So two types of signals have been classified as a result of this type of ion movement. Graded potential, which we'll talk about now, and then next we'll be talking about an action potential. So graded potential is a diffusion potential.
So graded potentials result from ions moving through either a ligand-gated or mechanically-gated channel. There's other channels too that can also cause graded potential. but these are going to be two main ones that we'll focus on for now.
So these things are called graded potentials because they can vary in magnitude, meaning they can be small or they can be large. also vary in polarity, meaning they can be hyperpolarizing or depolarizing. So in this example, let's say that we've just opened up a ligand-gated sodium channel. That allows sodium to move in, so that's going to cause the cell to move towards sodium's equilibrium potential.
So this is a depolarizing event. Sodium is moving down its electrochemical gradient. This is at graded potential.
If we create a stimulus that causes an even greater increase in sodium permeability, then that means more sodium is going to move, which means we're going to have a stronger graded potential. So this is an example of different sizes of graded potential. If we have another stimulus that say rather than open up sodium channels causes us to open up potassium channels.
In this case we are going to have a hyperpolarizing event. This is still a graded potential. So this is an excitatory stimulus. This is an inhibitory stimulus. These are different sizes.
These are all graded potentials. So a stimulus that results in depolarization is excitatory. A stimulus that results in hyperpolarization is inhibitory.
And what those are referencing is in regards to their ability to bring us closer to an action potential, which we'll talk about in our next lecture. So when we initiate a graded potential on a cell membrane, those effects are going to occur right at the point that the stimulus occurs, but they're also going to have effects as that signal is conducted. Remember, this is an electrical type of signal.
The thing with the graded potential is that the strength of the graded potential weakens the further we get away from the side of origin. So imagine that this was a nerve and we had a stimulus here in the middle and this is an excitatory stimulus. So closest to the stimulus we'll see we're going to have the largest graded potential, but as we move down the neuron in different directions we see that that graded potential becomes weaker.
And the reason that's occurring is because in essence this is an electrical current and it's leaking out of the neuron. It's losing its magnitude as it travels. This is called decremental conduction. So here's a basic neuron and you need to go back and review your basic neuron anatomy from your freshman biology. I will assume you know that.
But here we have cell body, the axon, and then the axon terminal. and it's on the cell body where we're going to have receptors. So imagine that this is a receptor and we have a ligand that's going to come and bind to that receptor. In this case I have it drawn in green indicating that this is going to be an ex... excitatory stimulus.
So we have a ligand gated channel that is now bound to that ligand and that channel opens. That's going to allow the movement say of sodium into the cell. That then will result in a excitatory potential as we've shown here at the top. Now the thing about graded potentials is that they can be added together. So here we have the same type of neuron.
We have excitatory receptors. on the cell body. We have our ligand coming in. Remember that when we had one of these ligands bind, we got a small blip, a small graded potential. In this case, if we have two ligand-gated channels opening, that's going to allow more sodium in and that's going to give us a stronger graded potential.
So this is called summation. Now remember that graded potentials can be depolarizing or hyperpolarizing, and that depends on the channel that it opens. So here in red, we have a channel that opens.
that's going to be an inhibitory channel. So this would be a channel that when it binds to its ligand might allow potassium movement. Potassium, because of its electrochemical gradient, is going to go out of the cell.
So when potassium leaves the cell, it's going to hyperpolarize that cell, which is shown here by this negative curve. It's moving toward potassium's equilibrium potential. So realize that in our typical neuron we may have lots of different synapses. A synapse is where one neuron touches another neuron.
This is where we're going to have the release of our chemical messenger and these are going to be acting upon receptors on the cell body of this neuron. So if this is a synapse, we would call this neuron the post synaptic neuron. So in this case we're showing the excitatory synapses in green and the inhibitory synapses in red.
Now each synapse always has the same chemical messenger, so it's always going to result in the same response. So if we were to stimulate excitatory synapse A, we would expect to see a small depolarization. Same with B. We would see a small depolarization. And C, because it's an inhibitory synapse, it's going to result in...
Now if we added both A and B together, meaning we stimulated them both at the same time, we would then see the combined effects. We would see them summate. This is called any of these things, all three of these are called an EPSP, and that stands for an excitatory postsynaptic potential. So remember, we're measuring the cell membrane potential in this cell, and this is the postsynaptic cell body.
So these are excitatory postsynaptic potentials. If we stimulate C, this is going to result in a hyperpolarization. So this is called an inhibitory postsynaptic potential. Now we can still summate excitatory and inhibitory postsynaptic potentials.
So in this case, if we stimulated A and C together, we would then take the net effect of those two together, and that would result in no change. That is still called summation.