The neuronal membrane is selectively permeable, primarily due to specific ion channels. At rest, the membrane is significantly more permeable to K ions than to Na ions. This means that K can move out of the cell more easily than Na can enter, leading to a net loss of positive charge within the cell. This permeability contributes to the negative charge within the cell.

Resting Membrane Potential (RMP)

The resting membrane potential of a neuron is typically around -70 mV. This negative potential is primarily due to:

The Sodium-Potassium Pump (Na/K ATPase)

One of the active transport mechanisms that maintain the ion concentration gradients and contribute to the negative charge inside the neuron is the Sodium-Potassium Pump (Na/K ATPase). This ATP-dependent pump transports 3 Na ions out of the neuron and 2 K ions into the neuron for each cycle. This activity helps maintain the concentration gradients of these ions and further contributes to the negative charge inside the neuron.

Anionic Proteins and Organic Anions

Neurons also contain large negatively charged proteins and organic anions that cannot cross the membrane. These negatively charged molecules contribute to the negative charge inside the neuron.

In summary, the combination of ion concentration gradients, selective permeability, and active transport mechanisms, such as the sodium-potassium pump, creates the resting membrane potential that is negative inside the neuron compared to the outside environment. This negativity is a convention and results from the sodium-potassium pump's activity, which uses ATP as an energy source.

Further Reading

If you want to delve deeper into the details of neuron physiology and membrane biology, here are some additional resources: