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Medical Stimulus detection during relative refractory period

  1. Sep 26, 2005 #1


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    I have a question about how neurons respond to stimuli during the relative refractory period of an action potential. During the relative refractory period, it is relatively more difficult for a neuron to undergo another action potential due to hyperpolarization, but it is possible. In general, the stronger the stimulus, the more frequently action potentials will fire, and thus frequency of neural firing encodes stimulus intensity.

    My question is as follows (in reference to the attached graph). Suppose a given neuron is exposed to two kinds of stimuli, call them S and W (for relatively strong and weak, respectively). S has a higher intensity than W, but S is intermittent at high frequency whereas W is constant. Might it be the case that for some intensty values of W and S, W actually causes the neuron to fire more rapidly than does S? If so, might this be a source of intensity coding error?

    The reasoning behind this question is that, since W's application is constant, it might be sufficient to cause another action potential early in the refractory period, e.g. at point A on the graph (although, suppose it is too weak to cause an action potential at point B). Meanwhile, S is sufficient to cause an action potential at any point during the relative refractory period, but because it is somewhat intermittent it might not cause an action potential until later on in the period, e.g. at point B.

    More generally, I am wondering to what extent the relative refractory period "overshoot" is really a limiting factor in the frequency of neural firing. The idea seems to be that overshoot puts a limit on how frequently a given stimulus can cause a neuron to fire, but on the other hand it seems like the stimulus might be able to "cut in line" so to speak by effecting an action potential early on in the relative refractory period.

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  3. Sep 26, 2005 #2


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    Yes, but does it happens in normal conditions?

    Yes, it is. You are thinking with inadequate means since you use electrical signals but the "negative" curve means an absence of positive ions inside the axon. This absence is like a car without gas. ions are the engine of action potential.

    BTW, what do you mean by constant? DC? if so, some neurons may fire automatically with DC but it does not change the response. the speed of occurence of action potential are liknked to ions channels and ions, not to electricity.

    Of course, neurons solve the problem of synchrony with pre and post synaptic connections.
  4. Sep 26, 2005 #3
    If a stimulus is strong enough (and of long enough duration) it can make the neuron fire after every absolute refractory period. How the firing rate exactly depends on the strength and the frequency of the stimulus is of course a complicated matter, depending on when exactly the stimulus is high and low, and also on how far from the axon the stimulus affects the neuron.

    Also a stimulus of high frequency will build up, it will keep depolarizing the neuron, giving it little time to get move somewhat towards its resting potential so it will be equivalent to a constant stimulus of a level that is a little lower than the peaks of the high frequency stimulus.
  5. Sep 27, 2005 #4


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    Do you mean that an action potential has not a definite amplitude at a same site?
  6. Sep 27, 2005 #5
    No I mean that that a stimulus/presynaptic-neuron that is active repetitively will keep depolarizing the postsynaptic neuron, not giving it enough time to get back to its resting potential in between repetitive excitations, and thus building up the postsynaptic membrane potential.
  7. Sep 27, 2005 #6


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    Sorry, I'm lost with your reply since it was not asked, in my view, any pre/post synaptic consequence but a simple axonal stimulation?
    I thought that we were speaking about a single neuron behaviour?
  8. Sep 27, 2005 #7
    An action potential will run along an axon if the membrane potential rises above a certain threshold at the origin of the axon (the axon’s hillock). The membrane potential at that point depends on everything that influences the membrane potential of the neuron. For a sensory cell that will be the sensory stimulus, and for an intermediate neuron it will be the presynaptic neuron(s).
  9. Sep 27, 2005 #8


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    Not sure. For many reasons. You cited a sensory neuron and if we take the example of a C fibre, origin is peripheral and AP occurs in the axon but the axon threshold was never met in the "free" endings of the fibres.

    Secondly, all AP are concerned by refractory periods. It may be interesting to understand why there is such refractory periods?
  10. Sep 27, 2005 #9
    Every neuron has incoming stimulation and based on that it will or will not release action potentials in each of its axons. Whether an action potential is released along an axon depends on whether a threshold is reached at the origin of that axon, it does not matter what the membrane potential is at the free endings (of the dendrites).

    Yes every axon can only release a new action potential after its refractory period, but if the membrane stays depolarized a new action potential is released sooner after that period, giving rise to a higher spike rate.
    (the reason for the refractory periods is clear: because of the slow closure of the K-channels)
  11. Sep 27, 2005 #10


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    hmm, it's just a part of the response IMHO.
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