 Hi, and welcome back to video 2.3 in the course Biological Psychology, in which we're going to talk about synapses and the action potential. Synapses are connections between neurons. So what you see here, that's a schematic of a synapse. As I said, a synapse is a connection between two neurons, and generally it's a connection between an axon that sends a signal, so in this schematic the axon would be depicted on the top, and a dendrite that receives a signal. As we've seen in a previous video, an axon is the output channel of a neuron, and a dendrite is the input channel of a neuron. Communication between neurons is chemical. The human brain uses a combination of both electrical and chemical communication, but at the level of synapses the communication is purely chemical, and it happens through neurotransmitters, and that's what you see here in this schematic. So the neurotransmitters are these little balls that are released into the synapse, into the synaptic cleft, as it is called. So you see that the synapse between an axon and a dendrite is a connection, but they don't really touch each other. There's a little gap in between, and that's called the synaptic cleft, and the axon releases these neurotransmitters into the synaptic cleft, and they attach to the receptors of the dendrite, of the receiving neurons. So these neurotransmitters basically carry the information that the axon sends to the dendrite. So it's a chemical form of information transfer. And very simply put, a neurotransmitter can either increase the membrane potential of the neuron that it receives. In a previous video we talked about the membrane potential, which is generally about minus 70 millivolts. Now if the membrane potential increases, so if for example this dendrite here would normally have a membrane potential of minus 70 millivolts, if it increases it could become either, for example, minus 80 millivolts, and that would make this neuron less likely to become active, as we will see in a bit. So that's why that would be inhibition. Or conversely, neurotransmitters could decrease the membrane potential of this dendrite, so the membrane potential could go for example from minus 70 to minus 60, and that would increase the probability that this neuron, the receiving neuron, will become active. So that's basically what a neurotransmitter does. In reality there are many complicated interactions, and there are also many many different kinds of neurotransmitters, and different kinds of biological molecules that act in one form or another as neurotransmitters. And those interactions are the basis of pharmacological drugs. Now we're not going to talk about that because it is a very complicated subject that I frankly don't know too much about. If you're interested there is a lot of fascinating literature about neurotransmitters and psychopharmacological drugs. For now I think for this course the basic idea is just to understand that communication between neurons is chemical and is mediated by neurotransmitters that go from an axon to a dendrite, and that act to either increase or decrease the membrane potential of the receiving neuron. Now let's now take a look at the action potential, and the action potential is what happens when a neuron becomes active. So the general idea of an action potential is very simple. If neurons A and B are connected, so if there is a synapse between neurons A and B, and neuron A fires, and neuron B is excited strongly enough, so what does that mean? Well it means that neuron A releases neurotransmitters onto a dendrite of neuron B, and those neurotransmitters would reduce the membrane potential of neuron B. And if that membrane potential is reduced sufficiently, that's what excited strongly enough means, then neuron B fires as well. And this is what basically gives rise to a cascade of neural activity in the brain. So neuron A fires onto neuron B, if neuron B is excited strongly enough neuron B will also fire, neuron B would again be connected to other neurons, if those neurons are stimulated strongly enough by neuron B they would also fire, and so forth, and so forth, and in that way neural signals propagate from neuron to neuron through our brains. But now let's take a look at how this works in a little bit more detail, right? So what is the action potential? Because what I've described now probably sounds a little bit abstract. So what do we have here? Here we have a figure, a hypothetical figure, with on the y-axis a membrane potential, and on the x-axis time in milliseconds. So we're going to take a look at how membrane potential can vary over time. Then we have indicated a few lines in the figure, so minus 70 would be the resting membrane potential. Minus 55 would be the activation threshold, so it would be a particular membrane potential that if reached, as we will see, will cause the neuron to become active. Then zero means that there is actually no membrane potential, right? So there's no voltage difference between the inside and the outside of the cell. And plus 30 will be, will is the maximum positive membrane potential. So it's the upper limit of how positive a membrane potential can be when a neuron becomes active. Now let's take a look at what actually can happen. So say that here what we are showing is the membrane potential of neuron B over time after it has been excited, so it has received excitatory input by neuron A. Then you see that when this excitatory input was received, the membrane potential of neuron B reduces a little bit. But you see that it doesn't reduce so much that it reaches this activation threshold. We call this sub-threshold activation because there is some activation, but it is not sufficiently strong for neuron B to become active. And then the membrane potential goes back to what it was before. So this is not an action potential. Here what happens here is that neuron A is fired onto neuron B, and it has not succeeded in causing neuron B to fire as well. Now let's compare this with the following situation. Here neuron B again has received input from neuron A. And this input caused the membrane potential to become less negative until at some point the magical threshold of minus 55 millivolts is reached. At this moment NA plus channels open. Now what does that mean? It simply means that if we take a look