Are Neurons Just Electric Circuits?

thanks to blinkist for helping support this episode must animate more clones wait is this even gonna work [Music] this episode was made possible by generous supporters on patreon hey crazies let's have some fun and apply what we know about electrodynamics to the nervous system brains it's a field of study called electrophysiology which mainly covers nerves but is also concerned with things like muscles in the heart because you know that nervous system thing is all over the place electrophysiology shares some important history with electromagnetism and physics to the timeline prior to the late 18th century models for mechanisms underlying the brain and movement involve various types of fluids and spirits in ancient greece fluidy spirity stuff was referred to as pneuma but that word has since become a prefix for other words like pneumatic and pneumonia which are related to gases and pressure in 1646 rene descartes argued that the neuromuscular system was essentially a hydraulic system where animal spirits flowed from parts of the brain into the muscles but by the 1790s we started to figure out what was really going on in 1786 luigi galvani entered the field of physiology it's a me luigi no not that luigi unbelievable in 1786 galvani did a few experiments with lightning rods and frog legs they were dead frog legs there was no animal cruelty going on here okay anyway he hooked up a lightning rod to some dead frog legs and they twitched in response to electric shocks galvani called this behavior animal electricity when he finally published it in 1791 in an essay called commentary on the effect of electricity on muscular motion you know a typical boring science title but descriptive eventually galvani discovered the lightning rod wasn't actually necessary he could make the dead frog legs twitch simply by touching them with a combination of two metals called a bi-metal galvani used this fact to argue that animal electricity was actually intrinsic to the frog itself and as we'll see soon he wasn't completely wrong but a rival of galvanize named alessandro volta argued that volta the volta the volta we named the volt after yep that's the one volta suggested that animal electricity wasn't intrinsic to the frog but came from the bi-metal somehow and as it turns out volta was right the bi-metal is capable of producing its own electricity the two metals reacted with the electrochemical fluids in the dead frog leg forming a primitive voltaic cell huh a battery the bi-metal turned the frog legs into a battery really yes really volta literally discovered electrochemistry and invented the precursor to the battery just to prove galvani wrong it's crazy where an argument will take you anyway back to the timeline voltaic cells set the stage for experimental physicists like orsted ampere and faraday to develop electromagnetism in the 19th century so the next time you charge your phone don't forget about the frog galvani stopped his research after napoleon invaded italy in 1796 but the idea of reanimating life with electricity captivated early 19th century society it was the inspiration for mary shelley's frankenstein in 1818 so is this gonna work nope definitely not frankenstein is just a story this is not how clones are made okay then luigi galvani was probably stimulating the frog's sciatic nerve a nerve is a biological cable consisting of a bundle of fibers called axons um what's an axon the part of a typical neuron that carries electrical impulses when it comes to the brain the neuron is usually the star of the show your brain contains about a hundred billion of them isn't that also the number of stars in the milky way galaxy yeah it's the right order of magnitude but why are you asking didn't you say it was the star of the show yes as a metaphor this video is about brain stuff so what's a neuron a simple model of a neuron has three parts the main cell body dendrites which receive signals and axons which send signals and can sometimes span an entire limb one of the things that makes neurons special is their ability to generate and transmit electrical pulses over relatively long distances and they do that reliably and fast enough to respond to the environment so galvani was on the right track kind of but we've come a long way in our understanding of electrodynamics since then back to the timeline we'd have to wait until the 20th century before scientists really pinned down the biophysics of this electrical axon thing in 1906 ramon ika hall received the nobel prize for his pioneering work on the general structure and function of the neuron but it wasn't until 1952 that alan hodgkin and andrew huxley teamed up to discover the biophysical mechanism for electrical pulses and neurons hodgkin and huxley were both biophysicists so they decided to tackle the problem and they did their experiments on the squid giant axon a giant squid axon like 20 000 leagues under the sea kind of giant squid no no no not a giant squid axon a squid giant axon the axon was giant not the squid that made it more accessible and easier to work with i mean who wants to work with a tiny human neuron am i right anyway if you look up the hodgkin-huxley model you'll probably see differential equations that look like this of course equations don't exude intuition so let's try to make a visual instead think of a neuron like a leaky bag of charged liquid it's mostly water but it has ions like potassium sodium and chlorine dissolved in it a neuron like all cells has a two layered cell membrane when it's not doing anything the neuron maintains a potential difference a voltage across the membrane how does it do that ion pumps ion pumps run on chemical energy known in the bioworld as atp this is a major reason our brains consume 20 of the energy we eat outside the cell there's a higher concentration of positive ions like sodium inside the cell there's a higher concentration of positive ions like potassium negative ions like chlorine are spread out all over the place the ion pumps are specialized protein structures embedded in the cell