Welcome to 2 minute neuroscience, where I simplistically explain neuroscience topics
in 2 minutes or less.
In this installment I will discuss membrane potential.
Membrane potential refers to the difference in electrical charge between the inside and
the outside of a neuron.
This is the plasma or cell membrane of the neuron.
It separates the inside of the cell from the outside environment; we’ll say this is the
inside and this is the outside of the neuron.
The difference in electrical charge develops due to the grouping of ions on the inside
and outside of the membrane.
Ions are atoms that have either lost or gained electrons and thus have a positive or negative
charge.
There are several ions that play an important role in the membrane potential of neurons.
There are positively charged sodium ions, represented by these blue circles and negatively
charged chloride ions, represented by these green circles.
When a neuron is at rest, the sodium ions and chloride ions are more prevalent outside
of the cell.
There are also positively charged potassium ions, represented by these yellow circles
and various negatively charged ions, often referred to as organic anions represented
by these grey circles (anion is simply a term for a negatively charged ion).
When a neuron is at rest, the potassium ions and organic anions are more prevalent inside
the cell.
At rest, the inside of the neuron is more negatively charged than the outside, causing
the resting membrane potential of an average neuron to be around -70 millivolts.
One way this potential is maintained is through a mechanism known as the sodium-potassium
pump.
This is a transport protein that uses energy to constantly pump three sodium ions out of
the cell while at the same time pumping two potassium ions into the cell.
Because there are more positive ions being pumped out than in, it helps to keep the membrane
potential negative.
Unlike other ions, potassium tends to move fairly easily across the cell membrane through
ion channels, which are membrane spanning proteins that allow ions to pass through.
Potassium will pass out of the neuron until it reaches the point where it is at an equilibrium
- when forces like diffusion aren’t pushing it in one direction or the other.
At this point, the membrane potential of the neuron is around -65 to -70 mV, which is known
as the resting membrane potential.