If you are doing the same experiments in a neuron. You will find that actually it's similar. Again, the neuron is a polarized cell with unique features. A typical neuron usually has a long, thin axon to send the information out, which we discussed in a previous session. It also has the elaborate so called "dendrite" process to receive information. So a neuron is a polarized cell that receives signals, and then sends signals out. In actual recordings, we wish to record every single part of a neuron. We can theoretically visualize how the information propagates from one cell to the other. People are still making progress in that direction and we will cover it in the future, okay? But for electrophysiology, usually people will record from the soma or the cell body which is relatively big and fat, so you know it is easier to insert electrode without damaging, okay? And in this case, you can see that the cell has a resting memory potential pretty close to that of an ideal cell that we just discussed, okay? In fact, most of the electrical signals that we know are first studied, or derived from neurons, and then applied to other cells because neurons use the sophisticated machineries to regulate its own electrical potential. The membrane potential and the generation of action potential and the synaptic potential, okay? And Those potentials can also be modulated by activities or by experience. For example, when you remember something, some cells in your brain are permanently changed. Their ion channels get changed, their excitability gets changed, okay? So after attending this seminar, or this lecture, I hope I'm permanently changing some cells and their excitability in your brain, okay? Maybe that percentage is only 50% which I would be already very happy. So, how do the ions go in to the cells. In the initial part we just discussed that this passive channel is a hole, okay? or is a pore, that the ions will passively (and there is electrical, chemical potential) go in or go out. So that is the passive ion channel. There is no such active process. But, for a cell to initially set up the concentration, they do have active transport systems. So, the channels and transporters in the membrane are the main determinants of the membrane potential between a cell, and they allow the materials to exchange between the cell membrane. One can think of the mechanism of how those things move. So the energy is conserved. There are three possibilities. For the active transport the energy is against the electrical- chemical gradient, okay? And where does the energy come from? From the cell. For the energy conservation, they have to use some energy. So, it turns out the cell will use the hydrolysis of ATP to pump this up, so that would be the ATP driven pump. So the ATP hydrolysis will provide the chemical energy to move the ions. And this energy is not necessarily from ATP hydrolysis. It can be from light, okay? And then this will be a light driven pump. Recently,in neuroscience there's some hot topic called optogenetics, okay? And it turns out that people hijack some of the light-driven pump, mostly from Bacteria, weird bacterias, okay? And using that to move the ions, okay? And it turns out to that a lot of bacteria in the ocean don't have the photosynthesis system. Okay? They can directly harvesting light by using a light driven pump to move the ions across the membrane. And that can change the concentration of the ions. And then they can use another process to couple this chemical electrical gradient to generate ATP, okay. So indeed, this is changing the way people understanding how the light energy is fixed in earth, okay. And some smart scientist can hijack this bacteria's machinery, put it into the neurons, and then control those neurons' activity in a genetic specific manner. And then you use light to control in a temporal manner and then you can essentially alter, or perturb the nervous system to start neuronal cell function in specific cells. And therefore, this is an interesting example of interdiscipline research. Who knows that the bacterial studies can help the neuroscience, right? So that tells you that people with diversified background, if you have a way to integrate it in a constructive way, you can help people, right? And there is aother combination, which is you can use some chemical energy, use some other ions, electrical chemical energy as an energy source and couple it to the material that you are interested in moving. Okay, so this will be the coupled transporters. Okay? Again, nature uses all of those combinations, although in different cells, in different species to move things around they cannot break the thermodynamic principle right? The energy has to be coming from somewhere right? So what are those machineries? Okay? Where do they localize? What kind of genes that encode for those proteins. Actually there's been active research for the scientists to identify the machineries that mediate those process, okay? Still now, okay? And we will discuss in later sections. So, this is summarizing what we have just been describing about. In the resting condition, a neuron can be thought of the asymmetry distribution of these three ions. So the concentration we're putting here, the number is a little bit small. It's similar to the ideal cell we put. Again, there's relatively high potassium concentration inside and low outside. And sodium, there's a much higher concentration outside than inside. Okay? And chloride is higher outside than inside. So in a typical cell, it's like that. So, some cells might have 130 mV, but it's in a range. So for here, they are just close enough knowing around the millimolar range that would be good enough, and with this concentration dependent difference. Actually, this will be good for us to understanding the subsequent regulations of action potentials, okay? And the initial concentration difference of those ions actually is maybe determined by the transporters. The active transporters either using ATP, or the co transporters of potassium and chloride on the membrane. So those transporters will use the energy to alter the concentration of sodium and potassium. Okay, and to maintain roughly the asymmetry distribution of those ions across the cells. Okay, and again besides these three ions, there are additional ions in the nervous systems, across the membrane. And some of them are also very important, that are not listed here. What might be them? So here we described only three ions. Potassium, sodium, chloride for the simplicity. But in the nervous system, there are more ions and they also have pretty important physiological functions. What might be other ions? Calcium is very important. Calcium is very important for electrical purpose. For some cells, calcium can replace sodium to drive action potential. For heart cells, calcium plays a role in shaping the waveform of action potentials, okay? And calcium is unique in a way, that it not only plays a role in the electrical signals. But once it comes into the cell, it actually serves as a biochemical messenger to trigger downstream signals. For example, phosphorylation by activating kinase. Phosphatase, by activating calcium-dependent phosphatase. Gene transcription, by activating, subsequently, transcription factors. Okay, and there's even a much larger driving force for calcium, which is not listed here. Outside, the concentration of calcium is about 2 mM, okay, for the human cell. And inside, it's about 10 nanomolar to 100 nanomole, okay, rather than millimole. Okay, it's more than 1000 times lower than, more than 1000 actually, about a million times lower than potassium, okay? So 1,000 times it would be 100 micromolar, and then divide that by 1,000, that would be 120 nanomolar. That would be a million times lower than potassium, okay? And that plays a very important role. Again, we will discuss specifically during the synapse transmission section. Okay? And now, let's go back to our fundamental physics. I mentioned that we need to discuss Ohm's law. Ohm's law. Okay, so, the student in physics lab is preparing to leave. Okay. That would be good. So, why do we want to discuss the Ohm's law? Because as you will see, we can model a cell membrane by the combination of those simple components. That is the resistance and the capacitance and their combination. And that can quickly, accurately describe the electrical property of the membrane. So, if you're still, again remember your junior high school physics In a circuit, electrical circuit, if you only have a resistance, what is the relationship between the potential and the resistance? That is described by Ohm's Law, which states that the resistance is equal to the voltage, divide the current. Or put it another way, that is the current is going to be determined by the voltage dividing the resistance. This is Ohm's Law.