Introduction

The nervous system is specialized to carry nerve impulses from one organ of the body to another. Nerve impulses are carried by neurons. The action potential is generated in a neuron when it is excited by some stimuli or by the nerve impulse coming from the surrounding neurons via synapses.

In most cases, a neuron receives several signals from other neurons that might be inhibitory or excitatory. Neural summation is the process by which a combined effect of all the signals impinging on a neuron is generated. It determines whether an action potential will be generated in the neuron or not.

In this article, we will discuss important concepts related to neural summation, its types, the role of convergence in summation, as well as its applications.

EPSP, IPSP, and Threshold

Let us first revise some important concepts regarding excitation and inhibition of a neuron before discussing neural summation.

Excitatory Postsynaptic Potential

When a nerve impulse causes the membrane potential of postsynaptic neurons to increase from the resing value i.e. to be less negative, it is called excitatory postsynaptic potential.

The resting membrane potential of most of the neurons is -65 mV. Any increase in this membrane potential in a positive direction due to the firing of one or more presynaptic neurons is termed EPSP.

It is a change in the potential of the postsynaptic neuron due to the opening of positive ion channels caused by an excitatory neuron. When an excitatory neuron such as acetylcholine binds to its receptor on the postsynaptic neuron, it causes the opening of sodium channels. The sodium ions diffuse into the neuron and neutralize the negative charges, making the membrane potential less negative.

It is called excitatory potential because if it rises high enough, it can generate an action potential in the postsynaptic neuron. In most cases, an EPSP of 20 mV will be able to excite the neuron. However, a single synaptic transmission is not high enough to generate an action potential. The simultaneous discharge of multiple presynaptic terminals is needed to excite the postsynaptic neuron. It occurs via the process of summation.

Inhibitory Postsynaptic Potential

A decrease in the resting membrane potential of the postsynaptic neuron due to synaptic transmission is called inhibitory postsynaptic potential.

It occurs due to hyperpolarization of the postsynaptic neuron when the resting membrane potential becomes more negative than normal, the potential rises in the negative direction.

This inhibitory potential is generated when an inhibitory neurotransmitter binds to the postsynaptic receptors and causes the opening of potassium channels. The potassium ions, being abundant in the neuron, diffuse to the extracellular environment. This loss of positive ions causes the membrane potential to be more negative. In some cases, the inhibitory neurotransmitters also cause the opening of chloride ions. These chloride ions diffuse to the inside of the cell causing hyperpolarization.

The hyperpolarization of the postsynaptic neuron inhibits the transmission of nerve signals across the synapse. Therefore, it is called an inhibitory potential. It may arise from the firing of a single presynaptic neuron or by the collective effect of multiple inhibitory signals via the process of neural summation.

Threshold Potential

An action potential is generated in neurons when the membrane potential becomes depolarized to a certain extent. This minimum rise in the resting membrane potential needed for the excitation of neurons is called threshold potential. For most of the cells, its value is 20 mV i.e. the membrane potential must rise 20 mV in the positive direction to excite the cell.

The membrane potentials that are below the threshold are unable to excite a neuron and are called subthreshold potential. multiple subthreshold potentials can excite a neuron by the process of summation.

Types of Neural Summation

Recall that summation is the process by which multiple nerve signals or impulses are combined to generate one common effect. Two types of summations are seen in the neural circuits of humans.

• Spatial Summation
• Temporal Summation

A brief detail of these two types is discussed below.

Spatial Summation

It is the process by which multiple postsynaptic potentials are simultaneously summed up by activating multiple presynaptic terminals. These neuron terminals are present on widely spaced areas of membranes.

It has been mentioned earlier that the excitation of a single presynaptic terminal never succeeds to excite the postsynaptic neuron. It is because a single presynaptic terminal releases only a small amount of neurotransmitter into the synaptic cleft that is capable of generating only 0.5 to 1 mV potential. This potential is very less as compared to the threshold potential of most of the neurons.

