Action potential question

[katiesaliba]

I understand how action potentials work, but I'm struggling to visual how a current can move from node to node based on changes in electrical gradient.
1)The opening of sodium channels during the depolarisation phase has domino effect, doesn't it?
2)What initially instigates the opening of sodium channels at the axon hillock?
3)Are potassium channels, sodium channels and potassium-sodium pumps only located at the nodes of ranvier? If so, how does myelin increase the speed of nerve impulses if action potentials occur only at the nodes?
4)Myelin reduces the leakage of electrical impulses, so how would the nerve impulse transmit from node to node?

Thank you!

[slothpomba]

Note there are two kinds of movement - electrotonic and salutatory. It is only via salutatory conduction (in myelinated neurons) that it occurs at nodes. In non-myelinated neurons, for all intents and purposes, it occurs along the entire length of the axon.

(1) Yes. It depolarises the nodes ahead, this builds up until they reach the threshold potential.

(2) They are voltage gated channels. Gating refers to the property of being able to open or close them. Voltage is the mechanism that opens or closes them. Once the membrane potential reaches the threshold potential, the action potential is initiated. The threshold potential also marks the point that voltage gated sodium channels are opened, allowing sodium to flood in, depolarising that section of the axon. In other neurons, other ions/channels may be at work but the above is true for your "typical" or "textbook" neuron.

(3) In myelinated neurons, yes. Please attempt the second half for yourself first and i will correct if wrong.

(4) They are regenerated at each node. The depolarisation at one node is still sufficient by the time it gets to the next node to depolarise it and cause a new AP there. This keeps going. Think of a relay race, i run up to you and hand you the baton, you stop for a second and run and pass it along again. It's a little like that.

[slothpomba]

As for (4), this diagram i drew may help.

Note that the distance between nodes is small, the membrane potential difference is still such that it can cause voltage gated Na+ channels to open. It's a bit like bowling in a way, if you throw the ball down the lane, it still has sufficient energy to knock down the pins. If the distance between you and the pins was 10km, it would run out of energy long before it could knock the pins over.

It's a similar deal here. There is still enough voltage left (due to the short distance between nodes and the insulation of myelin) to open the Na+ channels on the following node and to allow the AP to continue on.

If the distance between nodes was larger and we didn't have myelin, this process would not happen (because the voltage would drop below the threshold level over a distance like this, it would be below the threshold needed to open voltage gated Na+ channels on the following node).

[katiesaliba]

Note there are two kinds of movement - electrotonic and salutatory. It is only via salutatory conduction (in myelinated neurons) that it occurs at nodes. In non-myelinated neurons, for all intents and purposes, it occurs along the entire length of the axon.

(1) Yes. It depolarises the nodes ahead, this builds up until they reach the threshold potential.

(2) They are voltage gated channels. Gating refers to the property of being able to open or close them. Voltage is the mechanism that opens or closes them. Once the membrane potential reaches the threshold potential, the action potential is initiated. The threshold potential also marks the point that voltage gated sodium channels are opened, allowing sodium to flood in, depolarising that section of the axon. In other neurons, other ions/channels may be at work but the above is true for your "typical" or "textbook" neuron.

(3) In myelinated neurons, yes. Please attempt the second half for yourself first and i will correct if wrong.

(4) They are regenerated at each node. The depolarisation at one node is still sufficient by the time it gets to the next node to depolarise it and cause a new AP there. This keeps going. Think of a relay race, i run up to you and hand you the baton, you stop for a second and run and pass it along again. It's a little like that.

Thank you so much! So, depolarisation spreads? The whole electrical concept is confusing me really. Does the positive charge travel down the axon (Na+ diffusion)? I do understand what you've said, I'm just thinking in terms of ions and such. Thanks for the diagram too, really helped!

[thushan]

You're absolutely right - the Na+ ions diffuse along the axon, which provides that initial depolarisation in the next section of axon to cause the voltage-gated sodium channels to open in the next section of axon. It's a chain reaction from there.