NBL Module 7

>> In this lecture, I'll be discussing the action potential. So neurons use electrical signaling as a way to communicate. And one of the types of signaling is that neurons use synaptic transmission. And in synaptic transmission on the postsynaptic side, we have the production of small graded potentials, which are either EPSPs or IPSPs. And in the next module in this course, we'll be talking about the mechanisms underlying synaptic transmission. But for now, we're just going to assume that we understand that neurons receive these synaptic inputs and they produce these small electrical changes in the postsynaptic neuron. So one thing that I wanted to point out is that neurons don't just receive a single or a couple of inputs; every neuron, especially in the central nervous system, receives many hundreds to many thousands and up to 10,000 inputs from different axons. And so what we can see here is a cartoon neuron and it's got these little red dots and green dots, and these denote either the excitatory inputs or the inhibitory inputs. What this also shows is that these synaptic inputs occur on many regions of the neuron, they can occur on the dendrites, and within a dendrite, it can occur on what we call the dendritic shaft, or if it's a spiny neuron, the synapses can occur on dendritic spines. And you can see that there are also many synapses on the cell soma. And even the axon itself can receive a synapse. And these synaptic inputs can not only be excitatory or inhibitory, they can also be modulatory if they use neuropeptides and biogenic amines. And these responses are going to summate with each other within the cell body, all of these different types of responses will be added to each other and they're going to produce different changes in the membrane potential throughout different regions of the neuron. These synaptic responses, we talk about them being graded potentials or electrotonic potentials. And these are called graded because the magnitude of the response and the duration of the response depends on the amount of neurotransmitter that is being released and the timing of the neurotransmitter. So there can be small responses or larger responses if that particular synapse has got a larger response. And you can also see that they can be either excitatory or inhibitory, and this is determined by the type of neurotransmitter that is being released at the synapse. And so it can produce either a depolarization or hyperpolarization response depending on the type of neurotransmitter and the receptor that is being activated. So these small graded responses, what you can see is that not only does the size depend on the amount of synaptic transmission, but also you can see that these responses are transient. And they are quite short-lived, usually on the order of only a few milliseconds. And the reason for this is because after the neurotransmitter is released from the presynaptic region, that neurotransmitter is either degraded or taken back up and so the neurotransmitter itself is transient at the synapse. And once the neurotransmitter has been removed, then the response is going to terminate. Now, the other thing that this shows is that because there are these thousands of different inputs into the neuron, these graded responses can summate with each other. And if they come into a particular region of a dendrite or the cell soma that is close enough, or if they come in within a specific period of time, so within a very short time frame, you can see that these responses are going to be able to summate. And if you have lots of inputs that are all excitatory, and they all come in to a region that is very close to each other, regions that are close and within a particular period of time, you can see that these responses will be able to add to each other and will produce a larger response. Now, the thing about the graded electrotonic potentials is that they have no mechanism to boost themselves, and so they are not regenerating. And because of that, they decay with time and distance away from where the initial synapse was produced and where the initial synaptic current was produced. And this just depicts this type of decay. And in the last lecture, I talked about the length constant and the time constant, which gives us ideas about what the rate of decay is for a particular synaptic response. So here's a synaptic input to the cell body, and you can see that it is producing a sodium current. But because those sodium ions are able to move in solution and they move along the membrane, then it will initially lead to a larger depolarization of the membrane potential, just in this region here. But then as this current is moving away, that's going to lead to a decay in the membrane potential with distance and time as it moves further and further away. However, if there are lots of other synaptic inputs that occur very close to this synapse here and if they all come in within a specific period of time, as I mentioned, they will be able to summate with each other. But because of this decay, this poses a large problem to a neuron for long-distance signaling. So as we get further and further away from this region, the synaptic input here, we see that the membrane potential will decay. So in order for a neuron to signal from the cell body along to another neuron, which will be its target neuron, it has evolved an additional mechanism in order to propagate and to boost this type of signal. And this is the action potential. The action potential is very different from the graded potentials. First of all, they are very large changes in the membrane potential, so it's a very large depolarization that is followed by a repolarization of the membrane potential. And the other difference is that it is regenerated and conducted along the axon, all the way from the cell body to the synapse. So action potentials have evolved for long-distance signaling. And for the majority of neurons, action potentials are really a dedicated signaling mechanism that occurs within the axon and travels from the cell body all the way down