The information from inputs to a neuron is converted to the size and duration of excitatory and inhibitory graded membrane potentials in the dendrites and soma. Neurons process that information by summation of graded potentials at the trigger zone to determine if an action potential will be fired down the axon. Action potentials, however, are usually the same size and duration for any given neuron, so that the information contained in the graded potentials is instead converted to temporal patterns, or the timing, of action potentials. Some neurons, such as the motor neurons that synapse on skeletal muscle, fire few or no action potentials until there are sufficient excitatory inputs, and then the size and duration of depolarization over threshold is converted into the frequency and duration of a series, or train, of action potentials.

Other neurons, such as some in the brain, fire action potentials at a regular rate or in bursts without input because of differences in their leak and/or voltage-gated channels that cause them to spontaneously depolarize to threshold.

Excitatory input causes these neurons to fire action potentials or bursts of action potentials more frequently, and inhibitory input decreases their frequency.

The advantage of this system is that information passed along to the target cells of the neuron can be fine-tuned in either direction with changes in the frequency of action potentials. The different temporal patterns of action potentials are then converted into amounts and temporal patterns of neurotransmitter released at synapses.

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Level 3 Unit 1 Part 17: Synapse structure