassium ions through the wall of the pipe(nerve). They'll expell three Na+ ions, simultaneously import two K+ ions. The nett result? Sodium ions are removed and Potassium ions are introduced into the nerve(pipe).

 

But what is the whole point of this? Well, we'll figure that soon. Lets just see what all this results in.

 

Three sodium ions out. Two potassium ions in. Three sodium ions out. Two potassium ions in. Both the ions have positive charges. The result?:smart:

 

Inside the nerve wall, there is negative potential. Actually, there are also a few negatively charged proteins inside the nerve wall. These proteins are too big in size. They can't cross the wall, and just contribute to the negative potential inside.

 

The potential is pretty small (Can't cause an electric shock :D). Around -70mV only.

 

So what have we got? A pipe with pumps on it's wall. The pumps create a high concentration of potassium inside the nerve.

 

Now suppose you want to transfer a impulse. (Yaay, finally.:thumbs_up ) Here we get to see some more dramatic engines in action. You've been seeing them on the wall of the nerve, even wondered what the 'do not touch' on them is all about.:D Well, these engines are pretty sensitive. The moment they notice that the potential's changed suddenly, they get into action. These engines are called 'Voltage gated channels', and they make the nerve excitable. They make the nerve useful. It's for them to work properly that the Transmembrane pumps do so much work. Well, what exactly do they do?:confused:

 

 

Well, consider this. Every thing is calm. The transmembrane pumps have worked hard for some time, and the voltage inside the pipe is the usual -70mV. All of a sudden, somehow the potential somewhere along the pipe shoots up to -50mV.:lightning Let's assume god was responsible. The voltage gated channels (lets call them VGCs here;)), kick into action. -50mV indicates that it's time for them to start work. They don't waste any time.

 

With a rumbling sound, the VGCs that detected the -50mV begin to do their own type of ion transport. Without sparing a thought for all the hard work the transmembrane pumps had done, the VGCs recklessly suck in all the sodium ions they find outside the wall, and spew the ions into the nerve. Not potassium, they don't have the ability to touch potassium, they just go for the sodium. (The transmembrane pumps were importing potassium and expelling sodium.) In an instant, they completely wipe out the -70mV that had beeing built. Within half a millisecond, the potential changes from -70mV to +40mV!:eek: Now the inside of the nerve has become, astonishingly, positive.

 

***(See note below)

 

But after half a millisecond, the sodium ions imported by the VGCs develop a pretty high concentration. They choke the VGCs, and the engines suddenly shut down.:dead: But then another type of Voltage gated channels come into action.(We'll call em VGC2 eh? I love cutting their names short.:hihi: ) The VGC2s are interested in restroring the -70mV that was once present inside the nerve. Funnily enough, the VGC2s are pretty different from the VGCs. Unlike the VGCs, they can only transfer Potassium. So that's what they do.

 

With a shrill screech (Or maybe not), the VGC2s expell as many potassium ions as they find. The concentration of potassium ions inside the nerve drops during this time. The over-action packed VGC2s reduce the potential inside to about -70mV. The nerve portion returns to it's normal state. So the situation is pretty much like before?

 

Unfortunately, no. The potental is only half the story. Initially there were more potassium ions inside the nerve, and now there are only sodium ions inside. The impulse has been made, but the nerve is in ruins. There can be no second impulse now. What we have is a fatigued nerve.

 

Once again, the transmembrane pumps begin their duty. Remove sodium. Import Potassium. Remove sodium. Import potassium...

 

 

 

The part below kinda was supposed to fit into the three star area above, but it ruins the scenario.

 

***

Depolarisation. That's what this event is called. The positive burst of potential spreads to nearby reigions in the nerve, and the disturbance travels along the nerve.