Now that we have a basic understanding of
how membrane potential is accomplished and maintained, let’s look at the action potential. Obviously, we would see the action potential
passing along the axon of a neuron all the way, but in this example, we are just looking
at single spot along the axon over time. We will come back to connect this to the scale
of a full nerve cell at the end. So, there would be constantly a lot going
on at the surface of a nerve cell’s membrane wall. Think of what we le
arned about establishing
neuron’s membrane potential. Here, however, for the sake of simplicity
I want to draw our attention to two specific channels that are the main driving force behind
what we will see. These are voltage-gated channels. So, what this means is that they respond to
an electrical charge close-by this spot of an axon, and then cause a change, locally,
here, too. Let’s look at sodium (Na+) voltage-gated
channel. Its structure has this little polypeptide
ball on the bottom. So, wh
en the voltage approaches -55 millivolts,
it will open up. And, it opens up wide, which allows a huge
amount of sodium (Na+) to come in to the cell. We see this ‘rush of sodium.’ Eventually, when we go away from that voltage,
it is going to be inactivated. And, then, eventually it will close. So, once we hit that threshold of -55 millivolts
in an action potential, all of those voltage-gated sodium ion channels are going to open wide
open. As a result, there was this rush of sodium
into the cell.
As a result, it is going to depolarize the
voltage of the neuron at that spot. – So, the voltage increases, as we have
this influx of sodium into the cell. There are also potassium (K+) voltage-gated
channels. And, these are going to open when the voltage
becomes around +30 millivolts. With this voltage, they will open wide open,
and they only allow the movement of potassium (K+) outside from the cell. Okay, going back to this drawing of an action
potential. So, here we had reached the +30 mill
ivolts. Now, the potassium (K+) channels are wide
open, and we have rush of potassium out from the neuron. This causes the charge inside the neuron’s
this part to drop, so we have repolarization. Sometimes it takes a while for these potassium
voltage-gated channels to close, so it is common that we have an undershoot, so the
charge goes even below that -70 millivolts. At this point, the voltage has gotten really
low, and we need to give the normal membrane potential establishing processes to tak
e place,
to create that normal resting potential of -70 millivolts. And, only once this has been done, another
action potential can take place. So far, we have been only looking at the changes
in the charges in one particular spot of an axon over time. What is really going on is that we have depolarization
of a segment of a neuron locally, which then triggers this change to the next segment,
while at the same time the initial segment recovers from this change back to normal…
Comments
Innovative.