at this picture again here, here we have the membrane in the middle, and this membrane has NA plus channels, so there are little channels that allow NA plus to flow through them. And they are normally closed, but they open up if the membrane potential reaches minus 55 millivolts. Now what happens then is that a lot of NA plus starts to flow into the cell. NA plus is positively charged, so this will cause the membrane potential to go up to become more positive. The inside of the cell will become more positive compared to the outside of the cell. This will continue until at some point a positive membrane potential of plus 30 millivolts is reached in this case. And at that point these NA plus channels snap shot, so NA plus cannot flow into the cell anymore, and K plus channels open. So there are channels that allow K plus to flow through. They open up, and then K plus starts to flow out of the cell. And because K plus is positively charged and flows out of the cell, the membrane potential will become more negative again, until actually the membrane potential becomes even more negative than it was to begin with, and then the K plus channels snap shut. Now at that point, the membrane potential has almost gone back to what it was before, right? But of course, the balance of ions inside and outside of the cell has been disturbed quite a bit, right? Because a lot of NA plus has flowed into the cell, and a lot of K plus has flowed out of the cell. And also the membrane potential is not exactly what it was before. So at that moment, the sodium potassium pumps, so these transporter pumps that actively consume energy, start to pump NA plus out of the cell and K plus into the cell until gradually the original balance is restored again. So what you see here, this cascade of activity, this enormous bump of change of membrane potential, that's an action potential. If people talk about a neuron that spikes or a neuron that becomes active, this is what they mean. After an action potential, there is a little period during which a neuron cannot have another action potential. It's called a refractory period. And that gradually dies off until the neuron at some point becomes quite likely again to have an action potential, right? So the neuron, after it has fired once, it doesn't fire rapidly again. And all of this happens in a time scale of milliseconds. So here you see I've indicated the time scale of 5 milliseconds. So it's very, very fast. But it's not so fast that we cannot measure it. We're talking about milliseconds. An action potential generally starts at the dendrite, because that's the input channel of a neuron. So what happens, essentially, is that one particular spot on the membrane, on the dendrite, the particular membrane part of the membrane on the dendrite has an action potential. It's something that happens not on the entire neuron at once, but it happens at one specific location. And then the action potential kind of spreads along the neuron. Because you can imagine that as soon as the neuron has this enormous change in membrane potential, then that part of the membrane will ignite parts of the membrane that are next to it, right? Because they also suddenly reach that activation threshold of the membrane potential. So in other words, it's like the neuron ignites itself. It causes other parts of the neuron to fire until the action potential spreads, propagates throughout the entire neuron. And that's what you see here in this picture, right? So here on the left side, the dendrites, where the membrane potential has been triggered, and then the membrane potential spreads across the entire axon, up, until at some point it will cause a member, cause goes all the way to the end of the axon and the synapses of this neuron with another neuron. Now, this form of action potential conduction is called continuous conduction, right? Because it basically relies on a continuous opening and closing of NA plus and K plus channels along the neuron, especially along the axon of the neuron. For invertebrate animals, so animals that don't have a spinal cord, this is the only form in which action potentials propagate. But humans and other vertebrate animals have another way in which action potentials can propagate. And that relies on myelin sheets. Many neurons have myelinated axons. And here this is depicted as these little cylinders that encircle the axon. And what is basically what these myelinated things do, is they prevent NA plus and K plus channels from opening. So, basically there cannot be this kind of continuous conduction, this continuous propagation of an action potential, because it is blocked by these myelin sheets. What does happen is that a purely electrical signal jumps from before, say this point before the neuron, before the myelin sheet to this little node after the myelin sheet, which is called a node of Ranvier in there. An action potential is triggered again. And it jumps to the next node of Ranvier and an action potential is triggered again. So in other words, you can think of these nodes of Ranvier as kind of amplification stations. So an action potential spreads purely electrically, but at some point the electrical signal would become too weak. So the action potential would be too weak to in a purely electrical way go from the beginning of the action, for example, entirely to the end. So what we instead do, how the axon instead works, is that there are some electrical transmission and then at the node of Ranvier, a new action potential is triggered, the action potential is refreshed in a way and then it can jump again, jump again, jump again, jump again. And in that way, basically, the action potential never dies out, right? So there's a mix of electrical and chemical transmission. This is called cell-tutorial conduction from the Latin verb saltare, which means to jump. And it is much faster than continuous conduction because essentially electrical conduction is just much faster than this chemical conduction, right? So it allows us, it allows neurons to transmit information with far greater speeds than just then invertebrate animals do with this kind of continuous conduction. Okay, with that, let's move on to the next video, video 2.4, in which we're going to take a look at the terms that we use to describe where in the brain things are.