Action Potentials: How Does it Work?

[Roxy]

1. How does an Action potential start in the nerve. Does it just receive a stimulus and suddenly the inside of the nerve depolarizes?? How does the inside of the nerve become more positive??

2. In people with Multiple Sclerosis the myelin on the nerve is broken down & the AP doesn't tavel down the nerve..Why can't the AP move down the nerve just like it does when the myelin is there? How does the AP tavel down a myelinated nerve? I know there is no Na/K voltagate gated channels where the myelin used to be so how does it travel?


Thanks

 

[Roxy]

ANOTHER QUESTION: I read that "because of the absolute refractory period of the Na+ voltage-gated channels, two action potentials cannot be fired one on top of the other. Therefore, action potentials will almost always have a fixed height or amplitude"

Can't an action potential be fired because before the Na+ voltage-gated channel (the one in refactory period) there is a Na+ voltage-gated channel that can be excited right?? Could someone explain how the "potentials will almost always have a fixed height or amplitude"

Thanks

 

[zoobyshoe]

2. In people with Multiple Sclerosis the myelin on the nerve is broken down & the AP doesn't tavel down the nerve..Why can't the AP move down the nerve just like it does when the myelin is there? How does the AP tavel down a myelinated nerve? I know there is no Na/K voltagate gated channels where the myelin used to be so how does it travel?
Thanks

An action potential passes along an axon because the positive charge of an area slightly depolarises adjacent areas of the membrane. This sets off an action potential in this area which polarises surrounding areas etc. Thus the potential propogates down the length of the axon. Once an area has gone through an action potential, it becomes less permeable to the ions that are part of the potential. This stops the action potential from occurring and continuing forever or from moving wildly in any direction. The time it takes for an area to recover from an action potential is known as the refractory period. The refractory period thus sets a maximum on the firing frequency of the neuron. If the refractory period is short enough, several action potentials can move down an axon at the same time.
Some (but not all) axons are also sheathed in a substance known as Myelin. Myelin covers the length of the axon except for small nodes about 1mm apart. Myelin prevents the ions from moving through the membrane, but the nodes have many of the ion gates necessary for action potentials. When an action potential depolarises a node, the charge is strong enough to depolarise the next node, skipping all of the distance in between. In this way, the action potential quickly jumps from node to node down the length of the axon, much quicker than an action poptential will usually propagate. This jumping effect is known as saltatory conduction.

http://www.cyberpunks.org/freeside/mab_neuro.html
So the way the depolarization travels is pretty much the same way it does in an ummeyelinated neuron: each change in voltage depolarizes the next little bit of length whose voltage then changes. But by insulating a length of axon the change in voltage is instantly shunted much farther down the axon where it acts to depolarize the uninsulated nodes. These then fire and reinforce the change in voltage down to the next node. Saltatory means "jumping". The advantage of this is increased speed. When the myelin is destroyed, in most cases the neuron still fires, but is considerably slower than normal and loses all coordination with other neurons:

Both groups of diseases cause either a blockage or reduction of the electrical impulse traveling down the axon. One can imagine the effect this has on the transport of messages in the nervous system in general when imagining that while impulses normally travel around 225 miles an hour, they are slowed down to about half that rate or less wherever a section of myelin is destroyed (5). Thus impulses are either significantly slower than normally or completely blocked. This causes considerable disruption to the overall 'symphony' of firing neurons that create meaningful patterns together, either caused by the absence of a signal or by a late signal which results in imperfect timing with other neurons.

http://serendip.brynmawr.edu/bb/neuro/neuro01/web1/Blumenfeld.html
I'm not sure why the signal is only slowed in some cases and completely dissipated in others. Perhaps it's just a matter of how many segments of myelin in a row are missing. If only 1 mm is missing then perhaps the signal is merely slowed, but if 3 mm are missing then it completely attenuates. I'm not sure.

