Action Potential Explained - The Neuron

An action potential is the neuron's way

of transporting an electrical signal
from one neuron to the next.

This is a picture of a neuron,

where you have the dendrite on one end
and the axon terminal on the other end.

In the middle of the neuron,
you will find the axon.

This is where the electrical signal travels
from the dendrite to the axon terminal.

This is a picture of a myelinated neuron,
which is covered with a myelin sheath

which allows the electrical signal
to travel faster down the axon.

So instead of activating every
ion channel down the axon,

only the channels in the spaces

between the myelin sheath
(called the node of Ranvier) is activated

and generates an action potential.

So let's take a closer look at the axon.

On the membrane of the neuron,

you will find channels that are closed
when the cell is in its resting state.

The neuron is able to
create an action potential

because of the concentration
difference of ions

between the intracellular space
and the extracellular space.

There is a higher concentration
of sodium ions outside the neuron

and a higher concentration
of potassium ions inside.

The extracellular space is more positive
than inside the neuron.

This creates a voltage difference
of -70 millivolts (mV),

which is created by leaky ion channels

that are more permeable to
potassium ions than sodium ions,

which cause the potassium to leave the cell
and only a small amount of sodium ions to enter.

The sodium–potassium pump
also regulates the environment

by pumping out three sodium ions
in exchange for two potassium ions.

The -70 mV is the neuron's resting potential,

but when a neuron is stimulated
by a presynaptic neuron,

it causes sodium channels to open,
letting in positive ions.

This will change the electrical environment

and make it more positive inside the
neuron and less positive outside.

This is called depolarization,
which causes a chain reaction

where the next sodium channels will open,
letting in positive ions all the way down the axon.

Shortly after the channels have opened,
they will close again

and potassium channels will then open,
letting out positive potassium ions

to recreate the negative
environment inside the neuron

and the positive environment outside.

This happens at around +40 mV
and causes a repolarization,

where the intracellular space
becomes negative again.

The neuron then reaches a state
of hyperpolarization,

where the cell has let out too many ions

and has now become more negative
than the cell's resting potential.

This is quickly corrected

by leaky ion channels in
the sodium–potassium pump,

and the neuron then stabilizes
at the resting potential at -70 mV.

The action potential that has
traveled down the axon

reaches the axon terminal by
vesicles with neurotransmitters

and released out into the synaptic cleft

where the electrical signal
will stimulate the next neuron.

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