In this page we delve into four major concepts, all of which explain how we can produce larger amounts of force (for extended periods of time):
The first three concepts are all seemingly linked conceptually, so that's why I called this the summation page. The last concept, recruitment, is a bit different, but still explains a means to more force creation.
Intro to staircase effect, summation, and tetanus
One twitch is not enough to move well. We need steady contraction forces to hold things and move properly, not individual twitches. How do we do that? Well, if we have more than one twitch-- twitches following twitches following twitches-- we can do that. It all depends on how closely the twitches occur to one another.
Here, in Figure 9.16 from your textbook, is a drawing of what happens when twitches occur one after the other. In part (a), the twitches are evoked far apart from one another... this is what it would be like if our motor units were activated only every second, but no faster. It is thought that the force of the first few twitches continues to increase because there is some residual calcium in the sarcoplasm (not all the calcium had a chance to be reabsorbed). This is what the staircase effect is.
In part (b) the twitches are evoked by stimulating the motor neuron at a much higher frequency. Now, you see a new phenomenon. The second twitch is able to produce much more force than the first, and the third twitch produces much more than the second. Basically what is happening is that the second twitch comes so quickly that the first one never fully relaxed (and there is also some residual calcium in the sarcoplasm). So the second twitch occurs when there is still some force from the first one leftover. This shows summation of force (in other words, a building up or addition of force).
In part (c) the twitches are evoked by stimulating at such a high frequency (at least at the end of this record) that the individual twitches summate and blend into a smooth force production, called a tetanus. I hope this is clear... you can also use the A.D.A.M. Muscular CD (by Marieb), and view the chapter on "Contraction of Whole Muscle."
There are a couple of important terms in this summation stuff. When twitches summate to the point where the force produced is smooth (and useful in our movements!), this is called a fused tetanus. Some people also discuss an unfused tetanus, where the forces are summating, but the force graph still looks bumpy. It needs to be a fused tetanus to be really useful for movement.
So now you should see that in order for sarcomere shortening to be used for movements like walking and picking things up and posture, we need to cause it to occur for longer periods of time by stimulating our muscle fibers repeatedly and at short enough intervals between stimulations to produce smooth force.
Then, why even bother to have such short twitches? Because that way, we can pick and choose how long our contractions should be. We can have very fast movements, like when I type on the keyboard or blink, as well as longer movements, like for posture.
You do not have to go past slide number 13 on the Contraction of Whole Muscle chapter of the CD. Only do 14 to 18 if you want to. Also, you will see that on the CD there is a graph that looks like part (c) above, but that goes for a longer amount of time in order to show fatigue. Take a look at that on the CD, and I'll describe it a little more on the web page on different skeletal muscle types.
Finally, you can get more and more force if you stimulate more and more motor units. I hope that makes sense, since the more motor units you stimulate, the more muscle fibers you stimulate. As our muscles want to produce more force, our nervous system recruits more and more motor units. So recruitment just means that you bring more and more motor units into action. You would not want to begin a powerful force production with all the motor units you will need; that's because then you would start with a jerky motion. So, instead, we begin our movements with only small numbers of motor units, and build up our force production by adding more and more active motor units. There's actually a pattern to which motor units are activated when, but that goes beyond the scope of this class.
© 2011 STCC Foundation Press