And so how can Huxley, Understand the mechanism of action potential? In fact, there are at least technical breakthroughs and imaginative usage of various special samples, and there are insights in understanding the circuit issue. So they were using the squid giant axon. This axon is a huge cell that allow them to manipulate and record the voltage and current across the squid membrane, it's a nerve cell. And this illustrates their setup, okay? So they were using the so-called voltage clamp invented by Casey Clamp. So voltage clamp was not invented by Hodgkin-Huxley, but by an American scientist. So what I do here is that you can put the electrode into the squid, okay? And what it actually to measure the current, and one is actual is to deliver, sorry, one is actually to measure voltage inside of the cell, and one is actually is to deliver the current. And what you can do is you can put external command voltage into this amplifier. So this electrical circuit, whenever you have a command voltage and if there is a difference between the voltage inside of the cell and this command voltage, then this amplifier is going to generate current into this axon to make sure that this voltage here will follow the command voltage. So this is so called voltage clamp, that is, you can artificially through feedback system to inject the currents to keep the cell's membrane potentially constant. That's the reason it's called voltage clamp, okay? So in this experiment, they can control the voltage across the membrane. And therefore, they can study how the different ion channel or the conductance, is regulated as a function of voltage difference, as a function of time. So this is one of their experiments in 1950s, that reported in 1952, okay? So here is one of the experiment. And then I hoped you guys can travel with me, time travels back into the early 1950s to understand how those things are working, okay? So on the top is the voltage clamp, that is, they used their circuitry to control voltage across the squid axon membrane. So at the beginning, the intracellular membrane potential is -50 millivolt, okay? And then through voltage clamp, they can control a membrane potential jumping to plus 15 millivolt, okay? So what will happen in the membrane? You have a voltage step. This initially minus, so this is a positive membrane potential that you apply. Initially, it is -50 millivolt and then go to plus 15 millivolt, for example. And this is a cell or a squid membrane. If the squid membrane does not have any active component, the active component will be those specific battery or specific ion channel. What will be the response of the current? So let me just rephrase the discussion. We already have a piece of membrane, a biological membrane, and then we can artificially apply a membrane potential that is following initially -50 millivolt. Put it a little bit longer. And then jump into plus 15. What will be the current? Or what will be the waveform of the current looks like? If you have the electrode like Hodgkin-Huxley does, you have one measure of voltage and the other one to deliver the current, okay?
