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The term “summation” is used to describe postsynaptic potentials. There are two types of summation: spatial and temporal. Both EPSPs and IPSPs can be involved in the phenomenon of summation. This paper studies the involvement of EPSPs and IPSPs in spatial and temporal summation.
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The term “summation” is referred to when used to describe postsynaptic potentials. Spatial summation is a phenomenon of neural integration which means that the postsynaptic cell’s membrane potential is a result of the combined activity of EPSPs and IPSPs that are almost simultaneously caused by two or more synapses (Reece et al. p.G-33). Temporal summation is a phenomenon of neural integration which means that the postsynaptic cell’s membrane potential in a chemical synapse is a result of the combined activity of EPSPs or IPSPs that are produced in rapid succession (Reece et al. p.G-34).
The body of the cell and the dendrites of one postsynaptic neuron are able to receive inputs from chemical synapses that may have hundreds or sometimes thousands of synaptic terminals. The magnitude of the postsynaptic potential for each of the synapses is different depending on certain factors, for instance, the volume of neurotrnsmitter that is produced in the presynaptic neuron. As it is a graded potential, a postsynaptic potential is always lower when the interval from the synapse becomes longer. As a result, when only one excitatory postsynaptic potential (EPSP) at last comes to the axon hillock, it is often too low to lead to an action potential in a postsynaptic neuron. Therefore, excitatory postsynaptic potentials (EPSP) are involved in the activity of postsynaptic potentials. In some cases, two EPSPs can take place at only one synapse in a very fast succession. It means that the membrane potential of postsynaptic neuron has not come back yet to the remained potential before another EPSP came to it. When such situation takes place, the EPSPs are combined, and this effect is known as temporal summation. In addition, EPSPs that are produced almost at one moment by different synapses on one postsynaptic neuron are also able to combine together. This effect is known as spatial summation. Due to spatial and temporal summation, several EPSPs are able to add their effects in order to depolarize the membrane after the axon hillock to the threshold. As a result, this makes the postsynaptic neuron produce an action potential (Reece et al. p.1056).
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The inhibitory postsynaptic potentials (IPSP) can be also involved in thhe phenomenon of summation. The process is the following. Several IPSPs that occur almost simultaneously at synapses in the same region or in fast succession at the same synapse produce a higher hyperpolarizing effect in comparison with only one IPSP. Due to the process of summation, an inhibitory postsynaptic potential is also able to counter the influence of an excitatory postsynaptic potential. The cooperation of multiple excitatory and inhibitory inputs is the necessary part of the nervous system integration. The axon hillock is the integrating center of the neuron. It is the region in which the potential of membrane at each moment is a result of the combined activity of all IPSPs and EPSPs. The potential of membrane at the axon hillock can reach the threshold any moment. However, an action potential can be produced anytime, and it moves through the axon to the respective synaptic terminals. When refractory period finishes, the neuron can cause one more action potential, and the potential of the membrane at the axon hillock reaches the threshold again (Reece et al. p.1056).
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In conclusion, the term “summation” is used to describe postsynaptic potentials. There are two types of summation, spatial and temporal, which can involve both EPSPs and IPSPs combining their effects.
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