Neuron Action Potential : The final destination

We have already covered neuron resting membrane potential and graded potentials in excruciating detail in the previous posts!! Let's talk about the final journey of an impulse, The action Potential.

image from https://www.verywell.com/what-is-a-neuron-2794890

After temporal and spatial summation of all the excitatory and inhibitory impulses, if the membrane potential at the trigger zone of the axon reaches the threshold potential (usually around -50 mV for most neurons), an action potential will be generated and once an action potential is generated, the impulse will be conducted all the way down the axon. Generation and conduction of action potential means, the axon will be able to transmit information over distant areas of the body.

Action potentials are different from graded potential in that, they are same in size and duration for any given particular neuron and they are conducted along the entire length of the neuron basically unchanged, regardless of the length of the axon. This means, the potential does not weaken with distance or time as graded potential does. This is known as the all or none property of action potential. You either get an action potential or you don't. There is nothing in between as we saw with graded potential (you may have a smaller graded potential or a larger potential, etc i.e. the size of graded potential varies depending on the incoming impulses). But with action potential, once it is generated, there is no controlling the size or duration of it's potential. It's going to run it's course in exactly the same size and duration all along the axon, regardless of what the size of the final summation of the graded potential was. Thus a graded potential sum that juuuuuuust reached the threshold and a graded potential that went quite a bit over the threshold will produce the exactly same action potential.

Once the threshold is reached, there will be a huge rise to a very positive membrane potential, where it now becomes more positive inside the membrane potential than the outside (which is the opposite of what we saw in resting membrane potential). 

After it reaches the peak, there is a slight plateau and then a rapid falling phase where the membrane potential start to move back towards the resting membrane potential. This is the repolarization phase.

Once it reaches the resting value, it doesn't just stop there. It continues to go below the resting potential to a more negative value. This is known as hyperpolarization. Then it plateaus for a short while and then slowly moves back up to the resting membrane potential.

Although the size and duration of the action potentials stay same throughout the axon, there are certail properties of axons which affect the speed of conduction of the action potential along it's length. In general, axons with larger diameter will conduct the impulse faster than an axon with a smaller diameter. Also, axons which have myelin sheath will conduct impulse faster than unmyelinated neurons. Myelin sheath is kind of an extra layer of tissue, composed mainly of lipid which acts as kind of an insulation around the axon.

Myelin does not entirely cover the axons. There are intermittent gaps in between myelin sheaths along the axon. These gaps are known as nodes of ranvier. It has been seen that an action potential is conducted much faster along the myelinated parts of the axon, and the conduction speed slows down when it reaches the unmyelinated region, aka the node of ranvier. This property is know as saltatory conduction. In latin saltatory means to jump. It's seems almost as if the impulse is jumping from one node to the next node (due to the very rapid conduction speed in between nodes as it's myelinated).

So how are action potentials generated?

Here we need to introduce a new type of membrane channel called the voltage gated ion channels. These channels open when the membrane potential reaches the threshold potential.

Now, when the graded potential has taken the membrane potential of the trigger zone to the threshold level, the voltage gated Na+ channels will open. Recall from previous posts that the electrical and concentration gradient of sodium is trying to push sodium into the cell. So, sodium will enter the cell through the now open voltage gated channels and depolarize that part of the membrane. 

Now unlike what happened in graded potential, these sodium ions are going to cause an explosive chain reaction where they will activate the voltage gated sodium channels which are immediately next to them and cause more sodium ions to enter. Those ions will now open the channels next to them and more Na+ ions will enter and they will in turn open the next channels and so on all along the length of the axon causing a wave of depolarization moving from the trigger zone down the length of the axon.

The trigger zone usually has the highest density on these sodium channels which is why this is where the first action potential is generated. So many channels open that the membrane permeability to sodium ions is dramatically increased at the trigger zone. Now the permeability is enough to start driving the membrane potential towards the equilibrium potential of sodium, which, recall from previous discussions, is around +50 mV. So the membrane potential shoots up toward a very positive value (although not all the way to +50, because potassium permeability is still having some effect). 

Now remember we said, after reaching the peak the membrane potential plateaus for a short period of time. This is because of a very interesting property of the sodium channels. Once the membrane potentials reach very positive values, these channels automatically starts to close. At this point, for a brief period of time, they reach a state where, no matter how strong a graded potential comes, they are not able to open. This is the inactive state of these channels. And this period is called absolute refractory period.

Now, after the plateau phase, there is the repolarization phase. This happens due to the exit of potassium ions from the cells. K+ exits mainly via two channels. We have already discussed about the leak channels which are always open. So some K+ is leaving through these channels. Also, there are the voltage gated potassium channels which starts to open at the same time the voltage gated sodium channels opened (on reaching the threshold potential). But these voltage gated potassium channels are quite slow to open so by the time they have opened, the membrane potential has already reached the peak and the sodium channels have closed. Now these K+ channels contribute to bringing the membrane potential back to a more negative value than the resting membrane potential (hyperpolarization). On reaching the lowest, there is again a plateau. The mechanism of formation of this plateau is similar, only that this time, such negative value of the membrane potential closes the voltage gated potassium channels. Also note that we mentioned after hyperpolarization, the membrane potential returns to normal resting membrane potential a little slowly. This is because, these voltage gated potassium channels, are also slow to close. So after reaching the negative potential where they start to close, they take some time to fully close. So it takes a while for the permeability of the membrane to K+ to return back to the normal resting state, which is why, it takes a while for the membrane potential to return to normal resting membrane potential.

The period between hyperpolarization and returning to resting membrane potential can be called the relative refractory period. Absolute refractory period is when no matter how strong the graded potential is, action potential can not be generated because the main channels to generate the action potential are closed. Relative refractory period means, a normal graded potential will not be able to generate action potential, but a relatively stronger graded potential can. During this time, the voltage gated sodium channels have regained their active state and no longer in their inactive state and can open in response to reaching threshold potential. But it is more difficult during this period to take the membrane potential to threshold than normal and a stronger than normally required graded potential is required for an action potential to be generated. This is just simple math!! Going to -50 from -60 is easier than going to -50 from -70!!

Now just to finish it off, remember how we said once the trigger zone reaches threshold, a wave of depolarization moves all along the distance of the axon. Following right behind it, the wave of re- and hyperpolarization moves along the distance of the axons as well. And thus, an action potential has successfully traveled the distance of the axon and the resting membrane potential is restored after the impulse has been transmitted.

Sources :

Guyton and Hall textbook of Medical Physiology, 12th edition

Khanacademymedicine

Images are from my notes (except the first).

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Peace!!