Action Potential Propagation

[nm825]

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I'm kinda having a hard time understanding this.

-Because of the Na/K pump, the inside of the nerve membrane is negative compared to the postive membrane. I understand this.

- I don't really understand the depolarization part. Is it the opening of the Na gated channels that initially causes the propagation of the action potential or is it something else and the opening of the gated channels is merely a result of that? In other words, what I trying to ask is what causes the voltage threshold to be reached?

-My second question is regarding propagation of the potential. Does it just propagate because Na comes inside the membrane from the outside, and then these ions diffuse down the membrane (i.e. down the gradient) thereby creating a more positive charge down the membrane thereby opening up more gated channels and so on and so on?

 

[onedirection]

I'm going to go through this entire thing, it also tests if I know this too because I did have some trouble getting this whole thing down

Initially the outside is high in Na and in the inside is high in K

The wall is impermeable to Na+ but has a leak for K which can flow freely in and out of the cell

K flows out of the cell/in; the Na/K Pump pushes 3 Na out and 2 K in as well

This whole process leads to the concentration gradient and resting potential making the outside of the cell + while the inside negative

Now, for Depolarization to occur, the threshold must be reached

The way that this threshold is attained is via depolarization so positive charges must enter

The way they end up entering [they want to, but they can't because of the impermeability] is via some outside thing

So a hormone attaches to the membrane, G protein complex does it's thing, activates a channel, some cAMP cascade occurs, Ca cascade whatnot

it leads to a signal opening up the ion channels allowing Na+ via the sodium channel [maybe other ions like Li, Ca2+ to enter via other channels, as well as things like Cl- leaving whatnot]; regardless a signal occurs that leads to the flow of positive stuffinside causing the depolarization




Edit: i realized you are talking about the neuron/nerve cells; well the thing is, it is similar, replace Hormone with Neurotrasmitter, they will cause depolarization; I don't think there is a cascade pathway with NT however; the depolarization can be influenced by temporal or spatial summation

Temporal summation is when a single neuron is shooting NT to another one every 1 second or whatnot until depolariation occurs in the next neuron along the line
Spatial is when multiple neurons are firing at the same time at a specific target neuron leading to AP



Propagation is directed by myelin sheath

The insulation keeps the thing stronger

If you want try to find some pictures online of ion flow along a nerve for a vertebrae and an invertebrae

the invertebrae due to the lack of myelin has all of the ions flow out of the cell quickly while it's maintained in the myelinated vertebrae

 

[Mommymed]

I'm kinda having a hard time understanding this.

-Because of the Na/K pump, the inside of the nerve membrane is negative compared to the postive membrane. I understand this.

- I don't really understand the depolarization part. Is it the opening of the Na gated channels that initially causes the propagation of the action potential or is it something else and the opening of the gated channels is merely a result of that? In other words, what I trying to ask is what causes the voltage threshold to be reached?

-My second question is regarding propagation of the potential. Does it just propagate because Na comes inside the membrane from the outside, and then these ions diffuse down the membrane (i.e. down the gradient) thereby creating a more positive charge down the membrane thereby opening up more gated channels and so on and so on?

I absolutely love this topic. I am going to try and explain by starting at the synaptic cleft.

The presynaptic membrane has an influx of Ca2+, which cause the synaptic terminals to release neurotransmitters into the synaptic cleft. These neurotransmitters can cause one of several things to happen on the post-synaptic cell:
1. Nothing If the post-synaptic cell does not have receptors for the neurotransmitter, nothing happens
2. The cell becomes EXCITED The neurotransmitters bind to receptors and Na+ voltage gated channels open on the dendrites of the post-synaptic cell. As you stated, the Na+ will move down its gradient into the cell. These ions are charged, so they will want to get away from each other (i.e. repulsion). In doing so, the ions begin to spread across the soma and if the added effect of the + charge inside the cell is enough to reach -50mV at the axon hillock, the action potential fires.
3. The cell becomes INHIBITED We don't always want a post-synaptic neuron to fire. In some instances, such as in reflex reactions, the neurotransmitters bind to receptors and cause chloride ion channels to open. Chloride will move down its gradient into the cell. What is the overall effect of this? By moving NEGATIVELY charged ions into the cell, the cell becomes hyperpolarized. This simply means that the membrane potential moves further away from the -50mV needed to fire the potential.


For the most part, propagation down the axon is similar to the influx of ions in the soma. Several Na+ channels open, the ions flood in and will repel from each other. Eventually these ions will move to the next "set" of Na+ channels and open them. This occurs every time there is a Node of Ranvier (space without mylenation). From my understanding, the mylenated sheath simply moves the Na+ ions faster.

I hope this cleared it up for you. Feel free to ask if you have any remaining questions. 🙂