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|Date Signed Up:||4/04/2011|
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latest user's comments
|#116 - I really think there are too many pointless jobs as it is and … [+] (1 new reply)||07/20/2015 on Baernie is bae||0|
|#18 - thank you, I love learning ****||07/11/2015 on Random Medical Facts, #4||+1|
|#6 - wouldn't those myelin sheaths actually cause impulses to trave… [+] (5 new replies)||07/10/2015 on Random Medical Facts, #4||+4|
#53 - nanahara (07/11/2015) [-]
tl;dr The myelin sheet works as an insulator around the nerve. it decreases the capacitance and increases the resistance.
The Impulse doesn't actually jump outside the axon on top of the sheets. but this myelin keeps the sodium inside not allowing it to leak out, and propagates inside the axon, and the impulse is "refreshed" at every node of Ranvier.
so.. in an unmyelinated nerves the action potential (the electrical impulse) is propagated in a way where there are channels which open once the cells sense a high enough voltage (threshold). so as the electrical impulse progresses through the axon, more and more sodium channels will open, thus maintaining this "high" voltage, and like this, it will propagate further and open more channels along the way.
You can imagine with this kind of conduction the concentration of sodium outside the cell must be sufficiently high, so that it can enter the cell once the channels open. if the outside concentration isn't high enough, the propagation of the impulse wont continue, and the nerve wont pass on the impulse to the next nerve.
so if a lot of channels are opening up right next to each other it will cause a decrease in sodium concentration. this means that this kind of conduction is decremental. it will get weaker and weaker the longer it has to travel (which is not good). the impulse moves like a wave. In addition to that it is more energy requiring and way slower (2 m/s) than a myelinated nerve (200 m/s).
The myelin sheet works as an insulator around the nerve. it decreases the capacitance and increases the resistance.
The Impulse doesn't actually jump outside the axon on top of the sheets. but this myelin keeps the sodium inside not allowing it to leak out, and propagates inside the axon. The impulse is "refreshed" at every node of Ranvier. thus the impulse is much faster (200 m/s) and not decremental.
so you can imagine how much energy is spared in myelinated nerves, as they only use about 0.2% of the channels compared to unmyelinated.
Also, these nodes are far enough apart for the sodium concentration to be unaffected by nearby cells channels, or the same cells channels. Thus the concentration of sodium at each node is high. as well as the density of these channels being higher at each node, compared to an unmyelinated nerve.
it is a bit more complicated than this, but im not gonna bother getting into it, unless you care. but other ions than sodium play a part too, and the myelin sheets have some function when it comes to regeneration of damaged nerves and maintenance of the nerve cell itself.
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