Summation (Neurophysiology)

Summation, which includes both spatial and temporal summation, is the process that determines whether or not an action potential will be triggered by the combined effects of excitatory and inhibitory signals, both from multiple simultaneous inputs (spatial summation), and from repeated inputs (temporal summation). Depending on the sum total of many individual inputs, summation may or may not reach the threshold voltage to trigger an action potential.

Neurotransmitters released from the terminals of a presynaptic neuron fall under one of two categories, depending on the ion channels gated or modulated by the neurotransmitter receptor. Excitatory neurotransmitters produce depolarization of the postsynaptic cell, whereas the hyperpolarization produced by an inhibitory neurotransmitter will mitigate the effects of an excitatory neurotransmitter.

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.

The nervous system first began to be encompassed within the scope of general physiological studies in the late 1800s, when Charles Sherrington began to test the electrical properties of neurons. His main contributions to neurophysiology involved the study of the knee-jerk reflex and the inferences he made between the two reciprocal forces of excitation and inhibition. He postulated that the site of this modulatory response occurs at the intercellular space of a unidirectional pathway of neural circuits. He first introduced the possible role of evolution and neural inhibition with his suggestion that “higher centers of the brain inhibit the excitatory functions of the lower centers”.

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