Resting membrane potential can be defined as the potential difference that results from the separation of charges along the plasma membrane of a neuron or other excitable cell. A resting neuron is not conducting a nerve impulse. The plasma membrane of a resting neuron is polarized, the fluid on the inner side of the membrane is negatively charged with respect to the positively charged fluid outside the membrane. The difference in electric charge between the inside and outside of the membrane at any give point is due to the relative numbers of positive (K± , Nat) and negative ions (Cr ions, huge negative protein ions) in the fluids on either side of the membrane, and to the permeability of the plasma membrane to these ions. Fig. 2.3 The Na’ and e ions constantly diffuse through ion channels in the plasma membrane, moving from regions of higher concentrations to regions of lower concentrations (except larger CI ions and negative protein ions which cannot move easily from inside of the neuron to the outside). However, the concentrations of Na + and K+ ions on the two sides of the membrane remain constant due to the action of the sodium – potassium ATPase pump, which is powered by ATP. The pump actively moves Na+ ions to the outside of the cell and K+ ions to the inside of the cell. Because it moves three Na+ molecules out , for each two e molecules that it moves in, the pump works to establish the resting potential across the membrane. Thus resting potential is maintained till there is a stimulus strong enough (threshold stimulus) to initiate an impulse.
This threshold stimulus increases permeability to Na+ ions at the point where stimulus affects. Both Na+ and K+ inos leak back across the membrane down their concentration gradients. e ions, however move more easily back to the outside, adding to the positive charge there and contributing to the membrane potential of -70 mV. Fig. 2.4