 

[zoobyshoe]

1. How does an Action potential start in the nerve.

When the right neurotransmitters are received they tell the cell wall to open some gates and let the ions that have been pumped outside the cell back into it.

Does it just receive a stimulus and suddenly the inside of the nerve depolarizes?? How does the inside of the nerve become more positive??

The opposite of the way it becomes "less positive". It becomes less positive when the pumps push more ions out than are coming in. Like charges repell. Positive repells positive. All these positive ions want to spread themselves from each other evenly and won't concentrate anywhere on their own. Pumps have to physically push them from the inside of the cell to the outside where they are more crowded. The outside of the cell is then more positive with respect to the inside, and given the chance the ions will automatically spread back into the inside of the neuron where there is less pressure. When the first gates in the cell wall open and let ions in the next gates in line sense this increase in positive charge inside the cell and they automatically open to let more ions in.

A given neuron may get 20 different neurotransmitter messages before it fires. Some neurons give it the message "don't fire" and others give it the "go ahead" neurotransmitters. It has to get the right number and kind of "go ahead" signals before it crosses the firing threshold and the first gates open.

That's all a bit casually put, but I hope it helps.

 

[somasimple]

Zoobyshoe,

this explanation about saltatory condution is the actual and teached one but it remains absolutely illogical and against all the principles I know in Physics.

 

[zoobyshoe]

Zoobyshoe,
this explanation about saltatory condution is the actual and teached one but it remains absolutely illogical and against all the principles I know in Physics.

Maybe you should start a thread in Mind and Brain about it. Explain what the teaching claims with backup from links and then why you think this violates physics, with links.

 

[somasimple]

Zoobyshoe,

You're right!
I'll try to make a thread about these riddles/mysteries.

 

[Roxy]

thank you

 

[shruth]

1. How does an Action potential start in the nerve. Does it just receive a stimulus and suddenly the inside of the nerve depolarizes??

This part of Roxy's question has an intriguing angle to it. Since an action potential has to start with a stimulus, does it mean that there is no such thing as free will?...i.e. is every neuronal activity a response to some sensory stimulus? Makes me wonder...

 

[selfAdjoint]

This part of Roxy's question has an intriguing angle to it. Since an action potential has to start with a stimulus, does it mean that there is no such thing as free will?...i.e. is every neuronal activity a response to some sensory stimulus? Makes me wonder...

Question, is free will even a testable scientific concept?

 

[somasimple]

Hi All,

B]Free will[/B] is a questionnable scientific question!
And there is, of course, stimuli that bring neurons to create action potentials.
It may be answered with some logical and known theories/physics laws => Newton, Gauss, Coulomb...

 

[selfAdjoint]

http://en.wikipedia.org/wiki/Benjamin_Libet" did an experiment that shows your brain "knows" what you're going to do around half a second before you decide to do it. Libet tried manfully to square this fact with free will, but most people don't think he managed to bring it off.

 

[shruth]

At the outset, I apologise for my cavalier usage of the term 'free will'. Of course, I never meant to use the term as some phenomenon that went beyond physics! But the usage is unpardonable nonetheless.
What I actually wondered was whether there exists a mechanism other than sensory stimuli relayed to the CNS which can elicit an action potential in a neuron. Can the brain begin a train of thought without an external stimulus prompting it onto that line of thought? If it can, how are the first neurons excited? How are the first action potentials initiated? Or are thoughts only elicitable as a consequence (albeit extremely indirect) of sensory stimuli?
So in essence, I am defining free will here as the ability of the brain to initiate activity independent of sensory input. It does not, of course, mean that I am talking about any activity which is independent of the genetic and epigenetic phenomena that construct the brain and the life experiences that mould it.

 

[zoobyshoe]

I think that what people experience in sensory deprivation tanks is the best indication that the brain is always self stimulating with a greater or lesser amount of corrective or stimulatory imput from the outside. Once we remove as much sensory imput as possible, the brain continues thought and sensory processes with information from memory. More and more the result is imaginary and eventually hallucinatory: sensory experiences become stimulated by internal stimluli rather than external, while being just as vivid.