Process

The spatial summation is carried out when multiple presynaptic terminals are excited at the same time. Although these terminals are spread over wide areas of the neuron, the effect of these terminals can still be added.

One of the important properties of neurons is that the change in potential at any point within the neuronal cell body causes the potential to change everywhere in the neuron. It is possible due to the high electron conductivity inside the neurons. Thus, whenever a synapse discharges located at any point along a neuron, the membrane potential of the entire neuron increases by 0.5 to 1 mV.

The multiple synaptic potentials are added until the EPSP becomes greater than the threshold potential, and an action potential is generated in the initial segment of the postsynaptic neuron.

Spatial Summation in Signal Transmission

Spatial summation is also used to transmit increasing signal strength by using a greater number of nerve fibres.

Pain receptors are the free nerve endings of sensory neurons. A small section of skin is innervated by a large number of pain fibres that arborize to form hundreds of free nerve endings. The arborizing of pain fibres also overlap i.e. the nerve endings from different fibres are located together in a small section of skin. Therefore, a pinprick on the skin stimulates endings from multiple nerve fibres.

The stronger the stimulus, the greater number of fibres are used, and the signal is transmitted rapidly. This phenomenon by which a stronger stimulus spreads to multiple nerve fibres is also termed spatial summation.

Read more about Skin Cells

Temporal Summation

It is the process by which rapid discharges from a single presynaptic terminal are added to generate a combined effect. It is the process by which successive discharges of the presynaptic terminal are added together.

Process

When a presynaptic terminal releases neurotransmitters, the ion channels on the postsynaptic neuron remain open for only one or a few milliseconds. However, the potential generated by the opening of these channels stays for around 15 milliseconds after the channels have closed.

Thus, the opening of the same ion channel within this time can increase the postsynaptic potential to a greater level. Thus, the more rapid the discharge, the greater is the potential generated.

Temporal summation is increased when the time constant of the nerve fibre is increased. The time constant is the time for which the response to a particular input signal is present. If the time constant is increased, the rapid discharges during this time will get added and generate a cumulative effect by the process of temporal summation.

In this way, rapid discharges will lead to the generation of an action potential in the postsynaptic neuron, using a single presynaptic terminal.

Temporal Summation during Signal Transmission

Temporal summation is also applied for conducting signals of increasing strength by nerve fibres. It is done by increasing the frequency of nerve impulse generation in each fibre.

A strong stimulus causes nerve impulse generation at a rapid rate in sensory fibres. The rapidly generated high-frequency nerve impulses are carried to the CNS where they are perceived as a strong stimulus.

Summation of Inhibitory and Excitatory Signals

Sometimes, a neuron is receiving stimulatory and inhibitory signals at the same time. The stimulatory signals tend to increase to depolarize the cell while the inhibitory signals hyperpolarize it.

In this case, the simultaneous summation of inhibitory and excitatory potentials takes place. The EPSP that tends to increase the membrane potential can be complete or partly nullified by IPSP.

In this way, if a neuron is excited by a source, an inhibitory signal from another source will reduce the potential below the threshold level, turning off the activity of neurons.

The simultaneous summation of EPSP and IPSP also helps to regulate synaptic transmission and neuronal activity.

Facilitation of Neuron

Another important phenomenon associated with summation is the facilitation of neurons.

Sometimes, the summated postsynaptic potential is excitatory but is below the threshold level. In this case, the summated potential is unable to generate an action potential but makes the membrane potential close to the threshold. Such a neuron is said to be a facilitated neuron.

A facilitated neuron has a membrane potential nearer to the threshold for firing but is not at the firing level. Any additional excitatory potential will take the membrane potential to the firing level and an action potential will develop.

Properties

In this section, we will talk about the properties of neurons that make neuronal summation possible.

Large Field of Excitation

Most of the neurons, especially motor neurons, have dendrites that extend around 500 to 1000 micrometres on each side around the cell body of the neuron. This allows the neurons to receive nerve impulses or signals from a very vast spatial field i.e. they have a large field of excitation.

