neurons have three different kinds of potentials wresting graded and the action potential first let's see what happens at resting potential which is at minus 70 millivolts the neuron maintains this potential due to differences in permeability of ions on either side of its cell membrane as well as the sodium potassium pump the ions contributing to the charges on either side of the membrane are negatively charged proteins negatively charged chloride positively charged sodium and positively charged potassium now there are several kinds of channels found in cell membranes allowing for the transport of substances from one side to the other two kinds are important in the context of action potentials leaky channels and voltage-gated channels leaky channels allow the free flow of substances through them voltage-gated channels are only open at certain voltages back to the ions chloride and the proteins stay put as they don't have a means to cross the membrane however the neuron does have leaky sodium and potassium channels these are always open and allow some flux of these ions so what do the ions do first there's a lot more potassium in the cell than outside the cell the potassium wants to rush out of the cell because of the chemical gradient but wants to stay in the cell because of the electrical gradient like charges repel similarly there's a lot more sodium outside the cell than in it and the sodium has its own electrochemical gradient now if you were to just let the ions do their thing and flow through these leaky channels eventually a bunch of sodium would rush in and we'd lose that potential however the sodium potassium pump is at work the energy from one ATP hydrolysis reaction allows this pump to transfer three sodium ions out and 2 potassium zin as a result the neuron maintains its resting potential of minus 70 millivolts a graded potential is a change in potential that can vary in size with magnitude depending on the intensity of the stimulus and occurs when the neurons get excitatory postsynaptic potentials or epsps or inhibitory postsynaptic potentials or ipsps dendrites are a neurons input zone where a neuron can get epsps and ipsps from other neurons the neurons cell body is like a calculator integrating these signals when the summation of graded potential results in a potential of minus 55 millivolts at the axon hillock an action potential occurs hence epsps make it more likely that an action potential will occur while i PSPs make it less likely unlike graded potentials which are changes in potential varying in size action potentials are all or nothing more intense stimuli simply mean a higher frequency of firing so let's go through the steps of an action potential we start off at resting potential voltage-gated sodium and potassium channels are closed then a stimulus causes some voltage-gated sodium channels to open once we get to the threshold of minus 55 millivolts the action potential begins with lots of other voltage-gated sodium channels opening with sodium channels open depolarization occurs sodium rapidly rushes into the cell and the voltage soars up to plus 30 millivolts at which point the voltage-gated sodium channels closed potassium channels now open and repolarization occurs as potassium rapidly rushes out of the cell the voltage zooms down and overshoots the minus 70 millivolts before the potassium channels can close finally the sodium potassium pump restores the resting membrane potential it does a conformational change thanks to an ATP molecule being hydrolyzed again this conformational change results in three sodium atoms being shuttled out of the cell and two potassium atoms being shuttled in we've now seen what happens locally at one segment of the axon but how does the action potential propagate well when the sodium ions are rushing in during depolarization they repel each other and so spread out this makes the next section of the axon reach threshold and also have an action potential why doesn't the action potential travel backwards though at the same time as the first section is depolarizing the section of axon behind it is experiencing repolarization and the potassium rushing out results in a refractory period there is an absolute and relative refractory period during the absolute refractory period which coincides with most of the action potentials duration you can't trigger another action potential because the sodium channels are briefly inactivated the membrane needs to be hyperpolarized before that can happen during the relative refractory period only a very strong stimulus can cause an action potential since the membrane is below the resting potential and you need a stronger EPS speed to get to threshold another important point action potential propagation is slow that's why most of your neurons have myelin sheaths which are rich in lipids myelin sheaths are made by oligodendrocytes in the central nervous system and schwann cells in the peripheral nervous system they insulate the axon and prevent leakage of charged ions instead you get what's called saltatory conduction in which action potentials only occur in the spaces between the myelin called nodes of ranvier in addition to myelin sheaths neurons with larger diameters also have faster transmission speeds once the action potential reaches the end of the axon there are terminal buttons there the depolarization triggers the opening of voltage-gated calcium channels on the presynaptic membrane and calcium rushes into the terminal button this causes exocytosis of vesicles full of neurotransmitter molecules and neurotransmitter is released into the synaptic cleft where the neurotransmitters attach to the receptors of the postsynaptic neurons causing epsps or ipsps and potentially triggering another action potential if you liked this video please like and subscribe it would help me make more videos and make sure to comment with any topics you'd like me to cover in future videos also it would be really 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