Resting neurons usually have a stable potential across the entire cell membrane, which is often around -60 mV. Inputs from certain types of stimuli may increase or decrease the membrane potential of the dendrites and/or the soma a small amount for a brief time before it returns to the resting potential. These transient membrane potential changes of the dendrites and soma are called graded potentials, and their size and duration is determined by the size and duration of the input.
Graded potentials do not pass into the axon of most neurons; instead most axons have a different membrane potential change called an action potential.
Action potentials start when the combined effect of all the graded potentials at any moment in time brings the membrane of the trigger zone (the axon initial segment) across a certain value called the threshold potential, which is often around -50 mV. This adding together of graded potentials is called summation, and it is the way that neurons process the information from their inputs.
Most neurons respond to inputs in the form of neurotransmitter molecules released from other neurons at synapses.
Neurons and neuron-like cells that are sensory receptors may also generate graded potentials to physical stimuli such as light or odorant molecules. Graded potentials produced from a synapse are also called synaptic, or postsynaptic, potentials, and those generated by stimuli in sensory receptors are also called receptor potentials. A graded potential that moves the membrane potential to a less negative value closer to zero is called a depolarization, or an excitatory potential, and one that moves the membrane potential to a more negative value farther from zero is called a hyperpolarization, or an inhibitory potential.
Graded potentials decay with time and distance, so that their effect is brief and local. Because they decay with time, graded potentials separated by enough time have no effect on each other, but if they occur close enough in time their effects may be additive, which is called temporal summation. Because they decay with distance, graded potentials separated by enough distance may have no effect on each other, but if they occur near the same part of the membrane their effects may be additive, which is called spatial summation. For example, if two depolarizations of the same size occur at the same time and place, a depolarization twice the size will occur. The same will be true for two hyperpolarizations. If excitatory and inhibitory potentials of the same size occur at the same time and place, they may cancel each other out and the membrane potential may not change.
Because graded potentials decay with distance, the closer the starting point of a graded potential is to the trigger zone, the greater effect it will have on the likelihood of an action potential being fired. Therefore, the closer a synapse is to the trigger zone, the more influence it will have on the behavior of that neuron. Synaptic potentials are usually less than 1 mV in size, therefore most neurons require the temporal and spatial summation of many synaptic potentials to move the 10 mV or so that usually separate typical resting and threshold potentials.