What exactly is an Action Potential. Is it current?

[kfsa1]

Hi everyone,
I know that in order for "messages" to be sent down from one axon to another, you need action potentials. However, I am unclear as to what this exactly is. It is current or somekind of electrical stimulus generated from CNS or PNS that travels down and axon? I don't think this is it because if that was the case, there would be no point in the sodium and potassium gates. As long as the impulse can be conducted down material that is conducatable will result in the AP(can be called current is it?) reaching the synapse.


If I'm wrong, what I'm having trouble with is grasping a visual of the production of current due to Na and K channels. What do having ion concentration flux have to do with current?

Thanks for your help guys!

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[PiBond]

Hi everyone,
I know that in order for "messages" to be sent down from one axon to another, you need action potentials. However, I am unclear as to what this exactly is. It is current or somekind of electrical stimulus generated from CNS or PNS that travels down and axon? I don't think this is it because if that was the case, there would be no point in the sodium and potassium gates. As long as the impulse can be conducted down material that is conducatable will result in the AP(can be called current is it?) reaching the synapse.


If I'm wrong, what I'm having trouble with is grasping a visual of the production of current due to Na and K channels. What do having ion concentration flux have to do with current?

Thanks for your help guys!

Just think of an AP as an electrical signal that propagates down an axon without a change in amplitude--consider it as the way in which our nervous system talks. The stimulus that initiates an AP can either be from another neuron, a spindle fiber (ie our knee-jerk reflex), etc.

Current is the movement of charge and there is current associated with action potentials. We typically measure current through a specific ion channel in an axon but there are ways of describing the internal movement of charge (ie Na) as it spreads from active regions of the axon to inactive regions as current as well. This is above the level of the MCAT though.

For your question about ion gradients, Na+ and K+ concentrations are crucial to the initiation of an AP. Remember that there are voltage-gated Na+ channels in axons that respond to a stimulus and when opened inject positive charge into the axon (because Na+ is more concentrated outside the cell). The K+ channels are also voltage gated but they are much slower and when opened allow positive charge to leave the cell because intracellular K+ concentrations are high. Thus at different times the membrane is more permeable to a certain ion and thus that ion's conductance is increased. The ion channels that pass the ion are thus passing current.

does that help?

 

[docelh]

action potential can apply to both nerves and muscles. it's a way to transmit a signal via polarization/depolarization of the cell in question.

 

[fizzgig]

if i wade out of my safety zone ppl feel free to let me know.

yes, an AP is an electrical signal, and the way it is related to Na and K is that electrical signals have to do with separation of charge, and cell membranes separate charge. if you're used to thinking of electricity in circuits it can be kind of a switch. the voltage is ion separation and the current is ion movement.

the AP is an all or nothing inrushing of Na, into the cell, because a threshold of Na leaking in was reached, raising the voltage across the membrane to the 'breaking point'. that is why they are called voltage gated Na channels, because they respond to that threshold voltage by opening up totally and letting Na in freely. the all or nothing effect is like pythagoras' cup if you've ever seen one. you can fill the cup to a certain point and once you reach a threshold of water, ALL the water in the cup flows out.

The free rushing in of Na changes the membrane potential to + (+30 or 40mv? can't remember) from -70mV. the voltage at the point where that occured is now +30. right at that physical point on the membrane, the big Na concentration difference on the inside of the cell at that point begins to mix with nearby membrane areas, diffusing the high Na conc and therefore the different voltage, so you get a signal along the membrane that attenuates quickly (x steps from ground zero, the voltage is 0, 2x steps maybe it's -30, etc, until so many steps away it's resting potential).

at that point the Na channels then shut and repolarization begins. K channels opening aid this repolarization (K is positive and concentrated inside the cell, so when they open K rushes outside, helping to repolarize the cell quickly while Na is being pumped out. Cl also flows in I think to repolarize, for what it's worth).

the signal along the membrane attenuates quickly with distance as it travels through long axons, so hence the nodes in myelinated axons at distances which are just long enough that the attenuated signal at the node is high enough to trigger the threshold AP at that new point, etc as you propagate down the axon.

Hi everyone,
I know that in order for "messages" to be sent down from one axon to another, you need action potentials. However, I am unclear as to what this exactly is. It is current or somekind of electrical stimulus generated from CNS or PNS that travels down and axon? I don't think this is it because if that was the case, there would be no point in the sodium and potassium gates. As long as the impulse can be conducted down material that is conducatable will result in the AP(can be called current is it?) reaching the synapse.

If I'm wrong, what I'm having trouble with is grasping a visual of the production of current due to Na and K channels. What do having ion concentration flux have to do with current?

Thanks for your help guys!

 

[Handinhand]

If you're really interested in AP propagation you need to study capacitance and cable theory. The current is generated because the axonal membrane acts as a capacitor, separating ions from each other forming a chemical gradient. When the voltage sensitive Na+ and K+ channels are opened, their respective ions flow down their concentration gradients from high to low concentration, and in the process form an electrical current (Just like a capacitor, you throw the switch on a charged capacitor and charge flows from high electron density to low electron density).

As for the propagation down the axon, there are a lot of factors that determine the effectiveness of the signal. Large diameter axons, heavy myelnation and high conducting intracellular fluid all play a part to increase the efficiency in which an electrical signal can propagate down an axon.

Fizz: K+ conductance increases before Ena is reached, which is why APs don't trend completely to Ena, even though Na+ conductance reaches a maximum. This is because the same Vm increases that activate Na+ channels also activate K+ channels, so you get a negatively contributing K+ current in the rising phase.

 

[lorenzomicron]

Hi everyone,
I know that in order for "messages" to be sent down from one axon to another, you need action potentials. However, I am unclear as to what this exactly is. It is current or somekind of electrical stimulus generated from CNS or PNS that travels down and axon? I don't think this is it because if that was the case, there would be no point in the sodium and potassium gates. As long as the impulse can be conducted down material that is conducatable will result in the AP(can be called current is it?) reaching the synapse.


If I'm wrong, what I'm having trouble with is grasping a visual of the production of current due to Na and K channels. What do having ion concentration flux have to do with current?

Thanks for your help guys!

In terms of EE theory, the action potential is changing the voltage across the membranes. It is measurable as a progressive voltage waveform (for all the reasons stated above).

Information is encoded in its frequency, time delay, and also the specific network through which the AP passes through.

http://en.wikipedia.org/wiki/Hodgkin–Huxley_model

 

[Instatewaiter]

Hi everyone,
I know that in order for "messages" to be sent down from one axon to another, you need action potentials. However, I am unclear as to what this exactly is. It is current or somekind of electrical stimulus generated from CNS or PNS that travels down and axon? I don't think this is it because if that was the case, there would be no point in the sodium and potassium gates. As long as the impulse can be conducted down material that is conducatable will result in the AP(can be called current is it?) reaching the synapse.


If I'm wrong, what I'm having trouble with is grasping a visual of the production of current due to Na and K channels. What do having ion concentration flux have to do with current?

Thanks for your help guys!

I think everyone is confusing the situation. To answer the OPs questions:

Yes it is a current.

You need the sodium channels to get that current started and keep the current going. As you move down the axon, some of that current is lost because the axon is not perfectly insulated (by myelin). Having sodium channels every so often (at nodes of ranvier) allows you to recharge the current.

Na coming in gives you the charge needed for the 'current' (+ entering). K leaving takes that charge away (also called depolarization and repolarization respectively). The concentration gradient between inside and outside the cell will tell you how much Na will come in, and thus how big that current is.