neurons are the cells that make up our nervous system and they're made up of three main parts the dendrite which are

little branches off the neuron that receives signals from other neurons the soma or cell body which has all the

neurons main organelles like the nucleus and the axon which is intermittently wrapped in fatty myelin those dendrites

receive signals from other neurons via neurotransmitters which when they bind to receptors on the dendrite act as a

chemical signal that binding opens ion channels that allow charged ions to flow in and out of the cell converting the

chemical signal into an electrical signal since a single neuron can have a ton of dendrites receiving input if the

combined effect of multiple dendrites changes the overall charge of the cell enough then it triggers an action

potential which is an electrical signal that races down the axon up to a hundred meters per second triggering the release

of neurotransmitter on the other end and further relaying the signal so neurons used neurotransmitters as a signal to

communicate with each other but they use action potentials to propagate that signal within the cell some of these

neurons can be very long especially ones that go from the spinal cord to the toes so the movement of this electrical

signal within the cell is super important but what does this all have an electrical charge in the first place

well it's based on different concentrations of ions on the inside versus the outside of the cell generally

speaking there are more na + or sodium ions Cl - or chloride ions and CA 2 + or calcium ions on the outside of the cell

and more k+ or potassium ion and a - which we just used for negatively charged anions on the inside of the cell

overall the distribution of these ions gives the cell a net negative charge of close to negative 65 millivolts relative

to the outside environment and this is the neurons resting membrane potential when a neurotransmitter binds to a

receptor on the dendrite a ligand gated ion channel opens up to allow certain ions to flow in depending on the channel

ligand-gated literally means that the gate responds to a ligand which in this case is a neurotransmitter so let's like

the example of a ligand gated sodium ion channel which when it opens lets sodium flow into the cell the extra positive

charge that flows in makes the cell less negative since remember it's usually negative 65 millivolts and therefore

less polar so that's why gaining positive charge is called depolarization neurotransmitters typically open various

ligand gated ion channels all at once so ions like sodium and calcium might flow in while other ions like potassium might

flow out which would actually mean some positive charge leaves the cell in the end though when it's all added up if

there's a net influx of positive charge then it's called an excitatory postsynaptic potential or epsp in

contrast the opening of only ligand gated chloride ion channels would cause a net influx of negative charge creating

an inhibitory postsynaptic potential or IPSP making the cell potential more negative or repolarizing it now a single

epsp or IPSP causes only a small change on the resting membrane potential but if there are enough eps bees across

multiple sites on the dendrites then collectively they can push the membrane potential to a specific threshold value

typically around negative 55 millivolts although this can vary depending on the tissue when it hits this threshold value

it triggers the opening of voltage-gated sodium channels at the start of the axon which is called the axon hillock these

voltage-gated channels open in response to a change in voltage and when these open sodium rushes into the cell the

influx of sodium ions in the resulting change in membrane potential causes nearby voltage-gated sodium channels to

open as well setting off this chain reaction that continues down the entire length of the

axon which ends up being our action potential and when this happens we say that the neuron has fired once a lot of

sodium has rushed across the neuronal membrane the cell actually becomes positively charged relative to the

external environment up to about positive 40 millivolts this depolarization process ends when the

sodium channel stops allowing sodium to flow into the cells which is a process called inactivation

but this inactivated states different from when the channels closed or open for that matter which are the two states

that most other channels have the voltage-gated sodium channel though is unique in that it has what's known as

the inactivation gate which blocks sodium influx shortly after depolarization and stays in this state

until the cell repolarizes and the channel enters the closed state again and the inactivation gate stops blocking

influx even though that inactivation gates not blocking anymore the channel still closed so no sodium enters a cell

in this state the middle open states therefore the only state where sodium gets let into

the cell through the channel and this is a very short window of time now in addition to these sodium voltage-gated

channels we've also got potassium voltage-gated channels which are slow to respond and don't open until the sodium

channels have already opened and become inactivated the result of this is that after that initial sodium rush into the

cell potassium flows out of the cell down its own electrochemical gradient removing some positive charge in

blunting the effect of the sodium depolarization these potassium channels don't have a separate inactivation gate

and therefore they stay open for slightly longer which means that there's a period of time when there's net

movement of positive ions out of the cell which causes the membrane potential to become more negative or repolarize

during this repolarization phase the cell also relies on the sodium potassium pump which is an active transporter that

moves three sodium ions out of the cell and two potassium ions into the cell it's during this repolarization phase

that the cells in its absolute refractory period since the sodium channels are inactivated and won't

respond to any amount of stimuli this absolute refractory period keeps the action potentials from happening too

close together in time and also keeps the action potential moving in one direction

the combined efforts of this pump and the extended opening that potassium channels results in a small period of

over correction where the neuron becomes hyper polarized relative to the resting potential during this time the sodium

channels go back to their initial closed state and for a short period the potassium channels stay open at this

point we're in the relative refractory period since the sodium channels are closed but they can be activated

although because the potassium channels are still open and we're in a hyper polarized state it takes a stronger

stimulus to do so finally as the potassium channels closed the neuron returns to its resting membrane

potential all right as a quick graphical recap of this whole process with membrane potential on the Y and time on

the X first we start at the resting membrane potential of around negative 65 millivolts and voltage-gated sodium and

potassium channels are closed we then receive epsps enough to hit threshold at about negative 55 millivolts causing

voltage-gated sodium channels to open and we reach a peak of about positive 40 millivolts at which point the sodium

channels become inactivated and we're in the absolute refractory period voltage-gated potassium channels then

open and along with the sodium potassium pump start to repolarize the cell so much so that it overshoots in

hyperpolarizes the cell next the sodium channels enter their closed resting state while at the same time potassium

channels start to close meaning that were in the relative refractory period until finally they all closed and we get

to our resting membrane potential once again all right so this process of positive sodium ions moving in and

depolarizing the cell transmits the electrical signal down the length of the axon great but really this process isn't

that fast so that's where the fatty myelin comes in which comes from glial cells like Schwann cells or

oligodendrocytes these myelinated areas don't have voltage-gated ion channels spanning the membrane so ions can't

simply flow into the cell that only happens in the spots between the myelin called nodes of ranvier

so instead of propagating via channels the charge essentially jumps from one node to the next

that said though jumps isn't really the right term and these ions aren't just diffusing down the length of the myelin

to the other side that'd be way too slow what actually happens is more like the sodium ions rush in and bump other

positive sodium ions already inside the cell which bumps other ones and so on until this wave of positivity reaches

the next node the charge moving in this way down the myelinated areas moves really fast and is called saltatory

conduction which makes it look like the action potential jumps from one node to the next all right as an extremely quick

recap neuron action potentials happen when dendrites receive enough epsps to open voltage-gated sodium channels which

causes rapid depolarization of the neuronal membrane and propagation of an electrical charge from node to node down

the length of the axon thanks for watching you can up support us by donating on patreon or subscribing

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