Skeletal muscle experiments

0:00 Another way to increase muscle force, or the amount of force generated by a muscle to do work, is to take advantage of a process known as summation

0:09 Here we're taking advantage of the fact that a muscle twitch, or a muscle contraction, is a long-lasting event, in contrast to the action potential in the nerve which elicits the muscle contraction

0:20 If we were to draw these on a timescale... we'd find that the action potential in the nerve lasts about 1ms to 2ms

0:33 You'd have a delay - a *latent period* - before you start to get muscle contraction

0:42 The muscle contraction will take something of the order of 40 to 50 milliseconds just to reach its peak

0:51 And then gradually return back to rest, taking about 200ms between the start and the end

0:59 So because a muscle contraction is such a long lasting event, what we can then do is apply one stimulus to provide an action potential at that particular point

1:09 And we could produce a second action potential at this point by providing a second stimulus

1:14 In that case what we can see is here's the original muscle contraction occurring, then the second one is going to add onto it to give us an even larger muscle action potential

1:25 And that's the process we're going to be looking at in this particular part of the experiment

1:31 So we're going to take advantage of the process of summation, applying two successive stimuli, and bringing them closer and closer in time

1:39 What we'd expect to see is when the two stimuli are very far apart, let's say something like 400ms, we would get the muscle contraction to the first one, and then the muscle contraction to the second one

1:56 Because remember, each one only lasts in the order of 200ms, so if your stimuli are 400ms apart, then the two are not going to interact in any way

2:08 As we now change that interval, supposing you make the interval between the two pulses 200ms, again we would see no difference between the contraction to each of the two successive stimuli

2:27 When we decrease the interval between the two stimuli, now you are producing the second stimulus while the muscle is still undergoing the first one

2:39 Lets say we make the interval something like 100ms, now the second muscle contraction starts at about this particular point, adding on, and you see now that the muscle force generated there is greater than to an individual stimulus

2:56 If we were to decrease the interval even more, say to 50ms between the two stimuli, the sort of response we're likely to then see is a much larger amount of tension being generated

3:13 We can do that, because muscle contraction, unlike nerve potentials, does *not* have a refractory period

3:20 So in the summation part of this virtual lab, that's exactly what you will be doing; applying two successive electrical stimuli, progressively reducing the interval between them, and recording the amount of tension that you see to the second stimulus

3:39 So in this part of the experiment, we're looking at summation where we're applying two successive pulses to the nerve, and looking to see how the contractions produced by each nerve - each impulse to the nerve - results in the summation of tension

3:56 We start off with the two pulses applied 400ms apart, you've seen previous from the first video we showed you that a muscle contraction lasted about 200ms

4:08 So if we put two pulses 400ms apart, we would expect them to be completely independent, and you can see that on screen that each of the two pulses generates two different muscle twitches, and they appear to be equal in size

4:25 We then decrease the interval between the two to 200ms. Again we would expect little or no summation, since the first muscle twitch would have finished by the time the second pulse was applied

4:38 And that too happens! The two muscle contractions are equal in size

4:42 We decrease to 100ms, and now you will notice that the second muscle twitch is considerably greater than the first one, because they've started summing

4:51 And finally we decreased it to 50ms - the interval between the two pulses - and now we see a very large single muscle contraction

5:01 And that comes about from the summation of each of the two individual contractions