So now let’s talk about once
we have this post-synaptic response.
How do you propagate an action potential?
Probably, the best way to think about here, is that
you need to depolarize that postsynaptic membrane.
So what I mean by depolarize?
Remember from membrane potential,
a depolarization is moving up
or becoming less negative.
A hyperpolarization is moving down.
So with our depolarization component,
we have two things that could happen.
Either when we bind to our postsynaptic membrane,
we have a subthreshold event
or we have a threshold event.
If you are able to eclipse threshold,
you will get an action potential.
If you are subthreshold even though you
stimulated that postsynaptic membrane
you actually will not obtain an action potential.
So this threshold is a very
important concept to have in mind.
Each individual nerve will have a threshold limit.
And you need to get enough signal
up to that point to reach threshold.
And again, once that’s reached,
you will then propagate that action potential.
So let’s go through some processes here
that will make this more understandable
and how we quantify things like subthreshold
depolarization or hyperpolarization.
This is done by looking at excitatory
We often times abbreviate these EPSPs for short.
So there’s ways that we can maximize the amount
of potential that comes across the membrane.
If you have one stimulation,
you usually get one depolarization.
So maybe, away to get greater amount to depolarization
is use more than one nerve.
If you do that, you get a bigger response.
Finally, if you bombarded with a number of different axons,
bam, you will get a higher response.
And usually, when you get that higher response
you can then trigger a threshold event.
Another way to do this besides using multiple axon terminals
that will bought up against a particular neuron
is to have them occur very frequently.
So you could have one particular axon terminal
giving multiple signals.
And with that will allow to have happen
is to build up membrane potential
to again hopefully, do what?
Cause, if we can reach threshold, we will be
able to develop an action potential.
I would like to say that all
subthreshold events are positive.
Cause you know like we like to stay positive, don’t we?
We don’t like to get too negative on things.
But that’s unfortunately we have to add
both the positives and negatives in real life.
And that’s we have to do here for
excitatory postsynaptic potentials
and the opposite which is inhibitory post synaptic potential.
So we look at the positives first.
Let’s take a membrane potential
that starts out at -70 millivolt.
You can see here two green blips.
Those are EPSPs, a signal from another nerve
telling this particular nerve to fire.
At point A, we have not yet reach threshold.
In our example here, threshold is -55.
So no action potentials has occurred yet.
It will be nice if we got one more of these EPSPs
to let us have reach threshold, right?
Unfortunately, an IPSP just came in,
not our threshold down, not our threshold
but there membrane potential down
cause it hyperpolarized that particular response.
Then we are at baseline for a little bit.
We give another EPSP, its gets depolarized further.
We’re getting close to threshold.
Another IPSP just came in and hyperpolarize the membrane.
Luckily, the EPSPs didn’t give up.
They got another couple of hits.
and that way will move membrane potential
up to eventually reaching threshold and
right it be, we got an action potential.
And that is what’s going on constantly in the brain
and throughout many nerves in the body.
You’re getting all of these signals.
Sometimes, a signal will cause an event
like an action potential.
And sometimes they just buildup.
But you have to remember you to need to sum up
the total of EPSPs and IPSPs
to see if you can reach threshold.