In some point during our first few years of life we shift from constant reaction to outside stimuli to a preference for internal stimuli: we check things against memory, and conduct analyses of outside stimulation with processes, and "values" stored in memory.

I think the answer to your question is, not only can the brain stimulate itself from within, but that this is what it usually ends up prefering. Learning is essentially the process whereby we stop automatically reacting to external stimuli and modify our reactions increasingly with stored knowledge.

 

[shruth]

I think that what people experience in sensory deprivation tanks is the best indication that the brain is always self stimulating with a greater or lesser amount of corrective or stimulatory imput from the outside.

I do not think that the sensory deprivation tanks truly deprive us of sensory stimuli. The most important reason I can think of is the fact that the photoreceptors have their highest firing rates in darkness! Impinging photons drive down the firing rates. So I would suggest that the thoughts generated during 'sensory deprivation' are influenced by this input from the photoreceptors reporting the darkness that they are 'seeing'.

In some point during our first few years of life we shift from constant reaction to outside stimuli to a preference for internal stimuli: we check things against memory, and conduct analyses of outside stimulation with processes, and "values" stored in memory.

As you have yourself pointed out here, we evaluate the external input against our internal representation...so the drive is still sensory.

I think the answer to your question is, not only can the brain stimulate itself from within, but that this is what it usually ends up prefering. Learning is essentially the process whereby we stop automatically reacting to external stimuli and modify our reactions increasingly with stored knowledge.

I am not disputing the modulatory influences of learning To quote myself... are thoughts only elicitable as a consequence (albeit extremely indirect) of sensory stimuli? ...and...It does not, of course, mean that I am talking about any activity which is independent of the genetic and epigenetic phenomena that construct the brain and the life experiences that mould it.
So all I am wondering is whether there are mechanisms which can begin an action potential in a neuron by itself without the presynaptic neuron being active. (Of course we are not talking about random firing due to leaky channels or other such epileptic phenomenon)

 

[zoobyshoe]

I don't guess you ever had a question then, but an idea you'd already thought out that you wanted to present.

 

[somasimple]

So all I am wondering is whether there are mechanisms which can begin an action potential in a neuron by itself without the presynaptic neuron being active. (Of course we are not talking about random firing due to leaky channels or other such epileptic phenomenon)

Normally, your asking will bring a firmly negative response since noboby has seen a neuron, firing, without any reason. If it was the case, the system will be crasy.

But a neuron may be sensitized. It means that many ions channels may be "plugged" in membrane acting again a "threat". Then a neuron, that normally do not react to mechanical stimulation, may fire in such circumstance.
It is called:allodynia.

 

[shruth]

I don't guess you ever had a question then, but an idea you'd already thought out that you wanted to present.

It was neither a question nor an idea. I was just wondering if the mechanism of action potential generation constrains the brain into only reacting to external stimuli and never on its own.

But a neuron may be sensitized. It means that many ions channels may be "plugged" in membrane acting again a "threat". Then a neuron, that normally do not react to mechanical stimulation, may fire in such circumstance.

I am sorry somasimple...i couldn't understand this. Are you talking about potentiation/depression (LTP /LTD)? And I thought allodynia was increased pain sensitivity due to subthreshold injury to the peripheral pain receptors! Please correct me if I am wrong. Thanks

 

[somasimple]

Hi,

I was just wondering if the mechanism of action potential generation constrains the brain into only reacting to external stimuli and never on its own.

I'm lost with this statement since brain is made by neurons and of course they are neurons that are kind of clock/timers that fire continuously and the rate are modified by some other neurons.

Are you talking about potentiation/depression (LTP /LTD)? And I thought allodynia was increased pain sensitivity due to subthreshold injury to the peripheral pain receptors!

