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.