Neural Integration: Temporal and Spatial Summation

Neurons conduct signals to other neurons where synapse acts solely as conveyers of information. With the aid of various forms of synaptic activity, a single neuron can convey an array of different signals to the neural circuit.

Neural Integration

The neural communication is not usually only between one pre-synaptic neuron to a single post-synaptic neuron. But a neuron receives signals from hundreds and thousands of neurons (convergence) and a neuron, its axon with several collaterals communicates with several other neurons (divergence).

Hence one synapse doesn’t solely determine the generation of action potential but all the signals arriving from all the synapses add up. This summation process is called neural integration which occurs at the axon hillock.

Basic principle: If the membrane potential is depolarized to the threshold value, generation of action potential occurs. However, if potential lies below the threshold no generation of action potential occurs.

Summation: It can be defined as the phenomenon in which excitatory and inhibitory signals work together to generate an action potential. For the activation of the action potential, a limit of voltage has to be reached known as threshold voltage which depends upon the individual inputs to the neurons.  A single EPSP is not usually of sufficient magnitude to reach the threshold potential by depolarizing the neuron.

The inputs can be of two different types:

Excitatory neurotransmitter: It causes depolarization which increases the chances of initiating an action potential. The potential is termed as Excitatory postsynaptic potential (EPSP). Examples of excitatory neurotransmitters include glutamate, epinephrine, and norepinephrine. It is responsible for increasing the membrane potential causing sodium ion influx through sodium ion channels.

Inhibitory neurotransmitter: It causes hyperpolarization decreasing the chances of an action potential being initiated. The potential they generate is termed Inhibitory postsynaptic potential (IPSP). Examples of inhibitory neurotransmitters generating IPSP are GABA (Gamma-aminobutyric acid), glycine, and serotonin. They decrease the membrane potential causing chloride ion influx or potassium ion influx through chloride or potassium ion channels respectively.

Any neuron at any point in time will be receiving numerous EPSP and IPSP inputs simultaneously. To determine output whether the threshold potential will be reached and whether an action potential will be elicited or not, the algebraic processing of these EPSPs and IPSPs must be taken. These neurons will be receiving numerous inputs whether it be from multiple neurons (spatial summation) or multiple inputs from one single neuron (temporal summation).

Note

Defacilitation: removal of excitatory inputs

Disinhibition: removal of inhibitory inputs

Whether the membrane potential reaches a threshold value to elicit an action potential can be determined by adding up the signals received by post-synaptic neurons and the nature of the signal coming. This can be determined by adding each signal from two different types of summation.

1. Spatial summation
2. Temporal summation

Spatial Summation

{spatium – space; graded potentials originate at different locations (spaces) on neuron}

• Here, the integration of signals from multiple neurons triggers an action potential, the potential mostly transferring from dendrites to other neurons. 
• It occurs simultaneously but at different locations. 
• The higher number of excitatory postsynaptic potentials (EPSP) increases the probability of achieving the threshold potential to elicit an action potential. 
• Spatial summation cannot always be excitatory. Sometimes the release of inhibitory neurotransmitters prevents the generation of action potential. The greater the number of inhibitory postsynaptic potentials (IPSP), the less the chance of eliciting the action potential.
•  Additionally, the closeness of the dendritic input influences the threshold reach. The closer the dendritic input, the more likely it is to cause an action potential.
• Spatial summation occurs when multiple pre-synaptic neurons release neurotransmitters (e.g. acetylcholine), which together can exceed the threshold of the postsynaptic neuron.
• Two or more postsynaptic potentials originating from different synapses are generated at approximately the same time, such that when they spread to the axon hillock, they overlap and sum.
• Example: Neurons A and B release insufficient neurotransmitters but when combined threshold is exceeded to create an action potential. Each neuron’s EPSP is too weak to trigger an action potential by itself, but if the two presynaptic neurons fire simultaneously, the sum of the two EPSPs is above the threshold and creates an action potential. Sometimes, if IPSP counteracts EPSP, might create an integrated signal below the threshold resulting in the absence of action potential at the trigger zone.

Temporal Summation

{tempus: time; two stimuli occur closer together in time; graded potential overlapping in time}

• It is the cumulative effect of multiple stimuli arriving at a neuron over time. When two stimuli occur closer in time, the graded potential overlaps, enhancing the depolarization effect.
• Temporal summation occurs when the neurotransmitter is released several times by a single pre-synaptic neuron over a period. The repeated release leads to the accumulation of neurotransmitters in the synaptic cleft, surpassing the threshold for generating an action potential.
• Higher frequencies result in a quicker accumulation of neurotransmitters, leading to faster threshold exceedance and action potential initiation.
• Two subthreshold graded potentials from the same presynaptic neuron can summate if they arrive at the trigger zone closely enough in time. This summation occurs when the depolarization caused by the second potential adds to the first, reaching the threshold for action potential generation.
• Temporal summation leads to excitatory potentials exceeding the threshold for action potential initiation. Similarly, inhibitory postsynaptic potentials can also summate temporally, resulting in hyperpolarization and inhibition of action potential firing.
• While multiple inputs on a neuron can be summed up easily, temporal summation requires inputs to be closely spaced in time to prevent decay. Inputs from a single source can summate temporally.
• Closer proximity allows for greater summation, increasing the likelihood of eliciting an action potential.

Differences Between Spatial summation and Temporal summation

References

• Byrne, J. H. (2014). Postsynaptic potentials and synaptic integration. From molecules to networks. (pp. 459-478). Academic Press. https://doi.org/10.1016/B978-012148660-0/50017-2
• How Neurons communicate- Signal. Libre Texts BIology. Retrieved from https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/35%3A_The_Nervous_System/35.07%3A_How_Neurons_Communicate_-_Signal_Summation. Accessed on 4^th April 2024.
• Moini, J., Avgeropoulos, N., & Samsam, M. (2021). Cytology of the nervous system. Epidemiology of brain and spinal tumors. (pp 41-63).Academic Press. https://doi.org/10.1016/B978-0-12-821736-8.00012-1
• Silverthorn, D.U. (2019). Human Physiology: An integrated approach. 8^th Edition. Pearson.
• Spatial and Temporal Summation. Retrieved from https://uen.pressbooks.pub/introneuro/chapter/spatial-and-temporal-summation/. Accessed on 5^th April. 2024.
• Summation in Biology. Definition, Types and Examples. Retrieved from https://study.com/academy/lesson/temporal-vs-spatial-summation-biology-overview-differences-examples.html. Accessed on 2^nd April. 2024.
• Temporal and spatial summation. A level Biology. Retrieved from https://alevelbiology.co.uk/notes/temporal-and-spatial-summation/. Accessed on 4^th April 2024.