In addition, around 80% to 95% of all the signals are received by a neuron via the synapses on dendrites. The majority of the presynaptic terminals make contact with these dendrites. Thus, the large excitation field of dendrites plays an important role in neural summation.

Uniform Distribution of Action Potential

It is an important property of all neurons. The intracellular fluid of a neuron is a highly conductive fluid as it contains a large concentration of ions. In addition, the diameter of the neuronal cell body is large, causing no resistance to the flow of ions and conduction of potential from one part to the other part within the soma.

Because of these properties, a potential generated at any point along the cell body immediately spreads to the entire neuron. This principle is essential for neuronal summation.

Summary

Neural summation is the process by which multiple excitatory or inhibitory signals are added to generate a cumulative effect on the postsynaptic neuron at a chemical synapse.

The excitatory postsynaptic potential (EPSP) is a potential change in the positive direction due to the opening of the sodium channels.

• It is generated by excitatory signals in the postsynaptic neuron
• It is capable of generating action potential if above the threshold potential

The inhibitory postsynaptic potential (IPSP) is a potential change in the negative direction due to the opening of potassium and chloride channels.

• It is generated by the inhibitory signals
• It inhibits transmission at a synapse by causing hyperpolarization

Threshold potential is the minimum rise in membrane potential needed to generate an action potential in the cells.

Neural summation is of two types; spatial summation and temporal summation.

Spatial summation is the summation of potentials generated by the firing of multiple presynaptic terminals.

• It occurs when multiple presynaptic neurons are excited at the same time
• The presynaptic terminals are spread over a wide area
• Strong electrical conduction within the cell body makes it possible
• Using the greater number of nerve fibers in conduction of nerve signals is also called spatial summation

Temporal summation occurs when rapid discharges of a single presynaptic neuron are added together.

• Firing of the same terminal before the earlier potential is over makes temporal summation possible
• It increases when the time constant increases
• Sending the same signal again and again through one or more nerve fibers is also called temporal summation

When both the inhibitory and the excitatory signals are present, they are added together to generate a resultant response.

If a neuron is excited below the threshold potential, any next potential change will cause an action potential in it.  Such a neuron is called a facilitated neuron.

Two properties of neurons that make summation possible are;

• Large excitatory filed of neurons dye to arborization of the dendrites
• Uniform distribution of potential generated at any point along the neuron

Read more about the Neuromuscular Junction

Frequently Asked Questions

What is neural summation?

It is a process by which multiple excitatory and inhibitory impulses impinging on a neuron are added together to generate a cumulative response. 

What are two types of neural summation?

The two types of neural summation include spatial summation and temporal summation. In spatial summation, multiple presynaptic signals are added together to generate a postsynaptic response. In temporal summation, repetitive firing from a single presynaptic neuron is added together to generate a collective response.

How does summation happen?

Summation happens by adding together multiple excitatory or inhibitory stimuli at a synapse. All these stimuli are subthreshold individually. When added up, they become suprathreshold and are able to generate a response in the neuron or target cell.

What is the importance of summation?

Summation increases the strength of the signal at the postsynaptic neuron or target cell. If it is happening at the motor end plate, summation will result in increased motor response or contraction.

References

1. Coolen, Kuhn, Sollich (2005). Theory of Neural Information Processing Systems. London, UK: Oxford University Press.
2. Bennett, Max R (2001). History of the Synapse. Australia: Hardwood Academic Publishers.
3. Purves, Augustine, Fitzpatrick, Hall, LaMantia, McNamara, White (2008). Neuroscience. Sunderland, MA USA: Sinauer Associates Inc.
4. Kandel ER, Schwartz JH, Jessell TM, Siegelbaum SA, Hudspeth (2013). Principles of Neural Science. New York: McGraw Hill. p. 229.
5. Levin & Luders (2000). Comprehensive Clinical Neurophysiology. New York: W.B. Saunders Company.
6. Carpenter (1996). Neurophysiology. London: Arnold.