What is Summation?

The additive effect of multiple electrical impulses on a neuromuscular junction (junction between a nerve cell and a muscle cell), is referred to as summation in physiology. Individually, the stimuli are unable to elicit a response, but when combined, they can. Temporal summation refers to the addition of successive stimuli on a single nerve; spatial summation refers to the addition of simultaneous stimuli from several conducting fibers.

Types of Summation

The process of determining whether an action potential will be produced by the combined effects of excitatory and inhibitory signals, both from multiple simultaneous inputs (spatial summation) and from repetitive inputs (temporal summation), is known as summation. Summation may or may not exceed the threshold voltage to cause an action potential, depending on the sum total of several individual inputs.

Depending on the nature of the neurotransmitter that binds to the specific receptor present on the postsynaptic membrane, the membrane potential is altered by inducing the opening of voltage-gated ion channels. Excitatory neurotransmitters cause depolarization of the postsynaptic cell, while inhibitory neurotransmitters cause hyperpolarization, which counteracts the effects of excitatory neurotransmitters. An EPSP, or excitatory postsynaptic potential, leads to the depolarization, while an IPSP, or inhibitory postsynaptic potential, results in the hyperpolarization of the postsynaptic membrane.

A neuron can receive postsynaptic potentials from thousands of other neurons at any given time. The spatial (i.e. from multiple neurons) and temporal (from a single neuron) summation of all inputs at a given time determines whether the threshold is reached and an action potential is produced. It has long been assumed that the closest a synapse is to the cell body of a neuron, the more impact it has on the final summation.

By incorporating (adding or summing up) the incoming potentials, the neuron cell body functions as a device. The net potential is then sent to the axon hillock, which initiates the action potential. The summation of excitatory and inhibitory synaptic inputs is another consideration. An inhibitory input's spatial summation cancels out an excitatory input. The inhibitory shunting of EPSPs is a commonly observed consequence.

Spatial summation:

With input from several presynaptic cells, the spatial summation is a mechanism for eliciting an action potential in a neuron. It is an addition of potentials from various input areas, typically on dendrites. Summation of excitatory postsynaptic potentials increases the likelihood that the potential will reach the threshold potential and generate an action potential, while summation of inhibitory postsynaptic potentials will prevent the cell from reaching the threshold potential and generating an action potential. The closer the dendritic input is to the axon hillock, the more the potential influences the likelihood of the postsynaptic cell firing an action potential.

Temporal Summation

When a high frequency of action potentials in the presynaptic neuron elicits summation of postsynaptic potentials, this is known as temporal summation. A postsynaptic potential has a longer duration than an action potential interval. The amount of summation is increased if the time constant of the cell membrane is sufficiently long, as it is for the cell body. The amplitude of one postsynaptic potential at the moment it starts will algebraically sum with the amplitude of the next one, resulting in a greater potential than the individual potentials. This allows the membrane potential to exceed the threshold for an action potential to be produced.

At the same time, two synapses in different parts of a cell are stimulated. The density of receptors in a receptive field is usually highest at the field's periphery. This is a clear indication of temporal summation.

Temporal summation for vibratory stimuli is similar to what has been found for spatial stimuli; that is, the temporal summation is visible when stimulation conditions support stimulating Pacinian corpuscles (encapsulated sensory nerve endings which respond to pressure and vibration). The most temporal summation is achieved with higher temporal frequencies and wider contact surfaces. The threshold for these vibratory sensations decreases as the period of stimulation increases up to 500 milli sec. Based on these and other findings, it has been proposed that the Pacinian system (Pacinian receptors and their peripheral and central connections) is capable of both spatial and temporal summation, unlike the other mechanoreceptor systems.

Integration of Successive PSPs due to Temporal Summation

By firing action potentials in a presynaptic neuron and tracking the resulting EPSPs, temporal summation can be demonstrated. A single action potential in sensory neurons induces a 1 mV EPSP in the motor neuron. Two EPSPs are formed when two action potentials occur in rapid succession but note that the second EPSP occurs during the dropping process of the first, and the depolarization associated with the second EPSP is added to the depolarization produced by the first.

Two action potentials combine to create a summated potential with an amplitude of about 2 mV. A summated potential of about 3 mV will be obtained by combining three action potentials in rapid succession. In theory, 30 action potentials in rapid succession will create a potential of about 30 mV, allowing the cell to easily reach the threshold. This summation is purely a cell's passive property. There are no special ionic conductance mechanisms needed. The difference in postsynaptic conductance caused by the second of the two consecutive action potentials is added to the difference caused by the first. The postsynaptic membrane also has capacitance and is capable of storing charge. As a result, the membrane temporarily stores the first EPSP's charge, and the second EPSP's charge is applied to the first.

However, the length of the postsynaptic potential determines the "time window" for this mechanism of temporal summation. Temporal summation is only possible if the presynaptic action potentials (and thus postsynaptic potentials) are similar in time to each other. The time frame is determined by the length of synaptic conductance shifts and the time constant. Even when the postsynaptic conductance is altered, temporal summation gets altered. In this case, the synaptic current is slightly less than the first PSP reduces the driving force for that particular second.

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for Bachelor of Medicine, Bachelor of Surgery.