in the previous video we saw how changes in the permeability of the membrane by the addition of or the subtraction of gated channels and leaky channels can alter the membrane potential to alter that electrical disequilibrium now at rest you have a resting membrane potential so again at rest means that the neuron is not sending any kind of signals the difference in charge between the extracellular fluid and the intracellular fluid is such that it is minus 70 millivolts so this is at rest now if you alter the membrane permeability with gated channels or voltage-gated channels then you can generate a type of signal that the neuron uses now overall we're going to focus on two different types of signals graded potentials which primarily occur at the dendrite and so they can occur elsewhere but we're going to oversimplify this and say greater potentials occur in the dendrites and cell body and these are short range signals they tend to decay the further away you go from the site of stimulus they're going to be reliant on chemically gated channels while action potentials occur exclusively along axons they are initiated at the axon hill lock or trigger zone and they will exclusively use voltage-gated types of channels so keeping in mind these different types of changes that occur all of this is relative to the resting membrane potential so if i look at the electrical activity on the y-axis or the membrane potential and this is in units of millivolts over the course of let's say time when the neuron is at rest the resting membrane potential of course as we've established already is minus 70 millivolts okay now if the membrane potential becomes more positive relative to the resting membrane potential this action is known as depolarization now what would cause the interior to become more positive so again if we draw the plasma membrane here and we've established that the inside is negative and the outside is positive so what would cause the inside to become more positive well if the membrane became more permeable to a positive ion like sodium this sodium influx would cause the interior to become more positive so depolarization usually is associated with sodium influx okay now if however we look at a scenario where you have the resting membrane potential and you become more negative relative to the resting membrane potential this action is known as hyper polarization so both the term depolarization and hyper polarization is relative to resting membrane potential so what would cause the inside to become more negative well if potassium leaves the cell potassium having a positive charge it's leaving behind even more negative charges so hyperpolarization is usually due to potassium efflux so these will represent the two major types of graded potentials that we'll talk about so these graded potentials are going to be either depolarizing or hyperpolarizing now there is a third term here that is uh not on this particular list and it's going to apply to action potentials so action potentials are always going to be depolarizing and in fact i i put in the caveat i would put quick depolarizing or rapid depolarizing and it is then followed by repolarizing or repolarization so what repolarization is is when you have depolarization it's a return back to the resting membrane potential so repolarization is returned to the resting membrane potential and usually this is going to be associated with potassium efflux as well all right so we have these generic terms depolarization hyperpolarization and repolarization so let's delve more into these graded potentials so as we had mentioned before graded potentials are primarily going to be occurring at the dendrites and they will utilize chemically gated channels so let's examine this let's pretend that here we have the plasma membrane at the dendrites so recall the dendrites are the receiving end of the neuron this is where signal reception is occurring now if we draw an entire neuron with the cell body the axon and let's say these are the dendrites when we look at the presence of proteins you're going to find the leaky sodium channels everywhere because they are involved with the resting membrane potential all right so leaky sodium channels found along the entire neuron likewise the leaky potassium channels are found along the entire neuron and lastly the sodium potassium pump is also found along the entire neuron because these are required for the resting membrane potential now when we examine graded potentials using graded excuse me using chemically gated channels these channels again we're going to simplify and let's just say that they are only going to be found so they chemically gated channels so both chemically gated sodium and chemically gated potassium channels are going to be found at the dendrites so if we're at the dendrites you have your leaky potato excuse me leaky sodium you have more obviously of the leaky potassium but then you also have your chemically gated sodium and you have your chemically gated potassium now keeping in mind these chemically gated channels are closed unless a chemical in this case a neurotransmitter binds the channel to open it up so if i have let's say a neurotransmitter that binds to the chemically gated sodium channel it will undergo a conformational change and now the chemically gated sodium channel is open now the membrane is even more permeable to sodium and there would be an influx of sodium that influx of sodium would cause the inside to become more positive hence you would depolarize okay so if i redraw this and let's say the chemically gated sodium channel is open the influx of sodium or the amount of sodium that rushes into the cell is going to be dependent on how long that particular channel is open and how long that channel is open is going to be directly indicative of the amount of neurotransmitter you have in that immediate vicinity so the amount of neurotransmitter locally correlates with oops correlates with the size of the stimulus or the intensity of the stimulus so the neurotransmitter is bound there's an influx of sodium so it's going to be highest right to the amount of depolarization or shall we say the strength of depolarization is highest at the point of stimulus so if you take a finger and you poke your arm you're going to feel the strength where there's an interface between your finger and the arm if you go two inches away from the point of impact you're not going to feel it as much and likewise graded potentials the further away you go from the stimulus the lower the intensity is going to be and that's because we have leaky channels all over the place so this sodium that actually came in well it couldn't trickle out through the leaky channels and so we see this level highest at the point of stimulus and the further away we go from the stimulus side the more likely we are going to encounter these leaky channels and lose the strength of that depolarization so this is why graded potentials are considered short range signals because they will dissipate in strength the further away you go from the site of stimulus and this applies to both depolarization via the influx of sodium versus hyperpolarization due to the efflux of potassium through chemically gated channels so this is going to be a very common difference between a graded potential versus an action potential which we'll delve more into in the next few videos but for now if we compare graded versus action the main differences are going to be the types of channels you use so if we examine graded versus action you're using chemically gated channels for graded potentials you're using voltage-gated channels for action graded are primarily going to be at the dendrites and the cell body action potentials are going to be exclusively along the axon graded potentials will be either depolarization or hyperpolarization while an action potential will be both depolarization and repolarization so that the membrane potential is actually going to go through a very large change from -70 to positive 30. so a difference of about 100 millivolts okay so graded potentials will decrease in strength while action potentials will maintain strength and we'll see how this is accomplished oftentimes action potentials will be known as all or nothing because in order to initiate an action potential you have to achieve a certain threshold or a certain change in the membrane potential to cause these voltage-gated channels to open so in the next video we're going to talk about the intimate relationship between greater potentials and how they can be converted into action potentials