ADDENDUM BELOW: Please see the addendum at the end of the post - it seems that KA has redone their video, correcting the mistake I pointed out.
In recently learning about Khan Academy and the debate over their place in the world of education and whatnot, I stumbled across this quote from Salman Khan's 2011 interview with Charlie Rose:
But the main thing I do... whether it's history or chemistry or anything is get the scaffold and then really immerse myself for however long it takes and then make sure that I can make intuitive connections between everything that happens. [...] Understand why something - if I'm doing it on the neuron [...] a biology book will tell you the signal goes across because there's a myelin sheath. I'm like yeah but how does putting a little tissue around the neuron make the signal go faster, right? And no biology book will tell you that answer. And I'd kinda ponder it a bit and then I was like it's kinda like a fiber optic system and one's kind of amplifying the signal but if you amplify too - and then I'd call up some buddies who are either biologists or communications engineers or whatever and I was like "Does this make sense?" and they're like "No, I think you're right." And so it's a big deal for me if I'm doing biology - why does the myelin sheath make the signal go faster?
As a fledgling neuroscientist, this struck me as a somewhat weird way to think about the purpose of myelination, so I went to check out the video in which he attempts to teach this concept. Here he is discussing what myelination does (skip to 8:35ish if you don't want to watch the whole thing):
Now - the axons have these huge spaces where they're covered up. They're covered up by these Schwann cells or by these myelin sheaths. So even if there are gates underneath them they're useless. They can't interface with the outside sodiums. They can't interface with the outside sodiums and potassiums. So over this - over this distance of the axon you cannot have a straight-up action potential.
But what happened is your action potential it made everything a lot more positive than it would have been had there not been that opening of the gate and now you can have an electrotonic effect again. So you kind of boosted your signal so now this is going to - So now this might get to +40 maybe over here we only get to -50 but -50 once again is enough to trigger - to trigger a sodium gate which then makes everything really positive again. It - it boosts the - it boosts the signal.
He seems to think that the purpose of the myelin sheath is to smother voltage-gated sodium channels on the axonal membrane. By preventing access of these channels to the extracellular fluid [this is his speculation, I don't think that's really 100% true] the myelin ensures that the signal can only propagate electrotonically in that region of the axon.
But what happens if we ask a student (or ask yourself if you don't know how myelin works and just learned from his video) the following question:
Suppose that voltage-gated sodium and potassium channels involved in action potential generation are clustered predominantly at the axon hillock and nodes of Ranvier, and are largely absent from the neuronal membrane wrapped in myelin [this happens to be true]. Now suppose we remove the myelin sheath, but keep those channels in the same locations. Would anything be different about how the action potential moves down the axon?
This illustrates one problem with Khan Academy: when you have just one guy learning about a lot of subjects and then trying to teach them, he's going to get some things wrong. And because his videos just gloss over the surface facts without explaining the concepts behind everything (to be expected because you can't teach everything in a short video), his students aren't going to know to ask the right questions to realize where he might have made a mistake.
Well one day after this blog went up, the video I initially referenced went private (you may have noticed) and a new video on saltatory conduction was added in its place:
This video corrects the earlier mistake! I don't know if it had anything to do with my little blog (this particular entry only had like 69 views...) or if others had pointed out the mistake earlier and they had the video ready to go and were simply prompted to get on it by this post, or if the timing was entirely coincidental. Still, credit to them for taking the time to redo the video and correcting the mistake!