The Resting Membrane Potential
Neurons, like other cells, are surrounded by a plasma membrane composed of a lipid bilayer that serves as both an insulator and a diffusion barrier to the movement of ions. Embedded in the lipid-bilayer are several types of protein channels and pumps that regulate the passage of ions across the membrane barrier.
While neurons are at rest, two types of membrane proteins create and maintain a membrane potential of approximately -70 millivolt (mV).
- Ion pump proteins use the energy stored in ATP (adenosine triphosphate) to pump Na+ out of the cell and K+ ions into the cell, creating opposing concentration gradients.
- Leak channel proteins allow many of the K+ ions to passively move back out of the cell.
As positive ions accumulate outside the membrane, the inside of the membrane becomes relatively more negative (= less positive). Additionally, the cytoplasm along the inner surface of the membrane also contains a high concentration of negatively charged proteins.
The growing external positivity and internal negativity slow the outward leakage of K+ and the membrane comes to an equilibrium around -70 millivolts (mV), which is called the resting membrane potential.
Therefore, while the neuron is at rest, both electrical and chemical potentials are created. As long as the voltage-gated ion channels remain closed, the membrane stays at -70 mV. This is somewhat equivalent to pumping water uphill and storing it behind a dam while the floodgates remain closed.