The binding of neurotransmitters will either increase the likelihood of an action potential or decrease the likelihood of an action potential occurring in the postsynaptic cell. This is a result of changes in the membrane potential of the postsynaptic cell, and these changes are due to what are called excitatory postsynaptic potentials and inhibitory postsynaptic potentials. So, if there is a depolarization of the postsynaptic membrane, what we have is an excitatory postsynaptic potential, or an EPSP because, boy, is that a mouthful. This is a depolarization of the membrane, which increases the chance of an action potential occurring. Now, inhibitory postsynaptic potentials, or IPSPs, will hyperpolarize the membrane and decrease the chance of an action potential. Remember, this can happen through ionotropic or metabotropic receptors. But because it's probably a little easier to think about this in terms of ionotropic receptors, if an ionotropic receptor allows positively charged ions in, we're going to have depolarization, so it's going to be causing EPSPs. If an ionotropic receptor allows negatively charged ions, or anions in, it's going to cause hyperpolarization or an IPSP.
Now, EPSPs can be summed together. They can add to each other and depolarize the membrane potential up to the threshold of an action potential. So, you can add EPSPs together to cross the threshold. EPSPs aren't going to generate the action potential; they're going to get the cell to cross the threshold, and then it'll generate the action potential on its own. The sodium channels that will actually trigger the official action potential are located in the axon hillock. So, these excitatory postsynaptic potentials have to carry their depolarization from the dendrites to the axon hillock to actually cause the action potential.
There are two ways that you can combine these postsynaptic potentials: temporal summations or spatial summations. Temporal summations are when a bunch of these postsynaptic potentials occur in quick succession and add together to become a larger depolarization. So, here we have this scenario: this is our presynaptic cell, here's our synapse, and here is our postsynaptic cell. Basically, if a high frequency of action potentials comes in, like one, and then right after that another, and so on, it’s going to lead to a lot of neurotransmitter release. That's going to cause a bunch of small depolarizations, eventually summing together and crossing the threshold potential. When they cross the threshold potential, it's action potential time. Essentially, with temporal summation, you have a quick succession of action potentials that lead to a quick succession of depolarizations that add together to cross the threshold.
Spatial summation is when things happen at the same time but in close proximity. So, we have simultaneous action potentials coming together to the same postsynaptic cell, which will release neurotransmitters, generating an EPSP over here and an EPSP over there, two excitatory postsynaptic potentials that will sum together. Here’s number 1, here’s number 2, and that will allow them to cross the threshold, leading to an action potential. These are two ways that postsynaptic depolarizations can add together to lead the postsynaptic cell to generate an action potential of its own. However, remember that postsynaptic potentials can also be inhibitory and can decrease the likelihood of an action potential. With that, let's go ahead and flip the page.
