noun, plural: graded potentials
A change in the electrical potential on the membrane of an excitable cell (e.g. a nerve cell) in response to a stimulus, and where the magnitude of change is proportional to the strength of the stimulus
All cells have a membrane potential, i.e. the difference in electric potential between the interior and the exterior of the cell. However, only the excitable cells (e.g. neurons and muscle cells) are capable of generating changes in their membrane potential. A nerve cell, in particular, has special ion channels called gated ion channels. The effect of the stimulus on the membrane potential of a cell depends on which gated ion channels that the stimulus opens.
When a stimulus opens a sodium channel, more sodium ions will flow into the cell resulting in depolarization. This reduces the electrical gradient across the membrane of the cell. The magnitude of depolarization will depend on the intensity of the stimulus. The stronger the stimulus, the extent of depolarization increases as more sodium channels open and thus, more sodium ions flow into the cell.
When the stimulus opens a potassium channel, potassium ions will leave the cell. As a result, the membrane potential becomes more polarized (i.e. membrane potential becomes more negative). This is called hyperpolarization. Similar to depolarization, the magnitude of hyperpolarization will depend on the intensity of the stimulus. The stronger the stimulus, the more potassium channels open and therefore the efflux of potassium ions will be greater.