at this point it becomes very important that you have taken in all the lessons that we've been discussing up until this point the concept of ions existing in different concentrations on one side of the channel or the other and the concept of ions being ready to flow in one direction or another and especially the idea that sodium will readily flow into the cell or that potassium will readily flow out of the cell and exactly why that happens what exactly why it happens that way so we talked about the resting membrane potential it gave some different ideas about exactly why it happens the way it happens but it's important to note that membrane potentials can actually be changed they can be changed by number one anything that alters and ions concentration on one or the other side of the membrane and number two anything that changes an ions permeability or its ability to flow from one side to the to the other let's talk about the first case first anything that alters an ion concentration and one or the other side of the membrane would be something that changes for example the number of potassium ions or sodium ions inside of the cell or the number of potassium or sodium ions outside of the cell or perhaps even the number of proteins inside of the cell anything like that would start to change the membrane potential perhaps in subtle ways perhaps not by much perhaps instead of minus seventy it might be minus 68 or minus 65 but that would be a significant enough change that the cell would no longer function the same way that it used to function so one example of the way that this could occur would be if for example remember that outside of the cell is a very small number of potassium ions there are certain conditions in which all of a sudden the potassium ions would actually be increased outside of the cell this would be a disorder and this would be a disaster the body would no longer if the body was no longer regulating its potassium ions outside of the cell then the membrane potential would change significantly so potassium sodium ions chloride ions have to be regulated a particular number in order to make everything work correctly so that's kind of a rare instance it would probably be a disorder there are certain injuries and so on that would lead to this problem but number two the second situation actually happens all the time something that changes in ions permeability would be something as simple as a channel and one of the channels we're going to be looking at and one of the first sort of a case we're going to consider is the idea of a chemically gated channel also called a ligand gated channel or more generally would be called a a stimulus gated channel so this chemically gated channel will open when a certain chemical binds so what you'll see here is there's a closed ion channel and there will be some kind of a ligand or some kind of neurotransmitter that will bind to this receptor outside of the cell and that will allow for the channel to open and at that point the iron will full flow through an example of a channel like this would be something like a acetal colon gated sodium channel so you've named the ligand in the first part which causes the opening of the channel and in the second part you've named the ion that flows through so if you had for example a ligand gated potassium channel then when that particular ligand binds to those receptors it would cause the potassium to flow out another idea another version of this would be a mechanically gated sodium channel in that case basically a pressure change in the cell membrane would cause the channel to yawn open and sodium would flow into the cell so you get the idea here there's there's a nomenclature that's involved here where you have the naming of the channel takes on two parts the first part talks about what gates the channel the second part talks about the iron that flows through very common mistake that students make is that they will start to call channels they will start to call them for example sodium gated channels that's incorrect sodium itself is not gating the channel it's in this case it's some kind of a ligand it's a mechanical stimulus it's a neurotransmitter that's gating it that's causing the gate to open the I n is the thing that flows through so sodium chloride calcium potassium whatever the ion is is the second thing that is named because it's the thing that flows through so be very careful that you're that you know exactly what's gating the channel and then what flows through secondarily okay all right this is this is just another example of a stimulus gated channel in this case it is a channel that is permeable both to sodium and potassium when the neurotransmitter binds and the channel opens sodium flows in and potassium flows out and just so you know the drive is stronger for sodium and there's a few different reasons for that but in a general sense sodium tends to enter faster potassium than potassium flows out the second type of channel we're going to be looking at is a voltage-gated channel there's two for our purposes there's only two voltage-gated channels that we actually consider a voltage-gated sodium channel or voltage-gated potassium channel again note the nomenclature the gating is mentioned first and the ion is mentioned second so a voltage-gated sodium channel is shown here and what happens with the voltage-gated channel although it's not an easy concept to get I understand that it's not easy to understand this so you have to kind of sit with this information for a while is that a voltage-gated channel will open when there is a change in voltage so with a voltage-gated channel it says it right in the name both educating a change in voltage actually causes the channel to open so if you notice here then this is the way that a cell is typically set up where you have positive charges lined up along the outside of the cell negative charges line up along the inside of the cell and then something changes something changes so that there's a reversal all of a sudden there are negative charges outside positive charges inside that's actually going to cause this channel to change its configuration and flip open again it did not require a neurotransmitter it required a change and the membrane potential itself and since this particular channel is permeable to sodium what happens in this case is that the voltage gate opens and sodium flows in there are also examples of voltage-gated potassium channels and just so you know there is a third type of channel which is called a leakage channel which is open all the time the only one that we consider in the nervous system is a potassium leakage channel it's actually very very important channel that allows potassium to leak out at small increments on a regular basis that actually keeps potassium at a slightly higher concentration outside didn't cell then then would be otherwise down the line wait on the line we need when you talk about the cardiovascular system you're going to take into consideration some sodium leakage channels which have a very very important role in the pacemaker potential of the self so there are three types of channels then the ligand gated or chemically gated or stimulus gated channels the voltage-gated channels and then the leakage channels