Fundamentals of Neuroscience by Prof. David Cox - 3

Hey guys, I'm back with the third session of this series. Coming to lesson 3, we gain an insight into The Action Potential. As we've discussed on the first part, neurons communicate with each other by sending out electrical signals, these changes in the membrane potential have to be rapid, as we don't generally spend more than a minute just to pick up a pen, right? When ions channels are opened, Na+ would move into the cell to bring the membrane potential closer to its Nernst potential (which is relatively high positive) while K+ would move out of the cell to bring the membrane potential closer to its value (which is a little lower than the resting potential). So, if there is nothing more, the membrane potential would sit at its resting value and there cannot be any electrical signals in neurons. That's where voltage-gated channels come in, there are two kinds of voltage-gated channels: Na+ and K+ ones. These channels are especially sensitive to the changes in membrane potential, which mean they would have different conductances depending on the membrane potential. Sodium voltage-gated channels would open when the membrane potential is more positive, thus Na+ would rush through those channels and bringing the membrane potential even more and more positive. But doesn't it result in a loop? After this process, the membrane potential would have a very positive value, so will it be disadvantageous to us? Thankfully, Sodium voltage-gated channels have a special mechanism: at the end of its pore, there is a ball, when the membrane potential reaches a certain value, this ball will close the end of the channels, and therefore, even though the channels are open Na+ cannot go through, we call this these channels are in inactivation stage. The membrane potential would be reset by the potassium voltage-gated channels, when the membrane potential is more positive, they would be open and allow for the passing through of K+. But here, note that this is all dependent on probability, in other words, although voltage-gated Na+ channels are open when the membrane potential is more positive, it can also do that in very negative value (just less likely).Both these channels are open when the membrane potential is more positive, why don't they just cancel each other out? This is thanks to the channel kinetics, the inactivation of the sodium voltage-gated channels and the opening of Potassium voltage-gated channels have a lower speed than the opening of Sodium voltage-gated channels. Channels kinetics also contribute to what known as refractory period: absolute when the Sodium voltage-gated channels are inactivated (cannot stimulate another action potential right away) and relative when the both the Sodium and Potassium voltage-gated channels are open (hard to stimulate another action potential).

Okay, it's time for the content of the field trip. This field trips takes us an aquarium, and the professors and the caretaker of this aquarium shares with us some interesting information about neurotoxin (chemicals that affect the activity of neurons), in this case tetrodotoxin or TTX. This toxin is found especially on puffer fish, and it binds the Sodium voltage-gated channels, which is why we would feel a sense of numbness after eating this fish. Next is the trip to learn about the electroreceptors on some fish.

Below are some illustrations for you to better understand the content of my writing ^ ^.

Here's the shape of a Sodium voltage-gated channel at inactivation stage (you can clearly see the ball at the end of it)

This is the shape of an action potential with corresponding stages.

Here is the stages of Sodium and Potassium voltage-gated channels during each phase.

Thank you for reading my posts ^X^, hope you are all healthy and happy. Have a nice dayyyy!!!