|MadSci Network: Neuroscience|
This is an interesting question and I suspect that you may have been misled slightly from what you have read, although what you say IS essentially true. I am sorry if this answer seems a little long but there were many components to your question that all need to be dealt with individually.
As an introduction you may wish to read the answer to the question: How do cells generate electricity to give you some information on how cells are able to generate electrical currents and some other information you may find useful about How electical currents travel along a nerve.
Muscles are stimulated by electrical impulses called action potentials that travel down a nerve to the muscle. An action potential is similar to the flow of electricity down a wire. However, the major difference between a nerve and a wire is a nerve will only transmit a single packet of electric charge at a time. When the nerve is stimulated, there is a threshold of current required to activate the nerve before it is activated, and this is where your threshold comes in. Once the threshold current is achieved the nerve is activated and it will pass the message in the form of the electrical current on to the muscle. Even if the nerve is activated by 10 times the amount of current required to reach the threshold potential the nerve will still only carry the same amount of current to the muscle as it would if it had just reached the threshold current. This is because the amount of current passed by the nerve is determined by the ionic state of the nerve itself rather than the type of stimulus.
Between the nerve and the muscle is a neuromuscular junction. This is a very short gap between the end of the nerve and the muscle of about 10 nanometres or 0.00001 millimetres. Each nerve action potential stimulates the release of a specific amount of chemical transmitter called acetlycholine. This acetlycholine diffuses across this short gap and activates the muscle. This results in setting up a second action potential within the muscle, which propagates into the muscle fibres. The effect of this is that it stimulates the release of calcium ions within the cell, which activate the muscle fibres and starts the biomechanical interactions between the proteins in the muscle that cause contraction.
However, a single action potential will only activate the skeletal muscle fibres for about 0.002 seconds. During this time the muscle fibres will not be able to generate very much force before the action potential decays, calcium is removed from the cell and the fibre relaxes again. A single action potential will result in a single twitch of the muscle fibres of a similar duration, which is the all or none phenomenon that you describe.
Summary so far: A SINGLE NERVE IMPULSE WILL RESULT IN A SINGLE TWITCH OF THE MUSCLE FIBRES.
To perform the sort of contractions of the muscle that you think of as muscular contractions the muscle needs to be activated repeatedly. Each action potential will cause a twitch, but since the fibre is still partially activated from the previous action potential the subsequent twitch will be greater. Consecutive twitches add together until the twitches fuse together to form a 'tetanus'. This is how we think of our skeletal muscles as working. We do not feel individual twitches of the fibres caused by individual action potentials because the muscle is being repeatedly stimulated to give the smooth controlled contractions that we are used to.
See figure for an illustration of the effects of action potentials on muscle.
For an example, for you to lift a weight with your arm using your bicep muscle your brain will send a stream of impulses down a certain number of nerves to your muscle to activate a certain number of muscle fibres so your muscle can contract and you can lift the weight.
If you now double the weight that you want to lift your brain will send a stream of impulses down more nerves so that more muscle fibres will be activated and you will still be able to lift the heavier weight in a controlled manner.
So under conditions where the muscle is being maximally activated the control of contraction is by how many muscle fibres are activated to perform the necessary work.
Cardiac muscle still obeys the law of all or none, but it employs a different form of regulation and you only see the individual twitches and never get the tetanus situation.
The action potential generated in the heart muscle after the nerve impulse has passed across the neuromuscular junction is different from the action potential seen in skeletal muscle (see bottom part of the above figure). Following the upward phase of the potential there is a prolonged plateau phase before the potential reverts back to the baseline. This plateau phase is caused by the flow of calcium into the muscle, which as I mentioned above is necessary to activate the biomechanical processes of muscle contraction. A single cardiac muscle action potential lasts for about 0.25 seconds (compared with 0.002 seconds for a skeletal muscle action potential). Therefore, a single action potential initiates a contraction of the cardiac muscle. There are a number of processes inside the cell that are involved with regulating the total amount of calcium that enters the cell during a single cardiac action potential. This regulates how much the muscle is activated and how much force the heart muscle generates each heartbeat. Therefore, whilst the muscle is either activated or not activated it is not contracting maximally each time.
Smooth muscle is different again. I am not sure that such a rule could be applied to smooth muscle. The mechanisms involved in the regulation of smooth muscle are different and whilst contractions do occur as a response to calcium the rate of response is much slower and the muscles continue to be activated long after the calcium has been removed from the cell.
I hope that I have managed to clarify the situation for you and I apologise for making the answer so long but I found it an interesting question to answer. If you have any questions about what I have written you can contact me.
Best of luck
As a reference any good general physiology text book should provide you with further information. I used this reference to help me answer your question.
R M Berne and M N Levy. Physiology Fourth Edition 1998.
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