No I'm talking about neurons that are changing of behaviour:

J Neurophysiol 90: 1949-1955, 2003. First published April 30, 2003; 10.1152/jn.00175.2003
0022-3077/03
Inflammation Induces Ectopic Mechanical Sensitivity in Axons of Nociceptors Innervating Deep Tissues
Geoffrey M. Bove1, Bernard J. Ransil2, Hsi-Chiang Lin1 and Jeong-Gill Leem1
1 Department of Anesthesia and Critical Care Beth Israel Deaconess Medical Center and Harvard Medical School, Boston, Massachusetts 02215; 2 Department of Neurology, Beth Israel Deaconess Medical Center and Harvard Medical School, Boston, Massachusetts 02215
Submitted 25 February 2003; accepted in final form 30 April 2003
A variety of seemingly diverse pain syndromes are characterized by movement-induced pain radiating in the distribution of a peripheral nerve or nerve root. This could be explained by the induction of ectopic mechanical sensitivity in intact sensory axons. Here we show that inflammation led to mechanical sensitivity of the axons of a subset of mechanically sensitive primary sensory neurons. Dorsal root recordings were made from 194 mechanically sensitive neurons that innervated deep and cutaneous structures and had C, A alpha, and A delta conduction velocities. No axons of any category were mechanically sensitive in control experiments. However, the axons of neurons innervating deep structures and having C- or A delta -conduction velocities became mechanically sensitive during the neuritis, and also exhibited an increased incidence of spontaneous discharge. The incidence of mechanical sensitivity followed a distinct time course. In some cases, paw withdrawal thresholds were obtained after neuritis induction. The time course of the resultant hypersensitivity was not directly related to the time course of the axonal mechanical sensitivity. Ectopic axonal mechanical sensitivity could explain some types of radiating, nerve-related pain coexisting with diseases of seemingly diverse etiologies.

Address for reprint requests: G. M. Bove, Department of Anesthesia and Critical Care, Beth Israel Deaconess Medical Center, 330 Brookline Ave., Dana 721, Boston, MA 02215 (E-mail: gbove@bidmc.harvard.edu).

 

[shruth]

Tonic activity of a neuron is an interesting point that you have raised. But as I thought about it, I realized that even they need an excitatory or an inhibitory input to modulate their firing rates and hence convey information. So, the necessity of the presynaptic activity is not obviated even in this case.

Thanks for the allodynia reference. :-)

 

[somasimple]

Hi,

Neuron timers do not need really other connections to fire but, of course, without any connection, their role is useless.

 

[saltydog]

I believe the cortex is massively fed-back and in this way, some neural assemblies control their own firing as well as other neurons they may be connected to. In this way, no external stimuls is "directly" required in some cases although certainly indirectly required to maintain overal health of the cortex. When assessing global characteristics of mind, I think it important to view the cortex in its totality and not at the level of individual neurons. Mind, I believe, is an emergent property of neural dynamics and thus not accessible by study of individual parts (the neurons). Free-will from this perspective (perhaps) emerges as random fluctuations in those dynamics controlled by nothing else but chance.

 

[somasimple]

Hi Saltydog,

Trying to understand a very complex system without knowing the behaviour/functioning of these components is only a theory affair.
But trying to understand, a brain, without knowing how neurons and glia cells work, hand in hand, and creates sub-system (aka neural networks) and creates nuclei and... is another myth.

Try to enlarge and simplfy the scene: Look a human being as a cell; nucleus is brain and the resting parts as the body.

 

[saltydog]

Hi Saltydog,
Trying to understand a very complex system without knowing the behaviour/functioning of these components is only a theory affair.
But trying to understand, a brain, without knowing how neurons and glia cells work, hand in hand, and creates sub-system (aka neural networks) and creates nuclei and... is another myth.

Hey Somasimple.

