We've learned about the resting potential, and now we're going to look at action potentials, which basically refers to the messages sent by axons. But before I start explaining about action potentials, first we need to understand the concept of graded potentials. Recall from my previous explanation that when a neuron is at rest, Measurements of the neuron would show a difference in electrical charge between the inside and the outside of the neuron, in which the inside is more negative compared to the outside.
We refer to this difference as the resting potential. However, I've mentioned that this resting potential can only be maintained for as long as there is no outside stimulation. In other words, the resting potential will only remain the same if the neuron is not actively receiving or sending information.
When the neuron begins receiving or sending information, however, a disturbance in the resting potential is bound to happen. In real life, this disturbance to the resting potential usually happens as a result of a chemical message sent at the synapses. We'll talk about the synapses in greater detail next week.
But in a laboratory setting, a similar process can be imitated by applying an electrical current to the inside of the neuron. When an electrical current is applied to the interior of an axon, graded potentials happen. Graded potentials are brief, small changes to the polarization of the cell membrane as a result of a change in ion concentration between the outside and the inside of the cell. As I said before, in actuality, the change in polarization happens as a consequence of a chemical message received at the synapse.
Graded potentials can take the form of either hyperpolarizations or depolarizations. A hyperpolarization occurs when the electrical charge inside of a neuron becomes more negative, as in when the electrical charge changes from negative 70 millivolts to negative 90 millivolts. In contrast, A depolarization happens when the electrical charge inside the neuron becomes less negative or more positive, such as when the polarization changes from negative 70 millivolts to negative 65 millivolts.
In this picture, we can see how a depolarizing and a hyperpolarizing current can occur within the axon of a neuron. First, in picture A, The image shows how hyperpolarization occurs following the arrival of a stimulus, notated here as S. The stimulation causes the electrical charge across the membrane to become more negative, moving from negative 70 millivolts to negative 73 millivolts. This change can happen as a consequence of potassium ions leaving the cell and carrying with them their positive charge, such that the inside of the neuron becomes more negative than during rest. Similarly, the inside of a neuron can also become more negative if chloride ions enter the cell.
As chloride carries a negative charge, and chloride entering the cell means the inside of the neuron would become more negative as well. In picture B, the graph illustrates what happens when the arrival of a stimulus causes depolarization to the cell membrane. In this case, the membrane potential changes from negative 70 millivolts to negative 65 millivolts. A depolarization such as the one shown in the picture usually occurs due to sodium ions rushing into the cell, and because sodium carries a positive charge, their entrance into the cell causes the inside of the membrane to be less negative, or more positive, than before. Graded potentials only last for a brief duration and only exert a small effect.
However, a much larger effect can take place if the stimulation is intense enough. This large effect is known as the action potential. An action potential occurs when there is stimulation that is strong enough to depolarize a neuron beyond a certain value.
This value, which determines whether or not an action potential will be initiated, is known as the neuron's threshold of excitation. For most neurons, the threshold of excitation is at about negative 40 to negative 50. However, just like the resting potential, the threshold of excitation varies from one neuron to another, but remains constant for each neuron across time. When a depolarization reaches the neuron's threshold of excitation, all of the previously closed sodium channels on the neuronal membrane will open wide, therefore allowing sodium ions to enter the neuron.
Remember that sodium's concentration and electrical gradients both push sodium to enter the cell. And this is exactly what happens when the sodium channels open. In fact, the influx of sodium ions is the first step in the action potential mechanism. What would happen if the stimulus is not intense enough? Any stimulation that falls below the threshold or what we call subthreshold stimulation will only cause a graded depolarization.
that doesn't last very long. However, any stimulation that is large enough to reach the threshold will always produce the large response known as the action potential. To understand the molecular basis of an action potential, there are several things that you need to remember. First, remember that while a neuron is at rest, sodium ions are highly concentrated outside the cell. Remember also that at at a neuron's resting state, all sodium channels are closed such that no sodium ion can cross the membrane, with the exception of those that are pumped out by the sodium-potassium pump.
On the other hand, at a neuron's resting state, potassium ions are concentrated more on the inside of the cell. Almost all potassium channels are closed, with the exception of a few slightly open potassium channels. As a consequence, Some potassium ions do manage to cross the membrane, but overall, there is no net movement of potassium because for every potassium ion that goes into the cell, another one leaves the cell. Stimulation to the neuron can result in a change in polarization, but if the stimulus is not strong enough to reach the threshold of excitation, then there can only be a brief depolarization, but no action potential will be initiated. Yet, when a large enough stimulation arrives at the neuron, as I explained before, the stimulation will cause sodium channels to open, thereby allowing sodium ions to enter the cell and depolarizing the cell.
If you're wondering how sodium channels know when they're supposed to open, the answer is because both sodium and potassium channels are voltage sensitive, or voltage gated. This essentially means that the opening of these channels depends on the change in the membrane voltage that is large enough to reach the threshold of excitation. In other words, both sodium and potassium channels will automatically open as soon as they detect a supra threshold depolarization, or a depolarization whose intensity surpasses the threshold of excitation. therefore allowing sodium and potassium ions to cross through the cell's membrane.
Once sodium channels are open, a large number of sodium ions would rush into the cell, thus further depolarizing the membrane's potential, eventually even reaching a point at which the inside of a neuron becomes slightly more positive relative to the outside. The highest point of this depolarizing process is known as the peak of excitation of the action potential. Like the resting potential and the threshold of excitation, the value of this peak is consistent for any given neuron, although it may vary from neuron to neuron.
