Summation (neurophysiology)

Summation (neurophysiology)

Summation, also known as frequency summation is the method of signal transduction between neurons, which determines whether or not an action potential will be triggered by the summation of postsynaptic potentials. Neurotransmitters emitting from the terminals of a presynaptic neuron fall under one of two categories. Excitatory neurotransmitters produce further depolarization of the postsynaptic cell, while an inhibitory neurotransmitter will mitigate the effects of an excitatory neurotransmitter. While some neurotransmitters are known to predominately produce one of the two responses, they do widely vary and how they vary ultimately depends on the type of channel to which the neurotransmitter receptor is attached.

Neurons can only excite or inhibit other neurons (or bias the excitability of each other through modulatory transmitters). Given these two basic actions, a chain of neurons can produce only a limited response. A pathway can be facilitated by excitatory input; removal of such input constitutes disfacilitation. A pathway may also be inhibited by inhibitory input. Removal of such input constitutes disinhibition, which, if other sources of excitation are present in the inhibitory input, can augment excitation.

When a given target neuron receives inputs from multiple sources, those inputs can be spatially summated if the inputs arrive closely enough in time before the influence of each has decayed. If a target neuron receives input from a single axon terminal and that input occurs repeatedly at short intervals, the inputs will summate temporally.