Fundamentals of Neuroscience by Prof. David Cox

So, it's the last part of sharing, huh? The content of this post would be amazing, let's go and see it! Hmmm, a wire transmits electricity on the principle of the high conductivity of copper. However, neurons' cytoplasm is a great deal less conductive than copper, so it would be hard for a neuron to transmit signal by this approach. So, we are left with two options: to increase the amplitude of the current or to repeatedly regenerate it. This first option seems to be more easy work, but it may cause the breakdown of neuron's membrane. That's why nature chooses the latter, instead of moving current down an axon, neurons move a state of depolarisation down an axon. Similar to wave, whose propagation of up-and-down movements results in the movements of wave, in neurons, an action potential in one patch of membrane depolarises the next region, which in turn fires an action potential, and this process continues like a chain reaction (domino effect). The shape of voltage vs distance would be similar to standard action potential shape but flipped left to right due to the wave effect (each part of the membrane will continually experience an action potential). To easier understand, HarvardX allows us to do interactive in their lessons, which is a great help in understanding these abstract concept. Okay, so let's ignore it for now and take a look at some scenarios. First, let's imagine stimulating an action potential in the middle of an axon, what would happen? Of course this will be propagated at both ends of the neurons, since there is no filter of direction in a neuron. So how about we stimulate an action potential at each end of the neuron? They would then run into each other and stop, this is due to the inactivation stage of the voltage-gated Sodium channels (or absolute refractory period). Below is the illustration for you to easily understand this. This phenomenon is exploited by researchers to validate where a neurons projects its axon. To speed up an action potential propagation, we can either use a giant axon (which allows for high speed transmission of an action potential) but it would consume a large amount of space, or using myelinated axons. This is where things start to get interesting. Glia cells (the supporting cells of neurons) are responsible for the creation of myelin sheath wrapping around an axon. When an axon is wrapped up by myelin, the membrane's thickness would increase and therefore its capacitance would decrease -> speed up propagation speed. But, doesn't wrapping myelin around membrane make it more resistive to electricity, why is that advantageous to us? Well, when we wrap myelin around an axon, there would be less ion channels (these channels are wrapped up) so current could not leak through these channels, and therefore current would be able to travel longer distance. Oh, so wrapping myelin all along an axon's length would be the best idea, right? Well, on the contrary. Although current can travel longer distance, it would be lost anyway. That's where the nodes of Ranvier come in (the area between two parts of an axon that are wrapped by myelin). The node of Ranvier allows the current to be reboosted, since there are ion channels in this area, and therefore ensure high speed propagation of action potential. So if these myelin sheath are damaged, the current would leak through the membrane and therefore be lost before reaching a node Ranvier. This would cause sufferings to patients of demyelinating diseases (these patients often have paralysis as a result). Sad, not? Scientists are working days and nights to eradicate these diseases, let's help them, why not?

The field trip takes us to see sheep brain dissection, which is so fascinating. Hmmm, I will not go into this field trip now. Enrol in the course to see it for yourself ^ ^.

So, that's the end of my sharing, take care of your health and have a good day ;). Thank you for supporting me.

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