 Hi, and welcome back to this video 2.2 about neurons and the membrane potential, which is part of the biological psychology video course Let's start with a very basic question. What is a neural? A neuron is, as you probably know, a brain cell, right? This is a cell and that makes up your brain The human brain has about 100 billion neurons. Now, this is a ballpark figure and you will find quite varying estimates if you would try to keep 100 billion as an easy ballpark figure in mind and Neurons communicate with each other through synapses, which are connections between neurons and the human brain has about 100 trillion Synapses, so there are about a thousand synapses for every neural Right, so that is really a lot a lot of information a lot of neurons and a lot of synapses and this mass of neurons Is one of the reasons also why we still haven't been able to, for example, simulate the human brain on a computer Right, we still don't have the computing power to do that effectively with those numbers of neurons Of course, there are other reasons or more fundamental reasons why we cannot simulate the human brain For example, we just don't understand how a human brain works. If anything Please take learn that from this video course that we don't know how the human brain works But the sheer size of the brain and the sheer volume of neurons is also one of the reasons So here we have a picture of a neural Let's take a look at a few important parts of a neuron A dendrite is the part of a neuron that receives information So you can think of a neuron as an input output station, right? So it is it is a cell that receives input from other neurons and sends output to other neurons And the dendrites is where the neuron receives input. It's the input station of the neural Now then a neuron has a very long accent generally speaking and that's the output station So the output the exon is essentially a cable that that leads to other neurons that it connects to And the connection with another neuron is a synapse, right? So a synapse is where two neurons connect So you can think of basically a neuron in a simplified way You can think of a neuron as receiving input through its dendrites Then the information travels through the exon of the neuron and then it is sent to other neurons Through is through the synapse that a neuron has with other neurons Now neurons are biological entities and we have a lot of different neurons in our brain that work in slightly different ways But as a general model that applies to many neurons. This is a very accurate depiction The membrane potential Is an important concept To know about if you are thinking about neural communication So what is the membrane potential? Well, what is a membrane? Let's start there Well, the membrane is something that separates the inside and the outside of this of a cell Right is the the wall of a cell or the skin of a cell you could say and because a neuron is a is a cell Neuron also has a memory Now when a neuron is doing nothing so when it is not active the inside of the cell The inside of the neuron is negatively charged compared to the outside of the neural and This difference is about minus 70 millivolts This is the exact voltage the voltage differs from from neuron to neuron But minus 70 is generally taken as a as a as a ballpark figure Now why is there a membrane potential? Well, here's where we need a little bit of chemistry So the difference in volt or the different the voltage difference is due to ions and an ion is a molecule That has a positive or a negative electrical charge So basically there are molecules ions that have a positive or a negative electrical charge And they are not distributed evenly inside and outside of the neuron and that's what causes the membrane potential a Membrane or at least this membrane is selectively permeable meaning that it lets some stuff flow through but not everything and For example a lot of these Positively or negatively charged ions do not flow easily or at all through the membrane Some molecules flow through it, but other molecules don't right? So this membrane is like a block a blockage blockade that preserves the membrane potential by preventing ions from flea freely flowing into the cell or out of the cell Now there are three competing forces you could say that drive ions in or out of a neural and The first of these forces is diffusion and diffusion is just a force that drives toward equal concentration Inside and outside of the cell so if you look at this picture here And for example if you look at the inside of the neuron here on the right You see that there's a lot of K plus Inside of the neuron and very little K plus outside of the neural So diffusion will drive this K plus neurons a decade neural the K plus ions out of the cell Towards an equal concentration inside and outside So that's that will be a force operating on the K plus ions There's also the electrostatic force and the electrostatic force drives towards equal Voltage inside and outside of the cell so at rest the net charge inside of the cell is negative And what does this mean? It means that for example if we have a cl minus ion a chloride ion which we will meet later It has a negative charge So it is pushed outside of the cell because the inside of the cell is already negatively charged So there's already a surplus of negative charge So negatively charged ions are pushed outside of the cell and conversely positively charged ions like Na plus are pulled into the cell And then there is third force which is not really a force of nature actually diffusion is also not really a basic force Of nature, but we can think of them as basic forces in this context So third force is a transporter pump Now when transport of pumps, they are depicted here by this this thing with an arrow Actively transport ions across the membrane. So they are active pumps that also consume energy and they operate To carry ions from from the inside of the cell to the outside of the cell or the other way around and that way They are actually very important in maintaining this membrane potential right because there's always a little bit of leakage And the membrane isn't there's always ions flowing in or out of the cell that we don't really want So then these these transporter pumps maintain essentially the balance, right? They're essential in order to keep a stable resting membrane potential Now let's take a look at this in a little bit more detail So let's take a look at the chloride Cl minus iron So the Cl minus ion is negatively charged, right? That's what the minus means and it has a higher concentration outside of the cell than inside of the cell You can see that here in this picture that there are lots of Cl minus outside and very few inside Now this means that it is pushed into the cell through diffusion, right? Because there is a surplus of Cl minus outside of the cell so diffusion pushes Cl minus into the cell It is kept outside of the cell through the electrostatic pressure Because Cl minus is negatively charged the inside of the cell is also negatively charged So the negative repels negative so the Cl minus ions are pushed out of the cell by electrostatic pressure Now let's take a look at another ion sodium or Na plus also called natrium quite confusingly You may remember this from your high school chemistry lessons if you've had them Two terms are used in different parts of the word world to refer to the same the same molecule sodium and natrium and confusingly even more Sodium is generally used for the as a word and Na plus is generally used as an abbreviation Right, so that doesn't make any sense at all, but that is the way most people that's the consensus Sodium Na plus is positively charged That's what the plus means and it has a high concentration outside of the cell Now What this means is that it is pushed into the cell through electrostatic pressure Right, so because the inside of the cell is negatively charged Na plus is positively charged so Na plus is pulled into the cell and Also, it is pulled into the cell through diffusion because there is a surplus of Na plus outside of the cell So just through diffusion Na plus is pushed also inside into the cell, right? So electrostatic pressure and diffusion both operate on Na plus to pull it into the cell Now this also kept whether this kept outside of the cell through the sodium potassium pump So this active transporter pump Uses energy to pump Na plus ions that are inside of the cell outside of the cell, right? So that that the membrane is not permeable or hardly permeable to Na plus, right? So there's not that much Na plus actually flowing into the cell Despite the fact that it is that it is pushed into the cell through electrostatic pressure and diffusion But it's blocked by the by the membrane But still a little bit will leak in especially and also after an action potential as we will see later A lot of Na plus actually flows into the cell So this transporter pump then operates to restore the balance and and Pump the Na plus ions out of the cell again, right? So there's this loop Now then finally, let's take a look at the potassium or K plus also called Calium So again, we have this confusing double naming scheme where we use both potassium and Calium for the same thing and Even more confusingly we use potassium as a word generally speaking and K plus as the abbreviation. There you go And it has the highest concentration inside of the cell Now that means that K plus is pushed out out of the cell through diffusion, right? Because there's a surplus of K plus in inside the cell. So diffusion pushes it out And it is kept inside of the cell through electrostatic pressure because K plus is positively charged the inside of the cell is negatively charged So the electrostatic force pulls K plus into the cell and then there's again the sodium potassium pump That also pumps K plus into the cell, right? So this sodium potassium pump pumps any plus outside of the cell as we just saw and it pumps K plus into the cell All the while absorbing energy to do so, right? Because it's not really a force of nature. It is really an active biological pump Okay, now with that let's move on to video 2.3 in which we're going to take a closer look at Synapses connections between neurons and also the action potential, which is what happens when a neuron becomes active