Neuroscience Crash Course #2

Membrane Potential and the Action Potential

This is where things start to get interesting. The first part of this series was more or less a primer to make sure everyone has a base level of knowledge before we move forward onto the real meat and potatoes of neuroscience.

Knowing about the membrane potential and action potential makes the rest of neuroscience accessible. Things really start to make sense on a cellular level, and you gain a deeper understanding of how neurons actually function and communicate with one another.

Let’s get into it.

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Membrane Potential

If you remember from part one (link here if you missed it), neurons rely on internal electrical impulses in order to communicate with one another. These impulses are created by the movement of ions in and out of the neuron. 

Neurons, like all other cells, have a cell membrane composed of lipids separating its insides from the outside. This membrane doesn't allow any ions through it, so their movement in or out of the cell is entirely dependent on proteins that function as channels and transporters located within it.

Image Source: Typical plasma membrane of a cell with proteins embedded in it

Ions are just charged molecules that have an extra or missing electron, which means they have a positive (+) or negative (-) charge. The main ions at play in neurons are sodium (Na+), potassium (K+), and chloride (Cl-); however, there are also a large amount of negatively charged proteins (A-) inside neurons that contribute to the highly negative resting state of the neuron which we’ll talk about shortly.

These molecules are located in different concentrations across the cell membrane and generate the membrane potential, or difference in charge between the inside and outside of the cell. 

Image Source: The difference in ion concentrations across the cell membrane contributing to the membrane potential of a neuron

The inside of neurons are more negative than the outside at rest. On average it’s about 70 millivolts (mV) more negative, which makes the resting membrane potential -70mV. The ions and proteins mentioned above (Na+, K+, Cl-, A-) and their relative concentrations inside or outside the cell contribute to this resting potential.

Image Source: The inside of the average neuron is about 70mV more negative then the exterior

But why are neurons negatively charged at rest?