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Let's say that the threshold potential of a neuron is +40 mV to make a muscle twitch. Now suppose we have two signals of +20 mV. So, this signal will cause the twitch in muscle because it has reached the threshold potential.

Suppose that we have many signal of +20 mV. Will this cause a lot of twitch, the same twitch or no twitch at all in the muscle? In other words, how will the twitch be greater?

Please be a little simple.

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Are the stimuli arriving all at the same time or sequentially? If the latter, how far apart? You may want to look at this: en.wikipedia.org/wiki/Summation_%28neurophysiology%29 –  nico Aug 3 '13 at 8:13
    
Is the question something like: Given the mecanism of threshold potential, how can we produce various ammount of intensity of muscle contraction? –  Remi.b Aug 3 '13 at 9:52
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I think you are asking about cooperativity. In many biological systems the action of many individuals are coupled together such that when an individual is activated, the others are more likely to act as well.

The classic example of cooperativity is hemoglobin a protein which has four sites to bind oxygen (one in each of four protein subunits). When there is enough oxygen present for a single binding site to bind oxygen, the other three will have an increased affinity for oxygen. This makes the binding curve relatively steep. Take a look at the graph.

saturation of hemoglobin by oxygen

When one oxygen is bound, three others also become bound. The more individuals in a cooperative system, the sharper the curve becomes until a very small change in a few individuals creates a very large effect very quickly. "all or nothing" as you say.

Muscle contraction also is cooperative. In this case there are many individual muscle fibers, both the myosin thick filaments, and the actin thin filaments. Each fiber consists of chains of interwoven individual proteins. When a myosin individual protein acts against a thin filament other myosins in that filament and those on other thick filaments that are bound to the same thin filament will react to the change in tension and also be more likely to activate and pull. This causes the entire muscle to quickly start to contract together. Most researches feel that all the various other factors contributing to muscle contraction, calcium release and uptake, other proteins that are associated with the thick and thin filaments etc, are cooperative as well. A search for 'muscle contraction cooperativity' will give many references like these.

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