so in this video we're going to focus on physiologically what's happening that allows an electrical signal in a motor neuron to be converted into an electrical signal in the muscle fiber itself so we're going to call this the events at the neuromuscular junction and you can kind of read through the slide yourself but i'm going to draw this sequence of events out for you so if we draw the motor neuron here specifically the axon of the motor neuron so let's label this motor neuron and this is the axon and we have the axon terminus and here we have the muscle plasma membrane the area that we're interfacing so where the motor neurons axon terminus sort of overlaps with the muscle fiber cell membrane this region is known as the motor end plate so here we have our must skeletal muscle cell and lastly the gap in between which is really just isf this is the synapse no different from the interstitial fluid gap between neurons so cumulatively the axon terminus the synapse and the motor end plate this is cumulatively known as the neuro muscular junction or i'm just going to abbreviate it nmj for short okay so let's say you have an action potential propagating down the length of the axon of course we know that the action potential is propagated down the length of the axon courtesy of these special proteins the voltage-gated sodium channel and the voltage-gated potassium channel you do have leaky sodium and leaky potassium channels as well as the sodium potassium pump all three of which help to maintain the resting membrane potential in both the motor neuron as well as the skeletal muscle fiber itself but for the propagation of the action potential it's the voltage-gated sodium and the voltage-gated potassium that allows for your rapid depolarization and your repolarization of the membrane and as we learned in the previous module you have another voltage-gated channel the voltage-gated calcium channel that will open and then allow for the influx of calcium ions and this influx of calcium ions is important because it will trigger [Music] the exocytosis of the neurotransmitter and the neurotransmitter is acetylcholine this is the exclusive neurotransmitter found in motor neurons so the sequence of events then you have an electrical signal in the motor neuron at the axon terminus that electrical signal gets converted into a chemical signal and that's the neurotransmitter that's the acetylcholine now the acetylcholine is going to diffuse across the synapse and it's going to interact with a chemically gated channel so the chemically gated channels are found at the motor end plate but what is unique about these chemically gated channels is that when they open they allow both sodium and potassium permeability so sodium can come in right so you have influx of sodium down its electrochemical gradient but at the same time you will also allow potassium to leave so this one protein allows for both influx of sodium and e flux of potassium down its chemical gradient but we need to recall the concepts that we learned in module 4 surrounding diffusion for diffusion of course gradients matter size matters so if we're looking at sodium diffusion through this channel remember sodium is going down an electrochemical gradient versus potassium simply going down a chemical gradient so there would be more sodium coming in than potassium going out as for size sodium is smaller than potassium so based on the size as well there is more sodium coming in than potassium going out so in essence you do have a resting membrane potential which is maintained exactly the same as it was in neurophysiology but when this chemically gated sodium slash potassium channel is open in the presence of neurotransmitter you have increased sodium permeability relative to potassium permeability so the membrane potential becomes more positive now the term for this in neurophysiology relative to a resting membrane potential is depolarization now we called these electrical signals based on a stint chemical stimulus a graded potential so likewise this is a graded potential in the muscle cell unfortunately we don't call it a graded potential since it's depolarization specifically at the motor end plate we give it a new name we call it the end plate potential so we've converted a chemical signal into an electrical signal in the muscle cell and that electrical signal is a greater potential which we call the end plate potential or epp this is where the events at the neuromuscular junction end we've converted an action potential from the motor neuron into a chemical signal acetylcholine and then converted that acetylcholine into the end plate potential so there are some unique attributes here number one the chemically gated channel on the muscle allows for both sodium and potassium permeability the fact that you only have one motor neuron here means you're not going to have a summation of greater potentials like you did in neurophysiology now you might ask the question is this epp is this graded potential threshold well 99 of the time the epp will be [Music] super threshold so that is another crucial difference between greater potentials that you see in neurophysiology and greater potentials or epps in skeletal muscles okay the epp will almost always lead to an action potential but the events at the neuromuscular junction end at the neuromuscular junction so in the next video we're going to see how the epp is converted into an action potential and how this electrical signal ultimately causes a mechanical event the contraction of that muscle cell