Action potentials are a sequence of rapidly occurring events that decrease
and eventually reverse the membrane potential.
This is referred to as depolarization.
They then eventually restore it back to its resting state which is referred to as repolarization.
Action potentials are what result in nerve impulses.
Action potentials can only occur if the membrane potential reaches threshold.
Threshold is usually approximately -55 millivolts.
If a stimulus is at subthreshold strength, meaning there's not a very strong stimulus,
it can cause a local graded potential but if it does not go up to threshold,
it will not lead to an action potential.
If the stimulus reaches threshold strength, it will cause an action potential
and a nerve impulse will be able to be propagated down the axon.
If I use a super strong stimulus or a suprathreshold strength,
this does not actually change the size or amplitude of the action potential
like what we saw in graded potential.
Instead, it affects the frequency of the action potential.
Please note that action potentials are all or none and one size fits all.
The voltage across the membrane of an action potential can be correlated
with the ion channels found in the membrane.
In the resting state, the membrane potential is established
by leakage channels and the sodium, potassium pump.
Also in the membrane, we have voltage-gated channels
specifically sodium and potassium voltage-gated channels but during rest, these are closed.
Also, notice that there are two gates on the sodium channel.
There's an activation gate and an inactivation gate.
This actually is going to be very important for establishing an action potential.
Recall that at resting membrane potential, the voltage of the membrane is -70 millivolts.
When we depolarize to threshold, this happens when the activation gate
of the voltage-gated sodium channel opens allowing sodium to come into the cell.
The voltage-gated potassium gate however remains closed.
Remember, the membrane potential at threshold needs to get to at least -55 millivolts.
Eventually, the membrane potential will go up to +30 millivolts.
That is the peak of an action potential.
After the peak of the action potential, repolarization begins.
We begin our descent back toward the resting potential.
This can happen when the inactivation gate of the sodium channel closes
thus, stopping sodium from coming in.
As well, the voltage-gated channel of potassium is going to allow potassium to leave the cell
thus positive charges are leaving and we're now starting to return back to the negative state.
After we reestablish our resting membrane potential,
the activation gate of the sodium channel is going to close.
The inactivation gate is going to open and as well, the potassium channels are also going to close.
We are now back to our original resting membrane potential of -70 millivolts.
In order for communication to occur from one body part to another,
action potentials must travel all the way from the trigger zone of the axon
to the terminal of the axon or the axon terminals.
Action potentials do not die out and are able to keep their strength over the entire length of the neuron.
There are two ways in which the action potential is propagated
depending on the presence or absence of a myelin sheath.
We have continuous conduction which happens when there is no myelin sheath
and then we have saltatory conduction
which occurs in the presence of a myelin sheath or in the white matter.
In continuous conduction, the action potential
is going to proceed down the membrane toward the axon terminals.
However, in saltatory conduction, the action potential
is able to jump from node to node down to myelinated tissue.
These nodes are referred to as Nodes of Ranvier.
They are spaces in between the oligodendrocytes or the Schwann cells and in these spaces,
that is where the sodium channels are going to open up.
This allows for the nerve impulse to kinda hop down the neuron
as opposed to continuing along the neuron smoothly.
If you compare the distance of an action potential at the same time point so in this case,
10 milliseconds after the beginning of an action potential, you'll find that in saltatory conduction,
the action potential gets further toward the axon terminal
than it does in continuous conduction.
There are several factors that are going to affect the propagation of speed.
You have axon diameter where the larger the diameter of the axon, the faster the impulse will go.
The amount of myelination where the more myelination you have, the faster the impulse will go.
And finally, temperature where the higher the temperature, the faster the impulse.