membrane that pumps specific ions against the concentration gradient and against the electric potential if necessary find the gradient i know this is all sounding very biological but it's just electrodynamics it's just circuit stuff ion pumps use energy to move electric charges around that's what batteries and other voltage sources do in the hodgkin-huxley model ion pumps are like batteries remember how the cell membrane has two layers well that's just a capacitor the ion pumps build up electric charge on either side creating a voltage in the hodgkin-huxley model the cell membrane is a capacitor ion pumps are not the only devices that span the cell membrane though there are also ion channels when these channels are open ions can and will try to balance their concentration ions are just electric charges and when charges flow we call that an electric current it's positive charge too so the idea of conventional current isn't even a lie this time but what really matters here is what the ion channels represent in the circuit the ions don't flow through the channels all the time and they flow at different rates at different times that's a lot like a variable resistor one controlled by some kind of timer in the hodgkin-huxley model ion channels are variable resistors they control the electric current by ohm's law just like any other resistor each ion channel is selective though it'll only allow a specific type of ion to pass through sodium channels only left through sodium ions potassium channels only let through potassium ions so how are they variable because they're voltage-gated there are actually several different ways an ion channel could be gated chemically gated channels on cells that neurons talk to mechanically gated channels that are sensitive to pressure or stretch and even light gated channels in the eye but the ones in the axon are voltage-gated how open they are depends on the voltage across the cell membrane and that voltage can change with time in fact it will change with time as long as that neuron is trying to talk to other cells these voltage-gated ion channels are vital for getting a signal to the end of the axon the signal kind of rolls its way down the chain hodgkin and huxley modeled this process as a simple electric circuit the cell membrane is a capacitor the ion pumps are rechargeable batteries and the ion channels are variable resistors let's take a look at the voltage across the cell membrane aka the capacitor as the nerve tries to send a signal there are four stages resting depolarization repolarization and hyperpolarization jargon alert jargon alert i've been driving by awards this entire video now is the time you get on my case okay okay i suppose technically the membrane is polarized as long as there's a voltage across it which as you can see is true almost the entire time it blows right past zero twice but we have to tell these stages apart so it helps to think of polarization as just the resting state the membrane is stable depolarization is when the voltage rises towards zero but overshoots repolarization is when the voltage rebounds and drops toward the resting state hyperpolarization is when that overshoots don't get too hung up on the names wasn't this supposed to be about circuits oh yeah yeah right we were talking about the hodgkin-huxley model but i think this will make the most sense if we look at everything together let's sync up all three visuals first the voltage-gated sodium channels are triggered to open allowing a rapid flow of sodium ions into the cell the voltage across the membrane goes from negative 70 millivolts up to around positive 30 millivolts but remember they're on a timer so after the initial spike the sodium channels close when those close the potassium channels open rapidly sending potassium ions out of the cell the voltage across the membrane drops back down toward the resting state finally after an overshoot the potassium channels close and the membrane settles back into the resting state it does this by leaking a little and if necessary by turning on the ion pumps in short the channels balance the ion concentration and the pumps unbalance them again the hodgkin-huxley circuit successfully models this entire mechanism but only for a little patch of the cell membrane this patch only has a few things one sodium potassium pump one potassium channel one sodium channel and whatever other miscellaneous devices are around the signal process starts locally usually around where the cell body and the axon meet it then spreads as a chain reaction from one patch of the membrane to the next local regions depolarize their neighbors sending a pulse down the axon and this happens fast fast fast the entire spiking process starts and finishes in about a thousandth of a second but how fast does the signal take to get down the axon good question it actually depends it varies a lot depending on the type of neuron and the animal species but at top speed we're looking at about 224 miles per hour or hundred meters per second that's the length of an entire football field including the end zones in a single second compared to how fast signals travel through wires though this is actually pretty slow oh no does that give robots the advantage anyway so it took us a while to figure out neurons the real work began with luigi galvani and dead frog legs but culminated with alan hodgkin and andrew huxley in their circuit model a neuron is like a simple electric circuit or at least a chain of simple circuits it's made of capacitors batteries and resistors that obey ohm's law components that create a pulsing voltage all just to tell other cells what to do let's just hang here for a bit and take it all in so got any questions about this analogy please ask in the comments thanks for liking and sharing this video don't forget to subscribe if you'd like to keep up with 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Channel: The Science Asylum

Views: 376,124

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Id: YQsSrW5oX80

Length: 16min 13sec (973 seconds)

Published: Sat Aug 15 2020