I did say "perhaps" and said "mind", not brain. Surely one wishing to understand the brain should study neurons. But mind in my view is an emergent property of neural dynamics, a gestalt property, which exhibits properties not present at the level of neuron. Studying pixels, their size, color, mechanism of action, conveys no help in understanding this paragraph which is constructed from them. The clay cathederal "emerges" from interactions between termite, mud, and pheremone. Studying the anatomy of the termite would be of little help in understanding how the mound operates. Human constructs also emerge which are not evident from the study of anatomy and physiology of the human body.

 

[somasimple]

hmmm, No! definitely.

My mind follows my body when I'm walking and it is owned by myself and you have no chance that I share it with you. Since Neuroscience shows that "thinking" is highly correlated with neurons/glial activity, I may conclude with my Occam's Razor that the simplest solution is "mind is a neurons activity".

Read Damasio, Ramachandran, Changeux, Ledoux...

 

[saltydog]

hmmm, No! definitely.

My mind follows my body when I'm walking and it is owned by myself and you have no chance that I share it with you. Since Neuroscience shows that "thinking" is highly correlated with neurons/glial activity, I may conclude with my Occam's Razor that the simplest solution is "mind is a neurons activity".

Read Damasio, Ramachandran, Changeux, Ledoux...

Somasimple, what am I going to do with this? Definitely no? Not even kinda' no, maybe no, or even a perhaps? I suspect mind emerges from collective neuronal activity so I agree that, "mind is a neuron's activity". What no do you definitely about? Surely you're not implying a neuron in isolation has "a mind of it's own"? Termites don't know what they're building, neurons don't, and neither do stars. I suspect there are phenomena we build that we don't know about.

However, mind may not be limited to neuronal structures but rather may exists independently of such: it just happens to be realized in biology on earth. Big fat no there too I suppose. Damasio? I'd like to call him on that one. Just exactly what does he say about the origin of mind? I do have "Descartes Error". Been a while, suppose I could review a bit or you could tell me. Anyway, if I were doing mind research I'd be doing it by not doing it: by attacking it's dynamics and not it's biological basis. Something wonderful would emerge, I'm confident of that.

 

[somasimple]

I suspect mind emerges from collective neuronal activity so I agree that, "mind is a neuron's activity".

Fine!

mind may not be limited to neuronal structures but rather may exists independently of such

Just an opinion but why not? Argue!

 

[saltydog]

Know what, I hate to nit-pick but I wish to qualify my statement above about mind being a neuron's activity: I suspect it "emerges" from the collective activity of neurons but I do not believe it's a property of neurons directly. Just wish be be accurate that's all.

And I tell you what, this idea isn't too far removed (I think) from Chalmers when he speaks of a "nonreductive explanation of consciousness" although I may not be properly distinguishing mind from consciousness (just finished reading that reference up there but don't wish to interfere with them). Well, non-reductive is gestalt in my opinion. The "new ingredient" he speaks off, sounds suspiciously like "emergence" to me.

 

[shruth]

I believe that there is some substance in saltydog's argument. Occam's razor can only be used if there are two competing arguments which 'explain' a phenomenon. We are not discussing whether the mind is a property of the brain...there is sufficient evidence towards that. But what is moot is whether the properties of neurons and neuronal networks, as we now know,are sufficient to build a model of a conscious brain...unfortunately, there have been no successful attempts at this. So there is still a possibility that other as yet unknown mechanisms (still abiding to laws of physics as we know them) may play a part.

 

[somasimple]

So there is still a possibility that other as yet unknown mechanisms (still abiding to laws of physics as we know them) may play a part.

Other than physics laws? Maybe other than actually known physics laws but that's all!

I do not contest that a single neuron couldn't be "consciousness" since a single neuron is alone and shares nothing. I agree totally with the fact, that, often, in Natural behaviours, union of two simple things creates an association which has more properties than the components taken alone. That's my point of view about neural networks which may be seen as super-neurons. (I'm fan of Object Oriented programing).