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Before we come to know that we should try to understand what summation is! When multiple and repeated inputs occur, a combined effect of excitatory and inhibitory signals is produced which determines whether an action potential will be generated or not. This is called summation. The phenomenon in which an increase in pain is experienced by the repeated and equal-intensity noxious stimuli at a specific frequency.
Summation is of two types: spatial and temporal. Sometimes the edge voltage is enough to trigger a nerve impulse and it depends on the sum of the many individual inputs.
Temporal summation occurs when a high frequency of action potentials in the presynaptic neuron evokes postsynaptic potentials that summate with each other. It helps in stimulating Pacinian corpuscles. Synapses that are located closer to the axon hillock have a greater influence on the result of all inputs on a neuron. Temporal summation occurs when multiple subthreshold EPSPs (Excitatory Postsynaptic Potential) from one neuron combine and trigger the action potential at the axon hillock of another neuron. Postsynaptic potentials last for 4 milliseconds whereas action potentials last for only 2 milliseconds.
Temporal Summation Allows Integration of EPSP when the membrane temporarily stores the charge of the primary EPSP and therefore the charge from the second EPSP gets added to the first one. But the time for temporal summation depends if the presynaptic action potentials are close in time to each other.
Animals make use of temporal summation because it is liable for significant auditory threshold reduction where repeated signals can facilitate detection of signals. Whales increase their call duration or repeat them to enhance signal detection over long distances and during times of high background noise.
Acoustic temporal summation can be explained more simply by neural phenomena. According to Miller, different neural elements have different latencies and Loudness is correlated with the instantaneous level of neural activity within the upper centres. According to him, the ear is more of a delaying device than an integration device.
The phenomenon of ‘wind-up’ describes the abnormal temporal summation that corresponds with increasing neuronal activity by the repeated subthreshold stimulation of C-fibres which has impulses lesser than 5 Hz. Although repetitive low-intensity stimuli that acts on A-beta fibres elicit pain as there’s a summation of those impulses within the secondary neuron. Wind-up phenomenon occurs in the dorsal horn of the spinal cord in which repeated first – order neuron presynaptic inputs of different frequencies cause increased response of the postsynaptic neuron. This increased temporal summation occurs due to central sensitization. Pain research shows that there are differences in the temporal summation responses between healthy people and patients suffering from pain syndromes.
Temporal summation responds to smell and taste like the intensity of olfactory stimulants like H2S increase with a stimulus duration. For a mixed stimulus, both temporal summation and spatial summation systems contribute differently to sensations. Just like Nicotine produces different sensations like burning and stinging and they occur at different times; odorous and stinging sensation reaches a maximum after 3-5 secs whereas burning commences after 5 secs and does not end until 20 secs.
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