Nerve transmission requires ATP so it makes sense that nerves would get tired but I can’t think of a good example of this? I know that muscles lacking ATP do not contract but what about nerves that stop working for the same reason?
I once asked why hot water on an bad itch would stop the itch and I was told that the nerves would become overloaded. However, I do not have a cite.
Nerves fail to work, because of different reasons. They don’t get tired.
It’s been too long since I read around in the neurosciences, and this stuff is hurting my head, but it seems that things other than a lack of ATP are the main contributors to nerve fatigue.
Why not? Muscles get tired. Why don’t nerves?
Not sure what you mean by “tired”.
Stare at a colored picture or object for 30 seconds, and then suddenly look at something which is plain white. For a few seconds you’ll see an afterimage which has colors reversed from what you were staring at. I’ve interpreted this phenomenon as a result of the cones (rods?) in the eye getting tired, so only the ones from the opposite colors pick up the white, and you’ve got a reverse image. But I’m probably wrong.
Another very similar example: The nose gets tired of odors very quickly. You’ll think it has dissipated, but if someone else comes by, they can smell what your nose got tired of.
Same thing for taste. Never drink Coke after a chocolate bar. The Coke will not be sweet at all.
Tired as in need to shut down. Nerves don’t need to rest, they can have the receptors all full, and fail to respond until the receptors are freed up. Extreme use saturates the receptors, but they are not going to die because they can’t slow down. Muscles produce toxins and CO2 that requires them to slow down, or the cells could die.
Not all of them. Smooth muscle certainly doesn’t get tired, and I don’t really think cardiac muscle does, either. When’s the last time you had to stop working out because you felt your heart was tired? Odds are, never. You stopped because 1) it was time to stop, 2) your skeletal muscles were fatigued, or 3) your lungs were at capacity. That last one’s a biggie.
Well, lets say I’m starving to death. Will I no longer be able to run that marathon because my muscles won’t work, nerves won’t work or both?
Muscles. When you die of starvation (assuming it’s actually starvation, and not just malnutrition,) you die because your body has had to metabolize too much of it’s own muscle for energy, energy it keeps for more important tissues. Eventually, the diaphragm itself it broken down too much and you can’t breath.
I did just a few months ago. I jogged for the first time in a while and really pushed myself. I started feeling a sharp pain in my lower chest / upper abdomen. I stopped running and it gradually went away. As I jogged more and more and got in better shape, my lungs became the limiting factor. I’m not sure what exactly happened that first time, but it felt like I strained my heart, like it was about ready to stop, which would’ve been bad.
Are you sure it was your heart and not your diaphragm? And it should also be noted that chest pain, even if it is cardiac related, doesn’t always correlate to the heart being “tired.” I’m not saying your heart wasn’t tired, but consider this: the damn thing beats all the time. It would really suck if it got tired that easily. Some quick calculations and I figure that the average heart beats over 3 billion times in a lifetime. You really think a few minutes at maybe three times your normal rate will make it tired if seventy-five years at that rate won’t?
Yep, that sounds like a stitch – nothing to do with your heart.
IIRC, the heart rests between beats.
Experience some real torture sometime. Your nerves keep sending the pain, it never stops, your brain shuts down but until then, the nerves do not get tired of sending the pain signal.
Receptors and cones and rods and taste buds and what not can be overloaded, but the nerve signal path does not quit because it is tired.
But then again, I was not taught the sensory organs at the end of nerve lines were called nerves in the first place. “Nerve endings” at best.
YMMV
All of this is fascinating, but what I still don’t understand is this: If a nerve cell requires some amount of ATP to send a signal along, why doesn’t a nerve cell that’s been intensely used eventually run out of ATP and stop signaling?
Because when a nerve runs out of ATP to the point of not being able to signal that nerve is dead. Pure and simple. Nerves don’t get tired, they simply die of exhaustion.
