Intro | Inside Electrode at Rest | Outside Electrode at Rest | Inside Electrode with Action Potential | Outside Electrode with Action Potential
Part 1: Image-Mapped Tutorial
Part 2: Matching Self-Test
Part 3: Multiple-Choice Self-Test
When the cell is at rest the concentration of positively charged ions is slightly greater outside the cell membrane.
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The unequal charge across the neuronal cell membrane at rest is due primarily to the unequal distribution of sodium (Na+), potassium (K+), and chloride (Cl-). This difference in distribution of common ions is called the concentration gradient. The resting potential is maintained by a number of different mechanisms. Selective permeability of the cell membrane affects the ease with which molecules pass through the membrane; size plays a role in this mechanism. Gates or pores embedded in the membrane control the rate of passage for some important ions. When the neuron is at rest, potassium and chloride are allowed to pass at a moderate rate while sodium gates remain closed. The final mechanism for maintaining the concentration gradient involves active transport of sodium and potassium ions via the sodium-potassium pump. The sodium-potassium pump ejects three sodium ions for every two potassium ions pumped into the cell; energy is needed for this process. In addition to the concentration gradient, an electrical gradient plays a role in maintaining the resting potential. Charged ions are attracted to regions were the opposite charge is greater. The energy expended by the neuron in maintaining the resting potential is justified because the resting potential increases the neuron's ability to respond rapidly to stimulation. When the neuron is excited and the gates are opened, the concentration and electrical gradients for Na+ (maintained by active transport) result in an explosion of Na+ into the cell.