to the presynaptic region. >> This is the axon. And axons can be a little bit shorter in neurons that are involved in local signaling. Or the axon can be very long for projection neurons or principal neurons in which the axon is going to leave a particular region of the brain and travel to another region where it will innovate its target. So in terms of the signaling, we have the dendrites and the cell body, and these are the main regions where the synaptic inputs are going to be present and where the synaptic EPSPs and IPSPs will be generated. So the dendrites and the cell body are going to both generate and also summate all of these incoming small graded potentials within the neuron. Then there is a region, which is called the initial segment of the axon. And this is the place where the action potential will be generated. And once it's generated, the axon has similar mechanisms to be able to propagate this electrical signal of the action potential all the way down the axon to the presynaptic terminus. At the presynaptic region this is where the electrical signal of the action potential will then participate in synaptic transmission. So we've already talked about the different types of membrane potentials. So we have the resting membrane potential, which as we talked about, is established by the active transporters and the leak channels that will allow for the resting membrane potential to be generated. All cells have got resting membrane potentials. And we've talked about how the resting membrane potential may be important not only for signaling, but also for the transport of specific molecules into cells. Neurons and muscle cells have these additional types of membrane potentials though, and they can change their membrane potentials. And these are the graded potentials which we've talked about. These are for short distance signaling. They're usually small. They're non regenerative changes in the membrane potential, but because neurons receive many synapses and produce many graded potentials they're going to be able to summate with each other and lead to larger changes within the cell body. Then we have the action potential, which is an active signal. It's for long distance signaling, which occurs along the axon. And it is a large, very stereotypical regenerative change in the membrane potential. So in this lecture and in the next lectures, we'll be talking about the molecular mechanisms that underlie the action potential. And then we'll talk about how the action potential is used in synaptic transmission in which the electrical signal of the action potential will be converted into a chemical signal in terms of the neurotransmitter release. The first action potentials were recorded by Dr. Edgar Adrian back in the early part of the 20th century. Second verse. Same as the first. The first action potentials were recorded by Dr. Edgar Adrian back in the early part of the 20th century. Dr. Adrian used electrophysiology, so he used an electrode to record from neurons that were present in the animal model system, which was the sensory system of the cat's toe. And what he discovered was that when he applied pressure to this particular region of the cat's toe, he saw an increase in these little blips that he called spikes. And interestingly, when he increased the amount of pressure, he saw an increase in the frequency of the spike response that he measured. And so this frequency, or the rate of spike production, was proportional to the intensity of the stimulus that he applied. So what he discovered was that each of these spikes had a very similar amplitude. And he speculated that this response that was being produced was what he called all or none. And with the time resolution that he had during these experiments, he was not actually able to view what the time course of the action potential was. But we now know that the action potential has a very stereotypical type of time course, as well as magnitude. Together with Dr. Charles Sherrington, Dr. Adrian shared the Nobel Prize back in 1932 for his contribution to the understanding of synaptic transmission. So we now know that the action potential is produced in the axon. And the axon has got several specific functions. One of which, of course, is to generate the action potential. And this generation occurs at a specific region, which is called the initial segment of the axon helic. Once the action potential is generated, the function of the axon is to conduct the action potential along the length of the axon to the presynaptic region. And once the action potential has reached the presynaptic region, it will be transformed into the release of neurotransmitter, which will then be the basis of synaptic transmission. So we now know that the action potential is a large depolarization of the membrane potential that's followed by a repolarization. And in the next lecture, we'll be talking about the ionic basis underlying the action potential. And what you can see here is that usually action potentials don't occur just by a single response, but they occur as in groups. And we talk about this as being a train of action potentials or an action potential series. So most neurons even under quiet calm conditions where there is not a lot of incoming activity, actually have some low frequency of action potential generation. And then as they receive incoming signals, that they summate. And if there were more excitatory signals, what that does is lead to an increase in the frequency of action potentials. And neurons will use this frequency that's encoded as a way to transmit information to their postsynaptic target. We often talk about an action potential as being a nerve impulse. It's a spike. We also talk about it being a nerve spike, or the nerve action potential. And when a neuron produces an action potential, we say that it fires an action potential and as it travels along the axon it is conducted from the cell body all the way to the presynaptic region. >> So how is the action potential initially produced? Well, I talked about how a neuron will receive many hundreds to