Neurology question

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youngbiologist

Registered Senior Member
Myelin sheaths are one of the key reasons our body is able to function so well, however not all animals evolved this mechanism. Squid, instead of using myelin sheath instead of extra large neurons with axons called...large axons. Now I was just wondering if anyone in the neurology field could tell me what might happen if we were able to get extra large neurons to ALSO have a myelin sheath. Would the transmission rate of the nerve signal move even faster then in the current two systems? FYI, by having a large axon the transmission speed if also speed up but I forget the exact name of the effect that results in this increased speed.
 
Not a neurlogist am I (yoda talk ;) ) but let me see what your getting at:

1. Use Myelin to insulate the axons and prevent ions from leaking out and reducing the ion differential.

and/or

2. Just have very wide axons that some how retain their ions by shear size and width?
 
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well

Actually the larger the diameter of the axon, the faster the action potential moves. Like I said, has some electro-mumble jumbo word that I can't recall, and my neurology book is at home.
 
Really? I would think it would be slower considering there reduce surface area per volume. The ionic pulse (not electrical) is the rapid opening of pores and pumps on the axons that switch calcium and potassium ions from inside the axon to the outside and vise versa. I am not a neurologist so if I’m wrong about which ions don’t get mad at me!
 
Hey I know calcium is one of them!!! Which is outside the axon and which is inside I can't remember: youngbiologist should give us the details.
 
neuroscientist i am. cognitive though, not molecular.

that's an interesting question. in theory i would think that it would be faster.
but i don't know all the details of neurotransmission so if it would actually work in reality i don't know.

Do you know how squids and humans compare in speed?
 
Well I'll be a monkeys uncle!, It is Na and K then what the hell is Ca for in the nervous system? It was for something important but I can’t remember.
 
Na+ and K+ are involved in sending the action potential down the axon. Ca- then is released which leads (via various Ca dependant protiens) to the release of the neurotransmitter. Ca- also plays an important role in determining whether or not an action potential is initiated- it is sometimes allowed into the cell via inhibitory neurotransmitters, thereby hyperpolerizing the cell and preventing the cell from "firing".
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As mentioned, the action potential can be sped by both myelin sheaths and axon diameter. Having a larger axon AND a myelin sheath will speed up the transmition further up to a point. (Obviously, having ever larger and more myelinated axons will not keep speeding it up indefinitely- but some speed can be gained.)
 
thx cog

So that begs the question, how come our own nerve cells haven't evolved to be as large as the ones found in squid. While we do have single neurons that can be as long as a meter, I was under the impression from the liturature that our nerves were thinner then that of the squid. If I recall correctly, our transmission range is about 110 m/s( just heard this at my advanced cell bio), and the squid has about a 80 m/s transmission rate. Do you think the reason why our nerves don't have both adaptations may simply be cause of insignificant evolutionary pressure?
 
Re: thx cog

Originally posted by youngbiologist
So that begs the question, how come our own nerve cells haven't evolved to be as large as the ones found in squid. While we do have single neurons that can be as long as a meter, I was under the impression from the liturature that our nerves were thinner then that of the squid. If I recall correctly, our transmission range is about 110 m/s( just heard this at my advanced cell bio), and the squid has about a 80 m/s transmission rate. Do you think the reason why our nerves don't have both adaptations may simply be cause of insignificant evolutionary pressure?
Click to expand...


Good question. I do know that the speed they transmitt at should not alway be as fast as possible. There are some neurons (nociceptors), for example, that will have slower and faster speeds for different purposes. The faster nociceptors give rise to sharp pain immeadiately (so you can get away from the pain quick), while the slower, unmyelinated kind produce long lasting burning pain (so you can favour the damaged body part).

I don't think I answered your question. That is because it exceeds the extent of my knowledge. :)
 
I think it became difficult for evolution to increase the speed anymore with out having negative side effects. Evolution works to make something work well but not perfect... there is a point were something to perfect can become a detriment. Growing and powering such large neurons could become a problem. Already 1/3 of all energy produced in the human body goes to K/Na pumps in the neurons!
 
Something else (I should have thought of earlier)-


The critical factor is how many times per second a cell can "fire". After each time, there is a refractory period. My guess is that although the speed of transmition of a particular action potential might increase, the firing rate as a whole will not. I vaugely remember hearing somewhere that the firing rate of some cells is prevented only by the need for refraction. (This doesnt really explain a whole lot in terms of your question, but it is something to keep in mind.)
 
What is the fasts firing rate in humans? I think retinal neurons do about a max of 30per second.
 
Re: thx cog

Originally posted by youngbiologist
So that begs the question, how come our own nerve cells haven't evolved to be as large as the ones found in squid. While we do have single neurons that can be as long as a meter, I was under the impression from the liturature that our nerves were thinner then that of the squid. If I recall correctly, our transmission range is about 110 m/s( just heard this at my advanced cell bio), and the squid has about a 80 m/s transmission rate. Do you think the reason why our nerves don't have both adaptations may simply be cause of insignificant evolutionary pressure?
Click to expand...

speculation:

it is probably one of those chance things. Both squids and vertebrates needed a solution for fast neurons. In squids it was easiest to make large neurons. In vertebrates there already might have been some like myelin. That got adapted to make neuronal signals faster. Both systems attained the speeds they required and hence there was no need to evolve even faster ones.
 
Re: Re: thx cog

>> Both systems attained the speeds they required and hence
>> there was no need to evolve even faster ones.

Who said that we were done evolving? We haven't yet had the NEED for these superfast neurons, but I suspect that we will before long.
 
I say we start engineering neurons that use electrical impulse instead of ionic.
 
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