This blog will help biology students to understand the differences between temporal vs spatial summation. We will also see examples of temporal and spatial summation.

Neurons constantly receive electric inputs from the thousands of neurons around them. However, whether these inputs will result in action potential depends upon the types of summation of these inputs.

The summation is the process through which the excitatory and inhibitory signals are able to generate action potentials or sometimes not.

Let’s understand this in more detail.

Overview: Summation In Biology

Overview: Summation In Biology

It is a feature that results from various discrete electric inputs that cross synapses junction and axon hillock. Which are presynaptic and postsynaptic neurons. Summation occurs in all multicellular organisms having a nervous system.

Every single impulse is not enough to trigger the response. The summation of multiple electrical impulses across neuromuscular junctions collectively triggers a response or action potential.

The postsynaptic membrane needs to be depolarized before action potential is generated across the axon hillock of the neuron. When the cell membrane voltage threshold becomes more positive on the outside than outside of the cell depolarization happens.

Two types of signals regulate depolarization:

  • Excitatory Postsynaptic Potential (EPSP): EPSP signals make neurons more likely to trigger an action potential by increasing the positive charge within the post synaptic neuron. That further increases the chances of depolarization.

The threshold of the postsynaptic neuron receiving the action potential is thinned, progressively raising the membrane voltage between the presynaptic and postsynaptic neurons.  

  • Inhibitory Postsynaptic Potential (IPSP): IPSPs reduce the chances of the action potential by neurons. It lowers the membrane potential voltage between presynaptic and postsynaptic neurons. So the threshold of the postsynaptic neuron receiving the action potential is reduced.

It is like making a trigger on the firearm less sensitive and needing more force to pull. That results in ISPSs broadening the threshold away from the postsynaptic neuron which receives action potential.  

These two types of signaling methods across neurons interact with different kinds of summation.

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Types Of Summation

Types Of Summation

There are two types of summation in biology: Spatial and temporal summation. The difference between temporal and spatial summation is the number of neurons involved. Both these types use IPSPs and EPSPs to trigger the action potential at the postsynaptic neuron or axon hillock.

Spatial Summation Definition

Spatial Summation Definition

Spatial summation involves prompting the action potential in the synapses of a single neuron. The process involves receiving the action potential from axon hillocks of multiple neurons.

The postsynaptic neuron receives neurotransmitter signals from multiple presynaptic neurons, which send signals simultaneously or provide repeated inputs. As the action potential arrives simultaneously, the resulting EPSPs will summate at the axon hillock in the form of a large compounded signal.

Temporal Summation Definition

Temporal Summation Definition

Temporal summation happens when multiple action potential signals fire down the axon hillock of a single neuron. Which is further received by the synapses of the neuron. These impulses reach postsynaptic neurons one at a time and accumulate sufficient voltage to produce an adequate charge. 

Constantly arriving EPSPs combine sufficient positive charges to summate an action potential at the postsynaptic neuron’s axon hillock.

The mechanism of temporal summation achieving action potential is time-dependent. That means the action potential signals must be in rapid succession to accumulate effective charge. It happens due to signals losing voltages because they move from the synapses of the neuron to the axon hillock.

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Difference Between Spatial And Temporal Summation

Difference Between Spatial And Temporal Summation

Temporal summation is sensory summation involving the addition of a single stimuli over a short time. At the same time, spatial summation means the sensory summation that requires stimulation of various spatially separated neurons at the same time.

This is the main difference between temporal summation vs spatial summation. Let’s explore some other highlights of temporal and spatial summation.

Presynaptic Neurons

Presynaptic Neurons

Temporal summation occurs when a single presynaptic neuron is responsible for generating an action potential. While multiple presynaptic neurons are responsible for triggering the action potential in the spatial summation.

Mechanism

Mechanism

One presynaptic neuron generates sub threshold voltage in the temporal summation for a specific time period. While in spatial summation, multiple presynaptic neurons generate subthreshold. This is another difference between spatial and temporal summation.   

Efficiency

Efficiency

Efficiency is another aspect that differentiates spatial and temporal summation. Temporal summation is a less efficient mechanism as it takes some time to trigger the action potential. While spatial summation is a much more effective mechanism.

Now you understand the key differences between spatial summation vs temporal summation. You also understand the meaning of both types of summation. Now let’s move forward and explore more about the process.

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Summation Mechanism: Temporal and Spatial

The postsynaptic cell contains ion channels. These channels may open or close depending upon which excitatory neurotransmitter bind with the receptors. The process of opening and closing these ion channels creates postsynaptic potential. The postsynaptic potential has two types we discussed above.

These are excitatory postsynaptic potentials and inhibitory postsynaptic potentials. The excitatory postsynaptic potential enhances the chances of the action potential. While inhibitory postsynaptic potential decreases the chances of imitation of the action potential.    

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Excitatory Neurotransmitters (Glutamate)

Excitatory Neurotransmitters (Glutamate)

One of the best examples of an excitatory neurotransmitter is glutamate. The glutamate bind with the AMPA receptors on the postsynaptic membrane. This binding results in the influx of sodium cations and further depolarization. It is known as the excitatory postsynaptic potential or EPSP.

