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Speed of nerves

by Arsonade
Tags: nerves, speed
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Arsonade
#1
Jan7-05, 07:24 PM
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i know this isnt really biology so much as anatomy/medicine
but does anyone know the speed of nerve pulces, like, i know that nerves work by sending electrical signals to the brain, does it then follow that the speed of that nerve is the speed of electricity? i know i am phrasing this question very badly, but if anyone has an answer, that would be cool

Adam
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Janitor
#2
Jan7-05, 07:37 PM
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Quote Quote by Arsonade
...does it then follow that the speed of that nerve is the speed of electricity?...
Maybe somebody here can give you an actual range of numbers of feet per second or meters per second or whatever. All I can tell you is that nerve impulses are electrochemical, not purely electrical, and travel far slower than electrical signals that are sent in wires or coax cables.

I remember the high school biology teacher talking about the "sodium pump" in connection with nerve signals. Ions of sodium (and maybe potassium too) are key to the process, but more than that I cannot recall after all these years.
loseyourname
#3
Jan7-05, 07:51 PM
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Mammalian nerves vary from about 0.5 m/s in unmyelinated cells to 120 m/s in large myelinated cells. They are slower than wires both because myelin is not as effective as man-made insulation and because the current is carried by ions rather than free electrons.

Arsonade
#4
Jan7-05, 11:10 PM
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Speed of nerves

Quote Quote by loseyourname
Mammalian nerves vary from about 0.5 m/s in unmyelinated cells to 120 m/s in large myelinated cells. They are slower than wires both because myelin is not as effective as man-made insulation and because the current is carried by ions rather than free electrons.
thanks, this is exactly wat i wos loking for, i going to read up on this, i want to find out exactly how long it takes to do certain things like pick up a pencil, ect. but i meen to a tee, like speed of light lol, the more ithink about it the more i think im wating my time lol, o well lol.

Adam
Arsonade
#5
Jan7-05, 11:12 PM
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by the way i really like that signiture thing, good ol einstein lol
LURCH
#6
Jan8-05, 04:38 PM
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Quote Quote by Arsonade
thanks, this is exactly wat i wos loking for, i going to read up on this, i want to find out exactly how long it takes to do certain things like pick up a pencil, ect. but i meen to a tee, like speed of light lol, the more ithink about it the more i think im wating my time lol, o well lol.

Adam
The thing that for events such as these ("pick up a pencil, etc.") the great majority of the time is occupied by the brains Occasional work in processing nerve signals. In a reflex action (like jerking your hand away from some source of pain), the action takes place within the time it takes for the nerve signal to travel to the muscle. But in the case of a conscious decision (like when your hand accidentally makes contact with something you do not wish to touch), the reaction to pull weight takes several times longer, because the brain needs to do more than simply receive the signal, it must interpret what that signal means.
selfAdjoint
#7
Jan8-05, 06:00 PM
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And there's almost half a second between the brain's starting the action and your becoming aware of the decision.
DocToxyn
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Jan8-05, 09:25 PM
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Aren't many of the so-called "reflex" responses initiated by the sensory system picking up the stimulus, i.e. pain, sending it to its specific spinal ganglion and then the motor control shoots back to move the affected appendage? This closed loop effectively eliminates the brain and it "longer" processing time.
Quote Quote by loseyourname
myelin is not as effective as man-made insulation.
Is myelin truly less effective than man-made insulators? Think of the gauge of myelin in situ, do we have any synthetic insulators that can be drawn that thin and still function? I honestly don't know, but on a weight for weight comparison, I'm compelled to go for the myelin.
loseyourname
#9
Jan8-05, 09:52 PM
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Quote Quote by DocToxyn
Aren't many of the so-called "reflex" responses initiated by the sensory system picking up the stimulus, i.e. pain, sending it to its specific spinal ganglion and then the motor control shoots back to move the affected appendage? This closed loop effectively eliminates the brain and it "longer" processing time.
Yes.

Is myelin truly less effective than man-made insulators? Think of the gauge of myelin in situ, do we have any synthetic insulators that can be drawn that thin and still function? I honestly don't know, but on a weight for weight comparison, I'm compelled to go for the myelin.
You could be right. Per unit of mass, myelin might be better, but since there generally isn't as much myelin surrounding individual axons as there is whatever they put on wires surrounding the wires, it still lowers the signal speed.
Moonbear
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Jan9-05, 12:33 AM
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Quote Quote by DocToxyn
Aren't many of the so-called "reflex" responses initiated by the sensory system picking up the stimulus, i.e. pain, sending it to its specific spinal ganglion and then the motor control shoots back to move the affected appendage? This closed loop effectively eliminates the brain and it "longer" processing time.
Except reflexes are not entirely closed loops. The simplest of reflexes do exist as you've described, but also signal the brain of what just happened. Other motor functions are described as reflexes, because they can occur even if the spinal cord has been severed, but are also under supraspinal control from the brain. Ejaculation is one such example. Ejaculation can be achieved in patients with spinal cord injuries, but the brain usually provides an inhibitory input as well, which is reduced at appropriate times.

Loseyourname's estimates for rate of travel of an action potential seems about right. Keep in mind, however, it's not as simple to just calculate distance from your fingertip to your brain and back to determine the distance a signal is traveling. Neurons don't follow straight paths through the brain, and you need to take into account the many synaptic connections along the way as well.
gerben
#11
Jan9-05, 12:35 AM
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Quote Quote by loseyourname
since there generally isn't as much myelin surrounding individual axons as there is whatever they put on wires surrounding the wires, it still lowers the signal speed.
The insulating layer around a metal wire does not increase the speed of electrical signals being send through the wire.

