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Welcome to 2 minute neuroscience, where I simplistically explain neuroscience topics
in 2 minutes or less.
In this installment I will discuss the action potential.
The action potential is a momentary reversal of membrane potential that is the basis for
electrical signaling within neurons.
If you’re unfamiliar with membrane potential, you may want to watch my video on membrane
potential before watching this video.
The resting membrane potential of a neuron is around -70 millivolts.
When neurotransmitters bind to receptors on the dendrites of a neuron, they can have an
effect on the neuron known as depolarization.
This means that they make the membrane potential less polarized, or cause it to move closer
to 0.
This chart shows membrane potential on the y axis and time on the x axis.
When neurotransmitters interacting with receptors causes repeated depolarization of the neuron,
eventually the neuron reaches what is known as its threshold membrane potential.
In a neuron with a membrane potential of -70 mV, this is generally around -55 mV.
When threshold is reached, a large number of sodium channels open, allowing positively
charged sodium ions into the cell.
This causes massive depolarization of the neuron as the membrane potential reaches 0
and then becomes positive.
This is known as the rising phase of the action potential.
This influx of positive ions creates the electrical signal known as the action potential, which
then travels down the neuron.
Eventually the action potential reaches its peak, sodium channels close and potassium
channels open, which allows potassium to flow out of the cell.
This loss of positive potassium ions promotes repolarization which is known as the falling
phase of the action potential.
The neuron returns to resting membrane potential, but actually overshoots it and the cell becomes
hyperpolarized.
During this phase, known as the refractory period, it is very difficult to cause the
neuron to fire again.
Eventually the potassium channels close and the membrane returns to resting membrane potential,
ready to be activated again.
The signal generated by the action potential travels down the neuron and can cause the
release of neurotransmitter at the axon terminals to pass the signal to the next neuron.