You must remember that for the EPSP summation to reach the threshold, a large number of excitatory inputs are required. The effect of neurotransmitters lasts longer than the presynaptic impulses. 

Inhibitory Neurotransmitters (GABA)

Inhibitory Neurotransmitters (GABA)

GABA is the primary inhibitory input in inhibitory postsynaptic potentials or IPSP. When binding to the postsynaptic neuron receptors, GABA opens up specific ion channels. These ion channels differ from those opened in EPSP.

The ion channels allow an influx of negatively charged ions or an efflux of positively charged ions. The influx ions involved here are the chloride ions. The efflux ions are the potassium ions. Both ions have a similar role in lowering the membrane potential away from the threshold potential via hyperpolarization.

Algebraic Processing Of EPSPs And IPSPs

Algebraic Processing Of EPSPs And IPSPs

In a neuron, at any point in time, it is receiving multiple simultaneous inputs of EPSPs and ISPSs. It helps to determine whether the threshold potential will be reached to trigger an action potential. The algebraic processing of the EPSPs and IPSPs is taken into account. These neurons must be receiving multiple stimuli or numerous inputs.

It can be from the multiple presynaptic neurons (Spatial Summation) or multiple inputs from a single neuron (Temporal Summation). 

The output depends upon the number of neurotransmitters. It can be an excitatory neurotransmitter like glutamate or an inhibitory neurotransmitter like GABA.

These synapses also are referred to as the decision point, where algebraic processing of the IPSPs and EPSPs determines the output.

Axon Hillock

Axon Hillock

The part of the neuron cell body that connects with the axon is called the axon hillock. It is a sparse distribution of the nasal substance. Biologists identify it in the cell membranes using light microscopy. The axon hillock connects the axon and soma of the neuron. That’s why it is the final range where membrane potentials are summed from synaptic input. Then the summation is transmitted to the axon.

In the past, biologists believed that the axon hillock was only the trigger zone for action potentials. But today, it is known that the first segment is between the unmyelinated axon segment and the axon hillock peak. This is where the action potential occurs.

The positive point of the action where action potential occurs varies in different cells. In your free time learn about online vs in person classes.

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Shunting

Shunting

Most of the time, in a neuron, the excitatory postsynaptic and inhibitory influences in a dendritic tree are close to each other. This is known as shunting. The shunting also occurs in the soma of the cell in addition to the dendrites.

Temporal summation suggests us summating these excitatory postsynaptic and inhibitory postsynaptic potentials to find the resulting output, whether the threshold potential is reached for the action potential is triggered.

Sometimes in the event of this sequence occurring in the cell’s soma, the cell resistance is altered by the inhibitory inputs or response. The cell starts leaking, and the shunt will be created to oppose the effect of the excitatory response or input.

Therapeutic Application

Therapeutic Application

If we discuss the nociceptive stimulation, summate temporally creates a repetitive painful stimuli integration. While spatial summation is nociceptive input integration from larger regions. We know many chronic diseases have symptoms of long pain durations. That means both spatial and temporal nociceptive summations are found in chronic diseases. 

Many experiments have proved that temporal summation of nociceptive input is facilitated through spatial summation. Hence, the best way to treat chronic pain is to direct the treatment towards both temporal and spatial summation of pain.

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Similarities Between Temporal Vs Spatial Summation

Similarities Between Temporal Vs Spatial Summation

While both types of summation have different mechanisms and significant differences. There are some similarities between the spatial vs temporal summation, as listed below:

  • Temporal and spatial are the two types of summation mechanism that occurs in the sensory neurons of the nervous system.
  • Multiple stimuli are responsible for generating an action potential in one presynaptic neuron.
  • Every stimulus here is subthreshold and together forms a superthreshold through summation.

Conclusion

Temporal summation is the mechanism in the nervous system where a single presynaptic neuron generates a superthreshold. It happens over a period of time and triggers the action potential on postsynaptic neurons. While spatial summation is a different summation process. Here, multiple presynaptic neurons generate the threshold, which further causes the action potential on the postsynaptic neuron.

Both types of summation mechanisms have significant differences. The major among them is the multiple stimuli type involved in each type of summation. However, both of them have a few similarities also.

We hope you enjoyed this educational blog on temporal and spatial summation. For more such educational and informative blogs, always be in connection.

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Frequently Asked Questions

1. What’s the difference between temporal summation and spatial summation?

Spatial summation happens when multiple weak signals from different locations are converted into a single large signal. While temporal summation generates a rapid series of weak pulses from a single source to a large signal.

2. What is an example of temporal summation?

Temporal summation suggests us summating the excitatory postsynaptic and inhibitory postsynaptic potentials to determine the resulting output. To determine whether the threshold potential is reached and an action potential is initiated. Intensity is measured as energy per unit of time. This energy is integrated over time, which can be known as temporal summation.

3. What is a spatial summation example?

Suppose you have a color monitor. Get a magnifying glass and look closely; you will see a whole bunch of red, green, and blue dots that make all you see on the screen. This is the perfect example of spatial summation.

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George Orwell
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