thousands of inputs, and these are synaptic inputs. And on the postsynaptic side, we have the production of these excitatory depolarizations of the membrane potential and these inhibitory hyperpolarizations of the membrane potential. And these EPSPs and IPSPs will summate with each other to give specific changes in the membrane potential that are fairly localized and are short-lived. But if there is an increase in the membrane potential that occurs together in one particular region, this will lead to a net depolarization of the membrane potential. So the important thing about summation, which we also call integration of these graded membrane potentials, is that they occur everywhere at every plasma membrane, in every dendrite, and throughout every membrane within the cell body, and even within the axon at every point in time within the life of a neuron. So we can see here we have some incoming excitatory inputs and that's going to lead to a summated depolarization of the membrane potential or a hyperpolarization of the membrane potential. So in addition to occurring at all membranes, this type of integration of membrane potentials is essentially a passive type of integration. It doesn't require any active activity within the neuron. So this is just a passive addition and subtraction of all these membrane potentials at every period of time. So what happens then when there is more excitatory or less inhibitory synaptic inputs? So we can see here, this would be a neuron if we put an electrode into this neuron, it would be summating. We can see that there would be summation of all of these electrical EPSPs and IPSPs as a function of time. But then if we had more excitatory inputs and fewer inhibitory inputs, then the summated change in the membrane potential would become depolarized. It would become less negative or more positive. So it would go from the resting membrane potential about -67 millivolts here all the way up to -55 millivolts. And this membrane potential is very important and it is what we call threshold. At -55 millivolts, this is the threshold for the neuron to be able to trigger an action potential. But the only place that this threshold is important is at this region of the axon hillock, which is called the initial segment or the trigger zone. So even though the membrane potential will be summating at every membrane, throughout the neuron, the place where this summation is critical is only at this region here because this is the place where the action potential will be generated. So again, summation is occurring everywhere, all the time at every membrane, but the only place where it's important is here at the initial segment of the axon hillock. So this is quite a large region here, and this is the place where the action potential will be initially generated. When the summed membrane potentials of all the incoming synaptic responses, all the incoming EPSPs, and IPSPs, when that summed change in the membrane potential gets above -55 millivolts at the initial segment, the axon will generate an action potential. And then, because there are the similar mechanisms for regenerating the action potential along the length of the axon, the action potential will be conducted from the cell body all the way down the length of the axon to the pre-synaptic region. So in the next lecture, we'll be talking about the mechanisms that underlie the action potential. But before we go to that, I wanted to talk about the features of the action potential which are true for all action potentials. The action potential is a transient. That means that it's very short-lasting change in the membrane potential and it always has a characteristic large depolarization that is followed by a repolarization of the membrane potential back to the resting membrane potential, and then beyond it becomes hyper-polarized beyond the resting membrane potential and then eventually we'll come back to the resting membrane potential. We say that it is all or none because once threshold has been reached within the cell body at the initial segment, as soon as threshold is met, so when the membrane potential becomes -55 millivolts, it will initiate the action potential. So that's the only requirement for the generation of the action potential, is that threshold is reached at the initial segment. Once it's reached, it will then trigger this all-or-none response, this depolarization, and repolarization. The time course of the action potential is usually very short, and it's on the order of about 3-4 milliseconds. So this initial depolarization usually takes about a millisecond or so, and then the repolarization takes a couple of milliseconds. So this is an extremely short-lived response. We talk about the action potential as being a type of active response because once it has been produced in the initial segment, it will be regenerated. And the action potential is regenerated along the length of the axon and it will have exactly the same characteristics as it travels in space and time from the cell body down to the pre-synaptic region. Action potentials only occur in what we call excitable cells. And those are neurons and muscle cells. Other cells in the body are not able to produce an action potential because they don't express the specific type of ion channels that are required for the production of this depolarization and repolarization. And these are voltage-gated sodium and potassium channels and only neurons and muscle cells express these types of channels. Now, when will be dissecting the action potential, we're just going to show a single action potential. But it's important to remember that action potentials rarely only occur as a single action potential, but they usually occur in sets or what we call trains or series. And again, even under basal non-stimulating conditions, most neurons are what we call spiking and they spike at a low frequency. So even under resting conditions neurons will produce an action potential at a low frequency. And what will happen is when the input to a neuron through those synapses is increased, that's going to