A nerve cell is able to transmit because it maintains a very high electrical potential between the inside and the outside of the cell. It takes a shedload of energy to maintain that potential, but so long as that potential can maintained and reset the cell can transmit. When that potential can’t be maintained the cell can’t transmit.
Maintaining that electrical potential is done by lots of little pumps that move differently charged ions backwards and forwards through the cell membrane. The same types of pumps are also required to move nutrients and toxins backwards and forwards through the cell membrane. If a neuron doesn’t have enough energy to move ions then it sure doesn’t have enough energy to move much heavier and more complex organic nutrients and toxins. Ergo when a cell can no longer transmit the cell is mortally wounded. It is going to die.
Neurons naturally have various failsafes built in that stop them from becoming exhausted from transmission, but they do still die from exhaustion. One of the most common ways of doing this is exposure to certain poisons. Nerve agents such as Sarin and most household insecticides cause a neuron to keep transmitting constantly until it dies of exhaustions, which is how these poisons kill. But the nerve only stops transmitting shortly before death. It doesn’t actually get tired. Similarly if a nerve is pinched the distortion can cause neurons to fire incessantly eventually leading to death from exhaustions. But once again the nerve doesn’t stop transmitting because it is tired. Its stops transmitting when it is on the point of death.
Yep, you’re wrong. 
The inverted image is caused by the nerves becoming accommodated to the intensity of the input. Basically after a signal ceases to change for a while the nerves are programmed to cease responding to it. This allows the body to calibrate the nerves for the conditions they are operating under.
If this didn’t happen then you would be overwhelmed by meaningless stimuli constantly. It’s important to notice that it’s hot when you first step out of the air conditioning so you don’t; kill yourself through heat stress so the change triggers your body to scream “Shit it’s hot”. But you couldn’t function if your body screamed “shit it’s hot” at you a dozen times every second. So after a few minutes your nerves accommodate and they no longer remind you of the temperature unless you consciously ask for a report.
Your eyes do the same thing with bright objects. No point getting a report that says “shit that’s glary” a dozen times a second, so the nerves stop responding at that level. When you look at another surface those nerves that have ceased to respond will give no signal until they realise the input has changed. You interpret that lack of response as an inverse image.
Nope. That’s accommodation again. Nothing to do with the nerves getting tired. It’s just the body’s way of stoping itself being overwhelmed by constant meaningless input.
When you die of actual starvation you die ultimately because you can no longer manufacture sufficient ATP to maintain the membrane potential of the muscles and nerves. Nothing whatsoever to do with having “metabolized too much muscle”. Nothing to do with having “broken down the diaphragm”. You can easily starve to death with over 50% of your muscle mass still intact, and most victims do.
Don’t get the idea that the pictures of holocaust victims or Japanese POWs are in any way representative of the state of starvation victims. Those people were fed insufficient nutrients over an extremely prolonged period, allowing their body tot enter an extreme famine condition. And as a result none of them actually starved to death, rather they succumbed to infection and exposure.
A person who starves to death, ie actually dies from insufficient calories, will die of exhaustion. Quite simply they can no longer maintain the membrane potentials of the muscles and nerves. That can usually only happen of a person is given essentially no food and forced to continue exercising. It can happen in as little as a few days, which is way too little time to digest any significant amount of muscle tissue. Such people are usually in fairly good condition weight wise but have simply been unable to keep up with the body’s energy demands and have died as a result.
In contrast people who have sufficient tome to break down significant masses of muscle also have sufficient time to die of exposure and infection. Either way death from starvation doesn’t result from insufficient muscle mass.
Awesome Blake, thanks.
Just one more question.
According to my handy dandy textbook, the reason that sugar enters the urine of diabetics is because the nephron cannot actively recover all the excess sugar that has entered it. It runs out of ATP and the sugar enters the urine. This doesn’t sound right after your response regarding neurons. It seems more likely to me that the glucose recovery pumps just don’t have the capacity to get all that sugar out of the filtrate. So its not a lack of ATP, its a lack of pumps.
Correct?