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Home > Lecture Notes on Physiology > Physiology of the Nervous System > Physiology of Neurons > Neuronal Action Potential > Frequency Coding in the Nervous System
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We have actually emphasized that when the depolarization brought about by the stimulus is above thresorganize, the resulting neuronal activity potential is a finish action potential (i.e., it is all-or-nothing). If the stimulus stamina is increased, the size of the activity potential does not acquire larger (view figure). If the dimension (i.e., amplitude) of the action potential is constantly the exact same and also independent of the dimension of the stimulus, how then does the nervous mechanism code the intensity of the stimulus? The trick that the nervous mechanism supplies is that the stamina of the stimulus is coded into the frequency of the activity potentials that are created. Hence, the more powerful the stimulus, the higher the frequency at which activity potentials are produced (see Figs. 1 and 2 below). Thus, we say that our nervous system is frequency-modulated and not amplitude-modulated. The frequency of activity potentials is directly pertained to the intensity of the stimulus.
Given that the frequency of action potentials is figured out by the toughness of the stimulus, a plausible question to ask is what is the frequency of activity potentials in neurons? Anvarious other way of asking this question is just how many type of action potentials have the right to a neuron generate per unit time (e.g., action potentials per second)? Physiologically, action potential frequencies of up to 200-300 per second (Hz) are frequently oboffered. Higher frequencies are likewise observed, but the maximum frequency is inevitably limited by the absolute refractory duration. Because the absolute refractory duration is ~1 ms, tright here is a limit to the highest possible frequency at which neurons have the right to respond to strong stimuli. That is to say that the absolute refractory duration borders the maximum variety of action potentials generated per unit time by the axon. As defined previously, the strength of the stimulus should be extremely high in oder to ensure that the duration of the activity potential is as short as the duration of the absolute refractory duration. A stronger than normal stimulus is compelled to get over the family member refracctory period (see Refractory Periods for a review).
Because the absolute refractory duration deserve to last in between 1-2 ms, the maximum frequency response is 500-1000 s−1 (Hz). A sample calculation is shown below via the presumption that the absolute refractory duration is 1 ms in duration.
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Eq. 1

A cycle right here describes the duration of the absolute refractory period, which once the toughness of the stimulus is extremely high, is also the duration of an action potential. Similarly, if the neuron absolute refractory duration is 2 ms, the maximum frequency would be 500 Hz as shown below:
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Eq. 2

If a thresorganize stimulus is used to a neuron and preserved (top, red trace), action potentials happen at a maximum frequency that is limited by the amount of the absolute and loved one refractory periods (bottom, blue trace). Here, a threshost stimulus refers to that which is just solid sufficient to carry a relaxing neuron to threshost. Thus, with kept threshost stimulus, subsequent activity potentials happen just at the finish of the family member refractory period of the coming before action potential. The top and also bottom traces are on the exact same time range. The daburned line represents the threshold voltage (Vthreshold) of approximately −50 mV. ARP, absolute refractory period; RRP, family member refractory duration.
The over calculations correspond to the maximum frequency of action potentials, and also would only be current if the used stimulus is incredibly big in order to conquer the family member refractory period. Therefore, the maximum frequency of action potentials is inevitably restricted by the duration of the absolute refractory period. On the various other hand, if the used stimulus is just big sufficient to carry the neuron to threshost at remainder, the maximum frequency of activity potentials will currently be governed by the total duration of the neuron refractory duration (i.e., sum of the absolute and also loved one refractory periods) (check out Fig. 1). In a typical neuron, this is 1 + 4 = 5 ms. Under this condition, the maximum frequency of action potentials is 200 Hz as shown below:
Here, a cycle refers to the complete duration of the activity potential (absolute refractory period + family member refractory period).

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If a supra-threshost stimulus is applied to a neuron and also maintained (top, red trace), action potentials are not enabled to complete the relative refractory period (bottom, blue trace). Therefore, with kept supra-threshost stimulus, subsequent activity potentials take place during the family member refractory duration of the preceding activity potential. With enhancing stimulus strength, subsequent action potentials take place previously throughout the loved one refractory duration of the preceding activity potentials. With incredibly strong stimuli, succeeding activity potentials take place adhering to the completion of the absolute refractory duration of the coming before activity potential. Hence, the maximum frequency of activity potentials is ultimately limited by the duration of the absolute refractory duration. The peak and also bottom traces are on the same time scale. The daburned line represents the threshold voltage (Vthreshold) of around −50 mV. ARP, absolute refractory period; RRP, family member refractory duration.
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