A twitch is the force produced from the contraction event due to one action potential.  In order to understand this, we will have to spend a bit of time talking about this contraction.

    The stimulus that leads to contraction is an action potential on the sarcolemma, right?  Well, this electrical event (the action potential) takes only one millisecond to occur.  That's right, one millisecond.  What's that?  Well, in one second (sec), there are one thousand milliseconds (msec).  So, 1 sec = 1000 msec, and one millisecond is one-thousandth of a second (1 msec = 1/1000 sec).

    If an action potential only takes 1 msec to occur, you should understand that it is a very, very quick event.  Now consider what it does to the muscle fiber it runs along.  The action potential runs along the muscle fiber sarcolemma quickly.  This triggers calcium ions to be released from the SR.  When the action potential is done, the calcium ions are taken back up into the SR.  It takes a little longer for the calcium ions to all be taken back up, and until that time, the sarcomeres will keep shortening.

    Once the calcium is back in the SR, the muscle fiber now has shortened as much as possible for this event, so it begins to relax... More ATP binds to the myosin heads, releasing them from the actin, and allowing for relaxation.  This relaxation process is slower than the contraction.

    The contraction and relaxation due to motor unit activation by one action potential is a twitch.  It is shown in Figure 9.15 of your textbook, which is reproduced here.

    Please note that the twitch is a measurement of force... "Force of contraction" is the y axis.  This would be the force measured across the entire muscle when this motor unit is activated.  Here's a bit more explanation of this for you...

    Since the entire twitch started with an action potential, and that action potential was caused by the activation of a motor neuron, all the muscle fibers that receive information from that one motor neuron have to be active at the same time.   Right?  Think back to what a motor unit is (a motor neuron and all the muscle fibers to which it is connected).  Every time a motor neuron is activated, the entire motor unit will be brought into action.  So not just one muscle fiber will contract, but all the muscle fibers in the motor unit.  That could be hundreds of muscle fibers (or just 6 or 8 in small muscles) contracting at once.

    The contraction of this many muscle fibers can be resolved by measuring the force of contraction across the entire muscle.  I have tried to show this in this figure.  Imagine stretching a muscle across two clamps, where both ends were fixed in place.  One end of the muscle is attached to a measuring device that can detect the force being applied to it; this device could tell when it was being pulled upon and provide a quantitative (numerical) read out of the amount of force acting on it.  Now, if you cause a motor neuron to that muscle to fire an action potential, all of the muscle fibers it supplies (the entire motor unit) will get excited and contract.  In this figure, the one motor neuron was activated by electrically stimulating the nerve that supplies the muscle (you will learn about the anatomy of nerves soon).  Once the motor unit has been activated, the muscle fibers of the motor unit (5 of them in this figure) will be applying force for a brief interval (shorter than 1/2 sec).  That is a twitch.

    If we could take a look at the force read-out, it would look like Figure 9.15 in your book.  This type of read-out of force is called a myogram.   For a while, before you activated the motor unit, no applied force is measured.  But then, after stimulating the motor unit, force is applied, and then weakens and stops.  A couple of details... it takes time for the stimulation to lead to calcium release and contraction to begin; that delay is the time indicated as the latent period in the figure.  Then, the contraction begins, and that is called the period of contraction, when the force increases.  This is followed by the relaxation of the muscle fibers, called the period of relaxation, where the force decreases.

    The problem with this figure is that the entire process seems just like a bump on a graph.  You need to keep in mind that this process is occurring over time, and that the x axis is time.  If you were observing this as it occurred, you would see a flat line rise and then fall.  So, I have animated Figure 9.15 from your book to show you this.  Take a look:

    The twitch is the basis for all movement.  However, it, alone, is not very useful.  The twitch occurs very rapidly, ending in less than 1/2 second.   Therefore, if that were the only way we could contract our muscles, we would drop everything we picked up in less than a second!  That would be bad.  So, instead, you will see that we combine twitches to provide for useful movement.

    You can study this entire process again by using the A.D.A.M. CD that you bought with your book, but this time working through the "Contraction of Whole Muscle" chapter.  Please do this to understand the twitch (the first slides in this chapter).

    Now that you can understand how a single twitch occurs, let's build ourselves a contraction that occurs over a longer time interval...  one that would be useful for movement.  This can be done by adding up individual twitches, in the next web page on summation.

© 2006 STCC Foundation Press
written by Dawn A. Tamarkin, Ph.D.