14.8: Resting Potential Decay
The resting membrane potential of a neuron (-70mV) is sustained due to the selective ion permeability of the membrane. At the resting potential, the membrane is slightly permeable to ions like sodium (Na+) and chloride (Cl−) and highly permeable to potassium ions (K+). Differences in the ions' concentration inside the cell compared to the outside are maintained by membrane transport proteins like channels and pumps.
At rest, the K+ is the main ion that moves across the membrane through potassium leak channels. K+ flowing out of the cell makes the cell interior more negative. Na+ moving slowly into the cell makes it slightly more positive than if only K+ movement were allowed. Hence, at the resting membrane potential, the cells' negative interior is because of a much greater ability for K+ moving out than Na+ moving in. The driving force acting on the ions moves them down their electrochemical gradient. Though Cl− may not contribute significantly to the resting membrane potential, its permeability prevents abrupt changes in the membrane potential.
The efflux of K+ stops when the membrane potential reaches a value where this electrical driving force on K+ balances the effect of its concentration gradient. Therefore, to maintain the membrane potential, the cell expends energy to maintain the ionic concentrations. Therefore, the cell, which behaves like a leaky boat, allowing constant ionic movement, has the primary active transporter, the sodium-potassium (Na+/K+) pump. This pump moves Na+ out and K+ into the cell to counteract the constant movements of these ions down their electrochemical gradients. The pumps' movement ensures that the ions' concentration gradients are not dissipated, and the cell remains viable, even when not conducting a signal.
The Na+/K+ pump is not responsible for generating the membrane potential but for its maintenance, which it does by maintaining the normal intracellular K+ and Na+ concentrations. If the Na+/K+ pump fails to function, the entire ionic balance could be altered, sending the resting membrane potential to zero, detrimental to the cell. The resting potential is very crucial to the nervous system's functioning. The changes in membrane potential, such as the action potential, form the basis for neuronal signaling.
