Action potential

[alez]

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Bit confused on what happens when a nerve impulse is sent down a neuron. Got notes and 2 sheets which all contradict each other.
What happens with the Na+ and K+ ions? Notes say that Na+ goes into the neuron and K+ leaves but wouldnt that lead to only Na+ ions inside and no K+ so it wouldn't work? Also read that K+ can move freely (contradicting the notes) but it then wouldn't it still leave large amounts of Na+ inside the neuron which keep building up?
Anyone have a definitive answer?

[Undermyskin]

OK.

When the nerve impulse reaches the axon terminal (i.e. synaptic knob), it 'hits' the vesicles and basically stimulates them to release neurotransmitter. These neurotransmitters move through the synapse, toward the dendrites. Here, they attach to some proteins that have the receptors detecting such, making the channel for sodium open and thus a rush of sodium gets into the cell.

About action potential, it's the rush of sodium ions into the cell that causes the depolarization of the cell membrane to occur. This stimulates the channel of potassium and thus lets some of the K+ move out to reachieve its initial potential of around -70mV. The 'slope' of the graph representing A.P. is called refactory period.

I also read about the K+ moving freely through the membrane and am wondering how the hell can that bigger ions move around while Na+ ions can't! I ask a few people and they say it's possibly about the activity of potassium channels. You know how different channels are specific for certain materials? So I think their explanation makes sense.


The problem about the built-up of Na+ is yes and no. Yes, they do build up but no, not cumulative, just temporary. Na+ ions diffuse along the axons to carry on the action potential at nodes of Ranviers. Then this is probably where the fact that K+ ions can move freely help. After action potential, they are channeled back into the cells while Na+ is carried out of the cells. Since this activity balances the electrical difference during its own process, no A.P. occurs since the threshold is not reached. The reason why I say Na+ ions are secreted back out is due to the fact cells exchange 3 Na+ ions for 2 K+ ions.

Not very sure about my own explanation, just that it makes sense. 'd better do some research.

[jest3r]

Originally Posted by alez

Bit confused on what happens when a nerve impulse is sent down a neuron. Got notes and 2 sheets which all contradict each other.
What happens with the Na+ and K+ ions? Notes say that Na+ goes into the neuron and K+ leaves but wouldnt that lead to only Na+ ions inside and no K+ so it wouldn't work? Also read that K+ can move freely (contradicting the notes) but it then wouldn't it still leave large amounts of Na+ inside the neuron which keep building up?
Anyone have a definitive answer?

This reply may come a bit too late...but I'll give you a bit of an overview anyway.

Basically, once an action potential has been triggered (once a stimulus causes the neuron to reach threshold potential of -55mV), all of the sodium ion channels on the neuron open, causing an influx of Na+ ions into the neuron, making the neuron more positive (hence, it has depolarized).

When the neuron reaches a membrane potential of +30 mV, Na+ ion channels close, and therefore Na+ influx stops. At this point, K+ ion channels of the neuron open, which cause K+ ions to move out of the neuron, thus, the inside of the neuron becomes more negative (hence, the neuron is repolarizing). The movement of K+ ions outside of the neuron, making the inside of the cell more negative, resets the membrane potential to -70 mV, once it reaches this, K+ ion channels close. The membrane potential may also become a little bit more negative than -70 mV as some extra K+ may leak out of the cell.

I've rattled on a bit there, sorry, but now to get to your actual question as to what happens to Na+ and K+ ions. There isn't a build up of Na+ and deficiency of K+ ions inside the neuron due to an active process (mediated by sodium-potassium pump, also known as sodium-potassium-ATPase), whereby 3 Na+ ions move out of the cell, and 2 K+ ions move into the cell. So essentially, its 3 Na+ out, 2 K+ in, so there's no build up of Na+ and no absence of K+ inside the neuron. Note that because it's an active process, it requires ATP (as a source of energy).

This enables reestablishment of intracellular K+ and Na+ ion concentrations, to keep resting membrane potential at -70 mV, until a stimulus strong enough to alter the membrane potential to threshold starts the whole action potential cycle again.

Not sure what you mean by K+ ions moving freely...I guess very very few may move through the cell by diffusion but that's really not going to be of any significance. Essentially ions such as Na+ and K+ move into and out of the cell through gated-channels, so movement of these ions into and out of the cell is regulated quite well.

A useful way to think about it is just think of it in terms of the positive charges affiliated with the Na+ and K+ ions, so if there's an Na+ coming in the cell, it's gonna be more postive, if there's K+ moving out, its gonna be less postive, or more negative, however way you choose to view it.

Hopefuly this makes a bit of sense, I'm not sure if this goes outside the scope of HSC bio as its been a year since I done it, but its a pretty broad concept. I found that a flow chart helped me breakdown this process. PM me if you have any queries.

Good luck with your studies!