The difference between the speed of an electricty in metal wires versus action potentials in nerves is because of the different nature of the two: In a metal wire there are electrons moving trough metal while in a nerve axon there are ions moving through water.
DocToxyn
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Jan9-05, 01:31 PM
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Quote Quote by Moonbear
Except reflexes are not entirely closed loops. The simplest of reflexes do exist as you've described, but also signal the brain of what just happened.
I can't disagree with that, the complement to the sensory/motor loop is that signal which is sent to the CNS to aid in processes such as learning not to touch that hot burner again. Since we were discussing speed of conduction, etc., I guess I left that out. Thanks Moonbear.

Quote Quote by gerben
The insulating layer around a metal wire does not increase the speed of electrical signals being send through the wire.
This does have a profound impact on axonal conduction however. Non-myelinated axons, as stated by loseyourname, are not as efficient as those with the insulation. Since the action potential is carried by sodium and the potential "jumps" via saltatory conduction from one node of ranvier to the next, these spaces effectively speed up the rate of travel and make it possible for extremely long distances (feet to spinal cord) to be covered rapidly. Perhaps I don't have the physics of it down perfectly, but that's what I can recall at the moment. I would think insulation also cuts down on cross-talk (shorting?) between apposed axons.
gerben
#13
Jan9-05, 03:04 PM
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yes it does, I just wanted to point out that the amount of insulation around metal wires versus the amount of insulation around nerves is not important with respect to the difference in propagation speed.
Arsonade
#14
Jan14-05, 08:12 PM
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wow, where did all of this come from? lolol, anyway, i think ive read taht the max responce of a human mind is about 1/30th of a second, although as i can see there seems to be a bit more variables tahn i supected lol.

Adam
Moonbear
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Jan15-05, 11:47 AM
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Quote Quote by Arsonade
wow, where did all of this come from? lolol, anyway, i think ive read taht the max responce of a human mind is about 1/30th of a second, although as i can see there seems to be a bit more variables tahn i supected lol.

Adam
LOL! Well, you have a postdoc with a background in neuroscience (DocToxyn), a professor of neuroscience (me), and I think Loseyourname has mentioned a focus on neuroscience in his undergraduate studies, and you wandered in asking about nerves...it was inevitable!

Careful about calling it the mind instead of the brain or we'll have the philosophers in here breathing down our necks about the distinction.

Quote Quote by DocToxyn
I would think insulation also cuts down on cross-talk (shorting?) between apposed axons.
I'm not sure I'd use the analogy of "shorting" because closely apposed axons, even in the absence of synaptic specializations, may still communicate (I don't know if this has been clearly demonstrated to occur, but there is a camp who argue it is possible, possibly via gap junctions). So, this cross-talk may be functional for axo-axonal signaling, not just "leakage" as the term shorting would imply. I don't know if this would weaken the signal continuing along the axon toward the terminals.
DocToxyn
#16
Jan15-05, 08:18 PM
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Quote Quote by Moonbear
I'm not sure I'd use the analogy of "shorting" because closely apposed axons, even in the absence of synaptic specializations, may still communicate (I don't know if this has been clearly demonstrated to occur, but there is a camp who argue it is possible, possibly via gap junctions). So, this cross-talk may be functional for axo-axonal signaling, not just "leakage" as the term shorting would imply.
I threw in the "shorting" term as a nod to the physics people who may have been reading. I was using it purely, and potentially (no pun) incorrectly, as a jumping of signal from one conducting body to another. Although I haven't looked into the subject of cross-talk in a while, I would imagine that, under certain circumstances it can be an effective means of signal propagation. Nature continues to prove biologists wrong.
Janitor
#17
Jan16-05, 10:59 AM
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When the Sun is low on the horizon and my father is driving a vehicle, he tends to sneeze when he turns the vehicle quickly toward the direction of the Sun. I have read that in some people the nerves from the nose and the nerves from the eyes pass so closely that there can be cross-talk, and I guess that is what is happening in his case.
Moonbear
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Jan16-05, 12:34 PM
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Quote Quote by Janitor
When the Sun is low on the horizon and my father is driving a vehicle, he tends to sneeze when he turns the vehicle quickly toward the direction of the Sun. I have read that in some people the nerves from the nose and the nerves from the eyes pass so closely that there can be cross-talk, and I guess that is what is happening in his case.

I've experienced the photic sneeze reflex myself. I just argue I'm allergic to the sun and use it as an excuse to return to my vampiric night stalking.

http://www.straightdope.com/classics/a2_303.html

When they talk about crossed "signals", it's not the same thing a the cross-talk between neurons DocToxyn and I have been discussing. Neurons are the individual cells, nerves are large bundles of neurons.

Since the straightdope explanation mentions the trigeminal nerve, which was the first suspect in my mind as you mentioned innervation of both nose and eyes (the trigeminal nerve branches out to innervate most of the face in one way or another), here's a site explaining the anatomy of the trigeminal nerve:
http://www.meddean.luc.edu/lumen/Med...cn/cn1/cn5.htm

I can't tell you anymore than that as I found a total of 11 articles on the photic sneeze reflex listed in Pubmed, the vast majority of which have no abstracts associated with them (some may be letters to journals not articles), and I can't access the articles online. Based on titles alone, it doesn't look like much is known about it.


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