Neurons communicate by changes in electrical properties of the plasma membrane that move from one cell to another. The architecture of the neuron aids the spread of these electrical signals called nerve impulses. A potential difference exists across every cell’s plasma membrane. The side of the membrane exposed to the cytoplasm is the negative pole, and the side exposed to the extracellular fluid is the positive pole. This potential difference is called the membrane potential.
If the plasma membrane is depolarized (moved toward a polarity above that of the resting potential) slightly, an oscilloscope will show a small upward deflection of the line that soon decays back to the resting membrane potential. These small changes in membrane potential are called graded potentials because their amplitudes depend on the strength of the stimulus. Graded potentials can be either depolarizing or hyperpolarizing and can add together to amplify or reduce their effects. The ability of graded potentials to combine is called summation. The level of depolarization needed to produce an action potential is called the threshold.
