Sometimes, a single excitatory postsynaptic potential EPSP is strong enough to induce an action potential in the postsynaptic neuron, but often multiple presynaptic inputs must create EPSPs around the same time for the postsynaptic neuron to be sufficiently depolarized to fire an action potential. Summation, either spatial or temporal, is the addition of these impulses at the axon hillock. One neuron often has input from many presynaptic neurons, whether excitatory or inhibitory; therefore, inhibitory postsynaptic potentials IPSPs can cancel out EPSPs and vice versa.
Neurons are able to affect one another in a number of ways. Defacilitation is the removal of excitatory inputs which can facilitate a pathway. Disinhibition on the other hand is the inhibitory input removal. Multiple source inputs on a neuron can be summated spatially, however the inputs must be closely spaced so that none of the early inputs decay. If a neuron is receiving multiple inputs from a single source in the required close time interval so that no input is decayed, then these inputs may summate temporally.
Another very important factor in determining whether the threshold potential is reached and an action potential is achieved or not is the distance between the synapse and the neuronal cell body. The closer the distance between the synapse and cell body, the greater the summation and the more likely it is to elicit an action potential. As we know the means of travel for a postsynaptic neuron is a dendrite. These dendrites have few voltage gated ion channels.
Hence, on reaching the neuronal cell body, the postsynaptic potential attenuates. This neuronal cell body summates these incoming potentials. An action potential is then elicited on transmission of the net potential to the axon hillock. When inputs from multiple neurons trigger an action potential, this is called as spatial summation. These potentials are most commonly from dendrites, we add these inputs together to get the spatial summation. The greater the number of excitatory post synaptic potentials, the greater the chances of the potential achieving the threshold potential to elicit an action potential.
Similarly, the greater the number of inhibitory postsynaptic potentials, the lesser the chances of reaching the threshold potential to generate an action potential. The chances of eliciting an action potential are also significantly influenced by how close the dendritic input is to the axon hillock. The closer the dendritic input is to the axon hillock, the more likely it is to cause an action potential.
Shunting of an excitatory postsynaptic potential is the is the nullification of an excitatory inputs by the spatial summation of inhibitory inputs. When a large amount of presynaptic neuron action potentials triggers postsynaptic action potentials that summate with one another, this is called as temporal summation. In this case, the interval between action potentials is less than the postsynaptic action potential duration.
The summation may be increased if the cell membrane time constant is long enough. When the next postsynaptic potential starts, the amplitude of the previous postsynaptic potential will summate with it producing a greater potential increasing the likelihood of reaching the threshold potential.
The postsynaptic cells contain ion channels, these ion channels may open or close depending on which neurotransmitter bind to the receptors. There are 2 types of postsynaptic potentials. An excitatory postsynaptic potential is that which increases the chances of initiating an action potential. Similarly inhibitory postsynaptic potentials decrease the chances of an action potential being initiated. Furthermore, a single presynaptic neuron is responsible for generating the action potential in temporal summation while multiple presynaptic neurons are responsible for generating the action potential in spatial summation.
One presynaptic neuron generates subthresholds over a certain period of time in temporal summation while multiple presynaptic neurons generate subthresholds in spatial summation. Hence, this is another difference between temporal and spatial summation. Efficiency is another difference between temporal and spatial summation. Temporal summation is a less efficient process as it takes time to generate an action potential while spatial summation is an efficient mechanism.
Temporal summation is a summation process in the nervous system in which subthresholds generated by a single presynaptic neuron over a period of time are responsible for the generation of an action potential on the postsynaptic neuron. If all of the EPSPs are subthreshold then an action potential will not be fired once they reach the neurone individually.
Spatial summation in this example is known as postsynaptic inhibition. This happens when the summation of graded potentials originates from one presynaptic neurone or in other words, the signals overlap reaching a postsynaptic neurone. If a subthreshold EPSP reaches the neurone then no action potential will be generated, however, if multiple subthreshold EPSPs reach the neurone trigger zone close enough together in time then the two subthreshold EPSPs will sum up to cause a suprathreshold EPSP and an action potential will be generated.
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