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Neuronal Action Potential -
Frequency Coding in the Nervous System
We have emphasized that once the depolarization caused by the stimulus is above threshold, the resulting neuronal action potential is a complete action potential (i.e., it is all-or-nothing). If the stimulus strength is increased, the size of the action potential does not get larger (see figure). If the size (i.e., amplitude) of the action potential is always the same and independent of the size of the stimulus, how then does the nervous system code the intensity of the stimulus? The trick that the nervous system uses is that the strength of the stimulus is coded into the frequency of the action potentials that are generated. Thus, the stronger the stimulus, the higher the frequency at which action potentials are generated (see Figs. 1 and 2 below). Therefore, we say that our nervous system is frequency-modulated and not amplitude-modulated. The frequency of action potentials is directly related to the intensity of the stimulus.
Given that the frequency of action potentials is determined by the strength of the stimulus, a plausible question to ask is what is the frequency of action potentials in neurons? Another way of asking this question is how many action potentials can 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 routinely observed. Higher frequencies are also observed, but the maximum frequency is ultimately limited by the absolute refractory period. Because the absolute refractory period is ~1 ms, there is a limit to the highest frequency at which neurons can respond to strong stimuli. That is to say that the absolute refractory period limits the maximum number of action potentials generated per unit time by the axon. As described previously, the strength of the stimulus must be very high in oder to ensure that the duration of the action potential is as short as the duration of the absolute refractory period. A stronger than normal stimulus is required to overcome the relative refracctory period (see Refractory Periods for a review).
Because the absolute refractory period can last between 1-2 ms, the maximum frequency response is 500-1000 s−1 (Hz). A sample calculation is shown below with the assumption that the absolute refractory period is 1 ms in duration.
Frequency coding in the nervous system. Equation 1. Eq. 1
A cycle here refers to the duration of the absolute refractory period, which when the strength of the stimulus is very high, is also the duration of an action potential. Similarly, if the neuron absolute refractory period is 2 ms, the maximum frequency would be 500 Hz as shown below:
Frequency coding in the nervous system. Equation 2. Eq. 2
Frequency coding in the nervous system: Threshold stimulus.
Figure 1. Frequency coding in the nervous system: Threshold stimulus.
If a threshold stimulus is applied to a neuron and maintained (top, red trace), action potentials occur at a maximum frequency that is limited by the sum of the absolute and relative refractory periods (bottom, blue trace). Here, a threshold stimulus refers to that which is just strong enough to bring a resting neuron to threshold. Thus, with maintained threshold stimulus, subsequent action potentials occur only at the end of the relative refractory period of the preceding action potential. The top and bottom traces are on the same time scale. The dashed line represents the threshold voltage (Vthreshold) of approximately −50 mV. ARP, absolute refractory period; RRP, relative refractory period.
The above calculations correspond to the maximum frequency of action potentials, and would only be present if the applied stimulus is very large in order to overcome the relative refractory period. Thus, the maximum frequency of action potentials is ultimately limited by the duration of the absolute refractory period. On the other hand, if the applied stimulus is only large enough to bring the neuron to threshold at rest, the maximum frequency of action potentials will now be governed by the total duration of the neuron refractory period (i.e., sum of the absolute and relative refractory periods) (see 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:
Frequency coding in the nervous system. Equation 3. Eq. 3
Here, a cycle refers to the full duration of the action potential (absolute refractory period + relative refractory period).
Frequency coding in the nervous system: Supra-threshold stimulus.
Figure 2. Frequency coding in the nervous system: Supra-threshold stimulus.
If a supra-threshold stimulus is applied to a neuron and maintained (top, red trace), action potentials are not allowed to complete the relative refractory period (bottom, blue trace). Thus, with maintained supra-threshold stimulus, subsequent action potentials occur during the relative refractory period of the preceding action potential. With increasing stimulus strength, subsequent action potentials occur earlier during the relative refractory period of the preceding action potentials. With very strong stimuli, subsequent action potentials occur following the completion of the absolute refractory period of the preceding action potential. Thus, the maximum frequency of action potentials is ultimately limited by the duration of the absolute refractory period. The top and bottom traces are on the same time scale. The dashed line represents the threshold voltage (Vthreshold) of approximately −50 mV. ARP, absolute refractory period; RRP, relative refractory period.






Posted: Thursday, July 5, 2012
Last updated: Friday, January 17, 2014