lead to an increase in the rate of the action potentials so the frequency of action potentials will increase. And because an action potential, once it's generated, has this very characteristic of time course and amplitude then the information that is encoded in the action potential is not encoded in a single action potential itself, but it's encoded in the frequency and the pattern or the total number of action potentials. And we'll come back and we'll talk about the neural code because this is an important part of the integration of information. First the encoding of information, and then the decoding of information for how neurons communicate. For any given neuron, the action potential has a very characteristic shape so neurons can vary slightly for the magnitude or the amplitude, the peak amplitude of an action potential. So it can vary between about +20 and about +40 or so millivolts. And the time course of an action potential can be slightly different between neurons, but a given neuron will usually have a very characteristic shape of its action potential. And also, we'll have a similar type of speed of propagation of the action potential along the axon. [NOISE] >> Also, back in the 20th century, the mechanisms underlying the action potential were actively investigated by a number of laboratories. And the two investigators who discovered the ionic basis of the action potential were Dr. Alan Hodgkin and Andrew Huxley, who worked in England back in the 1930s and 1940s, and '50s. And they received the Nobel Prize for their discoveries back in 1963. These were the initial recordings of the action potential by Hodgkin and Huxley. And they used a preparation, which is called the squid giant axon. And this is a cartoon version of a squid, and you can see that it has this very large axon, which is one of its main mechanisms for communication. Now the reason that this preparation was so useful was not only were they able to get a lot of squid to be able to use in their experiments, but the squid axon is very large. And back in the 1930s and '40s, the types of electrodes that they used were much less sophisticated than they are today, and they were very large. But what was important was that they were able to insert an electrode into the squid giant axon, and were able to measure the changes in the membrane potential when they use different types of electrical manipulations. So here's the preparation that they used. This is the squid giant axon, and you can see that they were able to isolate the axon and insert an electrode inside the axon, and then they had the ground electrode, which is the reference electrode which would be out in the bath. And when they first measured the change in membrane potential, they saw this large depolarization up to about positive 40 millivolts, followed by this repolarization, and even a hyperpolarization beyond the resting membrane potential, which eventually came back to the resting membrane potential. And what Hodgkin and Huxley were able to do was to determine what currents were underlying the action potential. So using their electrode system, they were able to depolarize the membrane potential artificially, and then they could determine what the currents were that were crossing the membrane. And what they saw was that there was an initial inward current that was followed by an outward current. And we now know that this inward movement of positive ions is the influx of sodium across the membrane, which eventually stops, and then this is followed by an outflux of positive ions, which are potassium ions that are going to flow from the inside to the outside of the membrane. So by identifying these currents then, they were then able to next identify what the molecular basis of these currents were, and they discovered that sodium channels were activated first, followed by the activation of potassium channels. So we now know that the action potential has these specific phases and that the mechanisms for the depolarization and repolarization involves specific currents. So here's the resting membrane potential, and we know that the neuron receives lots of incoming stimuli, and if there are more excitatory inputs and fewer inhibitory inputs, then the summed membrane potential at the initial segment will cross threshold. And if that summed potential becomes more positive than -55 millivolts, then that will trigger the action potential. And the action potential involves this depolarization of the membrane potential. And we talk about this being the rising phase of the membrane potential, and during this phase, there is an inward current, an inward influx of positive ions, which is an inward current. This is then followed by the falling phase, which is a repolarization of the membrane potential, so it goes from +40 or so, it's going to repolarize back to the resting membrane potential and this involves an outward flow of positive ions. So this is an outward current. Then this is followed by a brief period in which there's actually a hyperpolarization of the membrane potential beyond the resting membrane potential. And this is called the undershoot phase. This hyperpolarization also involves additional outward positive ions moving out of the cell, and this is an additional outward current. Eventually this current will stop and the membrane potential will go back to the resting membrane potential, because all of the active transporters, the sodium potassium ATPAs and all those other active transporters will then return the membrane potential back to the resting membrane potential, which also involves the leak channels which will come into play. So each of these phases has got an underlying mechanism. And the underlying mechanism for the depolarization rising phase is the activation of sodium channels. And the next phase, the repolarization and hyperpolarization involves the activation of potassium channels. And in the next lecture, we'll talk about the specific channels and other mechanisms involved in the action potential generation, and then we'll talk about action potential conduction.