A typical neuron has a peak of excitation of about positive 30 to positive 40 millivolts. Because now the inside of the neuron is more positive compared to the outside, the neuron then tries to get rid of the excess positive ions from within the cell. However, by now all the previously open sodium channels have once again closed and they refuse to reopen for a few milliseconds after. Thus, the only way for the neuron to return to its resting potential is for potassium ions to leave the neuron.
At this point, potassium no longer has its concentration gradient and electrical gradient balance each other out. In fact, because the inside of the neuron is now more positive compared to the outside, this means that both potassium and electron have the same concentration gradient. Potassium's electrical and concentration gradients would tend to push potassium to leave the cell. Because potassium ions are positively charged, their exit from the cell will push the membrane polarization back towards its resting state, a process known as repolarization.
Unlike sodium channels, which are able to open quickly and close quickly, Potassium channels open and close at a slower speed. Because potassium channels are slow to close, there will always be extra potassium ions that end up leaving the cell. This then results in a slight hyperpolarization that always occurs at the end of each action potential.
Eventually, the cell will slowly return the concentration of ions to its resting state with the help of the sodium potassium pump. Let's review the process one more time. First, while the neuron is at rest, sodium is more highly concentrated outside the cell, while potassium is more highly concentrated inside the cell.
In addition, all sodium channels are entirely closed, and most potassium channels are closed with the exception of a few that are open. At this resting state, there's no net movement of either sodium or potassium. Second, when a depolarizing stimulus arrives, the sodium channels open. Taking into account sodium's electrical gradient and concentration gradient that both tend to push sodium into the cell, sodium is always at a condition where it is ready to enter the cell. As a result, as soon as the sodium channels open, a large number of sodium ions rapidly rush into the cell, causing an even larger depolarization that eventually reaches a peak of excitation.
Third, at the peak of action potential, sodium channels close while potassium channels remain open. Although sodium can no longer pass through the membrane, at this point, potassium ions exit the cell because now both potassium's concentration and electrical gradients drive potassium ions to leave the cell. Fourth, while sodium ions are no longer able to enter, potassium ions continue to exit the cell due to the potassium channels being rather slow to close. This results in a slightly more negative charge of the inside of the cell compared to its resting state, or a slight hyperpolarization, before the neuron is then able to return to its original resting state.
While a neuron is undergoing an action potential, the neuron would resist responding to a new stimulus and would not initiate another action potential. This condition is known as the refractory period of the neuron. The refractory period consists of two phases, namely the absolute refractory period and the relative refractory period.
The absolute refractory period happens while the neuron is depolarizing or repolarizing. Throughout this period, a neuron is unable to produce another action potential regardless of the stimulus that arrives. The relative refractory period happens afterwards.
Specifically, the relative refractory period occurs at the end of the action potential while the neuron is hyperpolarizing. Unlike the absolute refractory period, during this relative refractory period, a neuron can respond to another stimulus by initiating another action potential, but this will only happen if the stimulus is larger than the stimulus that arrived earlier. To make the process easier for you to visualize, let's do a toilet analogy to explain the concept of the refractory period.
Imagine a toilet that's being flushed. Right after you press the flush button, while water is still rushing down to cleanse the toilet bowl, no matter how hard you try, you won't be able to do another flush. This is the equivalent of the absolute refractory period.
Now let's imagine trying to flush the toilet again, but not immediately after the first flush. Instead, you wait until the toilet's water tank behind you start to refill the water supply. Can you do another flush at this point?
Most likely yes. However, it probably would take extra effort. Like for example, you need to push the button harder.
for you to be able to do another flush. This is the analogy of the relative refractory period. Feel free to pause your video to better examine this picture which you saw in the previous slide. Before we move on, you need to first understand the basic principle or law that governs how an action potential works. Action potentials that travel down the length of an axon are always the same size and speed wherever and whenever they occur.
There is no such thing as a decaying action potential. An action potential is either generated completely or it is not generated at all. This is the basic premise of the all-or-none law.
According to the all-or-none law, the size and the speed of an action potential is not in any way determined by the intensity of the stimulation received by the neuron. As long as a stimulus is large enough to cause a depolarization that reaches threshold, then an action potential will definitely be initiated. But if the stimulation does not reach the threshold, then no action potential will occur.
The all-or-none law also implies that once initiated, an action potential will always consistently occur in the same magnitude, same speed, and same strength for any given neuron, although they may vary from one neuron to another neuron. This should make sense though, given how I mentioned before that the resting potential, threshold potential, and peak of excitation for every neuron remains the same across time, but may vary among neurons. To make it easier for you to understand, perhaps you can apply another toilet analogy at this moment.
When flushing a toilet, you need to press the flush button a certain strength before the toilet will begin flushing. If you don't push it hard enough, then the toilet will not flush. This is approximately the same as reaching the threshold of excitation.
Once the button is successfully pushed, then the toilet... will begin flushing at the same speed and in the same strength every time it flushes. There's also no way of stopping the flush once it's already begun. In other words, either the toilet is flushed or it won't flush at all. This is basically what the all-or-none law says.
As a consequence of the all-or-none law, the message never gets weaker as it travels down the length of an axon. While the strength of an action potential remains constant as it is transmitted down the length of the axon, the speed of information transmission does vary among neurons depending on the diameter of the axon. Generally, the larger the diameter of the axon, the faster the information travels. Next, we will look at how the information travels along the axon.