>>Dr. Ketchum: In this segment of the neural integration lecture series, you will learn
about the ionic basis of an action potential. And so the goal is to view how a neuron generates
an electrical signal, and how that signal can be propagated down the neuron. So like
I mentioned earlier, we’ve already discussed that dendrites can generate graded potentials,
cell bodies can generate graded potentials. And if those graded potentials reach the axon
hillock, where I’ve put an “x,” then we can generate an action potential. So before
we begin, you should know some definitions and some terminology in order to understand
the phases of the action potential. And so on this graph you have the membrane potential
on the y-axis. If we use a term called “depolarization,” that means your membrane potential has become
more positive, or you can say less negative, than rest or than the resting membrane potential.
Remember that the resting membrane potential of a neuron is negative 70 millivolts. Repolarization
means the membrane has become more negative, until it’s reached negative 70 millivolts.
Repolarization is over once the membrane reaches rest. Then we have hyperpolarization. Hyperpolarization
tells us that the, the membrane potential is more negative than the resting membrane
potential. So these are three very important terms that we will be using throughout this
series of notes.
So with types of potential changes, we have two different types. You’re already familiar
with graded potentials and then we also have action potentials. So just so that you understand,
once again, the connection between graded potentials and action potentials, you have
to have a graded potential and it has to get the membrane potential to negative 55 millivolts
in order to generate that action potential. So threshold is negative 55 millivolts, be
sure that you know that number. Be sure you know resting membrane potential is negative
70 millivolts, and remember that this process is occurring on the cell body and the dendrites.
Once that graded potential reaches the axon hillock and the membrane potential is negative
55 millivolts, an action potential will occur. Action potentials are all or none; they either
happen or they don’t. So with the action potential, you have three different phases.
You have the depolarization phase, you have the repolarization phase, and we have hyperpolarization.
Now this is a very dynamic process, and so it’s very important to understand when we
explain this, I can only explain it in terms of the channel being open or the channel being
closed. However, it’s very dynamic. So we’ll begin here with our membrane at rest, so negative
70 millivolts. So the figure on the bottom left is showing you your graded potential
with respect to the action potential and how the two of them go together. And so the numbers—the
red numbers illustrated on the graph will correspond to the large number at the top
of your screen. So we’re beginning at rest, number seven. Okay, now at rest we know what
our ion concentrations are in the intracellular fluid as well as the extracellular fluid for
both potassium and sodium. Now at rest, let’s look at the configuration of the gates, of the
voltage-gated ion channels. So remember that these ion channels, these voltage-gated sodium
and potassium ion channels are located at the axon hillock and on the axon. So when
we look at the voltage-gated sodium channel, remember we have the two gates. At rest, the
sodium activation gate is closed and the inactivation gate is open, and you can see that in the
diagram. The voltage-gated potassium channel is closed. So at rest, is there any net movement
of potassium or sodium through a voltage-gated ion channel? You should’ve answered no,
because both channels are closed. You will note that when the voltage-gated sodium activation
gate is closed and the inactivation gate is open, this is the closed but capable of opening
configuration. This neuron at this point when the channels, the sodium channels are in this
configuration, that’s important. Doesn’t have to be at rest necessarily, but when ion
channels are in this configuration, this neuron is capable of responding to a stimulus. That
is very important. Now the other graph on the far right is illustrating the membrane
permeabilities to you for both the potassium and sodium ions, and they’re showing that
membrane permeability with respect to the phases of your action potential. So for example,
during depolarization, there’s a high membrane permeability for sodium and a decreasing membrane
permeability for sodium during repolarization. So we’ll assume that a stimulus has been
applied and an excitatory graded potential or an EPSP has occurred. The stimulus had
to have been applied where? If we generated an EPSP, what part of the neuron would the
EPSP be generated on? So it’s a graded potential, hopefully you said cell body and dendrites.
And the stimulus must be excitatory, and the reason we say it must be excitatory is if
you look at this graded potential here, the membrane potential is more positive. And when
the membrane potential is more positive, we say that that is an excitatory stimulus because
it made the memory potential more positive. So we’re assuming threshold stimulus occurred.
So the stimulus was strong enough to get the membrane potential to negative 55 millivolts
at the axon hillock, and now we have an action potential that occurs. You need to understand
what’s happening to the membrane potential during each of the phases. Is the membrane
potential becoming more positive, more negative, and how does that depend on the voltage-gated
ion channels? So if the voltage-gated ion channels are open, how does that change the
membrane potential? Or if they’re closed, does it change the membrane potential? And
what is the current (meaning the permeability for sodium and potassium) through those voltage-gated
channels? Remember that ions like potassium and sodium can only be permeable if their
voltage-gated channels are open. What causes the voltage-gated channels to open? And so
the answer to that is a change in the membrane potential. And then lastly, if a voltage-gated
channel is open and the current is zero, so let’s think about that. There’s a channel
that’s open, but there’s no ion movement through that channel. So as we go through
the notes and in—in lab as well, you’ll be understanding that concept. I am going
to stop here and then we will do a part two series for these—for this set of notes.