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Home > Lecture Notes on Physiology > Physiology of the Nervous System > Physiology of Neurons > Neuronal Action Potential > Refractory Periods
You are watching: Why is it harder to generate a second action potential
As stated in the last section, opening of the Na+ channels, spontaneously and quickly leads to their inactivation. At the optimal of the action potential, all Na+ channels come to be intriggered. When Na+ networks are intriggered, they cannot be automatically opened up again (view figure on Na+ channel inactivation). Recovery from inactivation is a time- and voltage-dependent procedure, and also full recoincredibly generally takes around 3-4 ms. Thus, it takes about 3-4 ms for all Na+ networks to come out of inactivation in order to be all set for activation (opening) aget. The period from the initiation of the activity potential to immediately after the optimal is referred to as the absolute refractory period (ARP) (watch Figs. 1 and 2). This is the moment during which an additional stimulus provided to the neuron (no issue just how strong) will not lead to a second action potential. Thus, bereason Na+ channels are inset off in the time of this time, extra depolarizing stimuli perform not bring about brand-new activity potentials. The absolute refractory duration takes around 1-2 ms.
Throughout the absolute refractory duration, a 2nd stimulus (no issue how strong) will not exmention the neuron. Throughout the family member refractory period, a stronger than normal stimulus is needed to elicit neuronal excitation.
After the absolute refractory duration, Na+ networks start to recoup from inactivation and if strong sufficient stimuli are provided to the neuron, it may respond again by generating activity potentials. However before, in the time of this time, the stimuli provided must be more powerful than was initially needed once the neuron was at rest. This case will certainly continue until all Na+ channels have actually come out of inactivation. The period throughout which a more powerful than normal stimulus is needed in order to elicit an action potential is described as the family member refractory duration (RRP). Throughout the loved one refractory period, because pK remains above its relaxing worth (view number on timecourse of pK in the time of the action potential), continued K+ flow out of the cell would certainly tend to oppose any type of depolarization brought about by opening of Na+ networks that have actually respanned from inactivation.
Considering the excitcapability of the neuron adhering to an action potential, it deserve to be checked out that the neuron is not excitable at all throughout the absolute refractory period, yet, neuronal excitcapability recovers in a time-dependent (and also additionally voltage-dependent) manner follwoing the absolute refractory period (Fig. 2). As pointed out above, the duration automatically following the absolute refractory duration till neuronal excitcapability is similar to that for a relaxing neuron is the family member refractory period. If the neuron is created through a stimulus strong enough only to lug a resting neuron to threshost, the neuron will only respond as soon as the relative refractory duration is over (i.e., the neuron is back to its resting state). Howerver, during the family member refractory period, the neuron have the right to be excited if a stronger than normal stimulus is applied. The stamina of the stimulus needed to expoint out the neuron throughout the family member refractory duration is extremely high initially instantly adhering to the finish of the absolute refractory period, however decreases throughout the family member refractory duration until it reaches that required to expoint out a neuorn at rest (i.e., at the finish of the loved one refractory duration (Fig. 3).
During the absolute refractory duration, the neuron cannot be excited to generate a 2nd activity potential (no issue just how intense the stimulus). As Na+ networks begin to recuperate from inactivation, excitcapacity is slowly restored. This recoincredibly duration is the family member refractory period throughout which a stronger than normal stimulus is needed to initiate a new activity potential.
Figure 3. Threshost stimulus toughness required to elicit an activity potential in the time of the relative reftractory duration.
No stimulus, no matter exactly how strong, will carry the neuron to threshost in the time of the absolute refractory duration. Throughout the loved one refractory duration, the neuron have the right to be excited with stimuli stronger than that needed to lug a relaxing neuron to threshold. The strength of of the stimulus required is extremely high at an early stage in the family member refractory period and slowly becomes smaller throughout the family member refractory duration as Na+ networks recover from inactivation and as K+ permeability returns to its relaxing level (view figure). At the finish of the relative refractory duration, once the neuron is earlier to its resting state, the stimulus stamina is at the minimum level forced to lug a resting neuron to threshost (daburned line).
In summary, inactivation of Na+ networks is exclusively responsible for the absolute refractory period. Both Na+ channel inactivation and also greater than relaxing pK worth are responsible for the family member refractory period.
The absolute refractory duration is responsible for setting the upper limit on the maximum variety of action potentials that deserve to be generated in the time of any kind of provided time period. In other words, the absolute refractory period determines the maximum frequency of activity potentials that can be created at any kind of suggest alengthy the axon plasma membrane. This action potential frequency, in turn, has actually necessary physiological effects for exactly how the nervous mechanism have the right to respond to high-frequency stimuli, and likewise for the capability of the nervous system to sfinish high-frequency signals to effector organs as soon as necessary (check out Frequency Coding in the Nervous System).
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One last note around the refractory period is in order. As stated before, the numbers reported in these lectures for miscellaneous physiological processes correspond to what has been established to be the "norm" or the best-studied instance of the procedure. Although we have actually reported the refractory period to be 3-4 ms lengthy, it should be noted that the hyperpolarization phase can last as much as 15 ms in some neurons. In these neurons, therefore, the family member refractory period is a lot longer.
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