When post-synaptic potentials reach the neuron, they can be added up through the process of summation.
Watch as Leslie explains this process and expounds on the 2 types of summation.
Transcript of Today’s Episode
Welcome to another episode of Interactive Biology TV, where we’re making biology fun! My name is Leslie Samuel. In this episode, Episode 19, I’m going to be answering the question “What is summation?” and I’m going to talk about the 2 types of summation. So let’s get right into it.
Summation is basically the addition of post-synaptic potentials. To illustrate this, I’m going to draw our membrane potential graph, with membrane potential on the Y-axis, Em, and time on the X-axis. I’m going to start with our resting membrane potential somewhere around -70 millivolts, so let’s say this is -70. Now, we’ve spoken about threshold, and we said that in order for there to be an action potential, the membrane potential needs to reach that threshold, and I’m going to say that’s around -55 millivolts.
So an action potential comes down an axon, it reaches the axon terminals, neurotransmitters are released, and that causes an excitatory post-synaptic potential in the post-synaptic cell. What you’re going to see there is there’s going to be a little bump in the membrane potential. We know that the threshold is up here, so that bump is not going to be enough to cause an action potential.
So what needs to happen is we get an excitatory post-synaptic action potential, and before the first one finishes, another one comes along, raising the membrane potential even more, and that process continues over and over until eventually, we have enough stimulation to cause the membrane potential to reach threshold. And then, we get the action potential with our depolarization, our repolarization, our hyperpolarization, and then the action potential is finished.
So that’s what we’re talking about with summation. We’re basically adding these post-synaptic potentials. Remember, sometimes we can get excitatory post-synaptic potentials, we can get inhibitory post-synaptic potentials that bring our membrane potential even further away from the threshold. But we’re basically adding them so that we can reach that -55 millivolt threshold and cause an action potential.
With that understanding, let’s look at the 2 types of summation:
1. Temporal summation
2. Spatial summation
I’m going to do a drawing to illustrate both. With temporal summation, what we have is a pre-synaptic neuron, so let’s just draw an axon, and I’m going to simplify it by making just 1 axon terminal, and that makes a synapse with a post-synaptic neuron, so here’s my neuron. Here we have the soma, and here we have the axon. What happens is a signal comes along this axon, comes to the axon terminal, releases neurotransmitters, and that causes an excitatory post-synaptic potential in this cell. That is where we see the first bump. Now, if it sends that signal and it sends another one quickly and it continues doing that, that’s going to cause the membrane potential to go up and up until it reaches the threshold and causes the action potential. This is temporal summation.
Now, that’s different from spatial summation. With spatial summation, we have an axon here, and I’m just going to draw 1 axon with 1 terminal, of course that’s simplified, that connects to another neuron. However, we also have another axon that comes and we have an axon terminal that connects to the same neuron.
What’s going to happen here is this one can cause a signal, an excitatory post-synaptic potential, and before that dies off, we can have another signal coming from this other cell. So we have 2 separate neurons causing responses in this cell, and of course, we get the same result where the membrane potential goes up, and when the next one fires, it goes up again, and this one can fire again, that causes it to go up, and then it eventually reaches the threshold, causing our action potential. That is spatial summation.
So an easy way for you to remember this, at least this is what works for me, spatial summation is separated by space, because we have 2 separate neurons that are firing. Temporal summation is the same neuron firing, but each signal is separated by time. So temporal summation is separated by time, same neuron. Spatial summation is separated by space, because there are 2 separate neurons.
That’s all the content for this video. We’ve looked at the 2 types of summation and we’ve answered the question “What is summation?” If you have any questions, go ahead and leave a comment below, and I’d be happy to get to it. That’s it for this video, and I’ll see you in the next one.