For a graded potential or an action potential to ever occur, the cells actually need to communicate with each other to send the signal to the next cell. Now what we're looking at in this picture is we're looking at what we call the pre and post synaptic neurons. The neuron that's sending the signal, as we can see that the signal is going down the neuron here, these are going to be your pre-synaptic neurons. They're presynaptic because they're before the space between the two neurons. The neuron that they are going to, this one right here, is going to be your postsynaptic.
Postsynaptic because post means after. So a really easy way to think about this is to draw just two little neurons, leave a little space between them. This is your synaptic cleft, or your synaptic space.
This neuron, the first neuron, is before the space, so it's pre. This neuron is after the space, so it is post. So presynaptic neurons and postsynaptic neurons.
Now, as you can see, there are different areas that the neurons can communicate with each other. We can have the axon. of the presynaptic neuron communicating with the dendrites of the post. We call that axodendritic. We can have the axon of the presynaptic neuron communicating with the soma, our cell body of the post, axosomatic.
Or we can have the axon of the presynaptic neuron communicating with the axon hillock of the postsynaptic neuron, axoaxonic. We are going to basically think about these two as the ones that's occurring. because it's easier to explain the steps in this particular fashion.
Now, even though it looks like all of these are touching each other, there's really a small space between these areas, the synaptic cleft. So in this particular picture, what we're seeing is the synaptic end bulb of the presynaptic neuron, and then we're seeing the postsynaptic neuron. Somodendrites, whatever area you want to call this.
So we're talking about how two neurons communicate with each other. So we're basically talking about a synapse. And this is very similar to the NMJ, neuromuscular junction, except for there's no muscle here. It's just two neurons communicating. So we just call it a neuronal synapse.
So the first thing is that the presynaptic neuron is going to be sending an action potential down it. Remember, action potentials are going to cause a change in charge inside the neuron. And when that action potential reaches the end of the axon, reaches that end bulb, that's going to cause the calcium voltage gates on the end bulb to open.
But when calcium voltage gates open, calcium is going to rush inside. That influx of calcium is going to cause... neurotransmitters to be released from the vesicles.
These neurotransmitters are being released into the synaptic cleft. So let's start again. That presynaptic neuron, we have an AP, an action potential, traveling down it.
That's going to change the charge inside the synaptic end bulb from negative 70 all the way to positive 30. With that change in charge, we're going to have voltage gates open. The voltage gates on the end bulbs are going to be calcium voltage gates. Since calcium is on the outside, when those gates open, calcium is going to rush in, and we call this the influx of calcium. The influx of calcium is going to cause the synaptic vesicles to release neurotransmitters into the synaptic cleft.
As those neurotransmitters are released across the synaptic cleft, they're going to diffuse to the postsynaptic neuron. Now on the postsynaptic neuron, we're going to have our ligand gates. Now, ligon gates are gates that open to chemicals.
When you look at this gate right here, this gate is closed. There are no chemicals on it. When the neurotransmitters bind to the gate, you'll notice that the ligon gate opens.
When the ligon gate opens, ions can either move in or out, depending on the type of gate that it is. So in this picture, we're saying that it's going to be a sodium ligon gate. Sodium carries a positive charge.
that sodium is going to rush into the postsynaptic neuron and going to cause a graded potential. Now remember, graded potentials are only tiny movements. So we need to add these graded potentials together, which we call summation.
And as long as they reach threshold negative 55 at the hillock, we will get a new nerve impulse. So let's do this one more time. We're going to have a nerve impulse and AP travel down the axon until it reaches the end bulbs.
It's going to cause a change in charge from negative 70 to positive 30. Change in charge causes voltage gates to open on the end bulb, which are calcium voltage gates. Calcium will rush into the end bulb, causing a release of neurotransmitters into the cleft. When those neurotransmitters get released, they're going to diffuse across the cleft and attach to ligand channels. When the channels get neurotransmitters attached to them, they will open, and when they open, ions can move either in or out.
In this particular picture, the ions are moving in, and it's a sodium ion, which will cause us to have a postsynaptic potential. This postsynaptic potential will only be a small movement from resting, so it's a graded potential. If we keep adding these graded potentials together, we're going to produce threshold through summation.
As long as it reaches the hillock, we're going to have an action potential form. Now here's a great little video from Wiley on these actions at the synapse. The number you have generated is currently is expected to report the number you have generated.
Over to you. The main thing to do is to use the IPXP and EPSP based arrays and result in hyper-formalization of the memory. In this case, no function detection will regenerate IPXP based on the EPSP array, but something is found to ensure the normalization of the memory.
Again, no function detection will regenerate it. So what was just explained was the different types of summation that can occur with graded potentials. So, what we're looking at right now is spatial summation.
The big thing about spatial summation is we will notice that we have two presynaptic neurons and only one postsynaptic neuron. With spatial summation, we're having multiple signals sent at once to the presynaptic neuron, so they're being added together very rapidly to meet threshold, so they're added together to meet negative 55 at the hillock. In temporal summation, notice we only have one presynaptic neuron and one postsynaptic neuron.
We still meet threshold, but it takes a lot longer. Here we met threshold in 40 milliseconds. Here we're meeting it in 60 milliseconds.
This one presynaptic neuron has to send signals over and over and over, and they're going to build off of each other. As they start building up, we'll eventually hit threshold at the axon hillock. and a new AP will be formed. So again, they both will cause action potentials over time, but spatial summations are quicker because it has multiple presubnautic neurons firing to one post-subnautic neuron. A good example is if you were going to dig a hole for a pool, one person could dig it, but it might take them a few months.
But if the entire class dug together, we could probably knock it out in a day or so. So the outcome is the same, the length of time is going to differ. Spatial summation will be faster than temporal summation. Now the other thing that can happen is, in this particular picture we have multiple presynaptic neurons firing to one postsynaptic neuron. Is that temporal or spatial summation?
If you said spatial, you're correct. We'll notice that presynaptic neuron 1 is sending excitatory neurotransmitters, so that means they're opening gates on the postsynaptic neurons membrane that cause EPSPs, that cause depolarization, excitatory postsynaptic potentials. The neuron number two is opening chloride gates or potassium gates, so the inside is going to be hyperpolarized.
This is an inhibitory postsynaptic potential, so it's keeping us from reaching threshold. But neuron number three is excitatory, neuron number four is inhibitory, neuron number five is excitatory. Overall, in this temporal summation, do we have more excitatory or more inhibitory?
If you said excitatory, you're correct. So this overall summation will be a super threshold to meet negative 55 in a depolarizing event. As long as it happens at the hillock. we will have a new AP occur. So as we can see, it's the net summation, the net gross of either excitatory or inhibitory potentials to determine whether we generate an AP.