Muscle fibre recruitment
One important way to grade the amount of force that a muscle can produce (if you want to do different amounts of work) is to activate more of the muscle fibres (the muscle cells) that make up the muscle. This can be done by the brain activating more and more of the motor neurons to that muscle. Each motor neuron controls a different set of the muscle fibres of the muscle, and thus activating more motor neurons to the muscle activates more muscle fibres.
You will see in the video that this activation of motor neurons occurs in a systematic order. Each motor neuron going to a muscle does not contact and control the same number of muscle fibres - the number can vary. Some motor neurons control only a small number of muscle fibres and activating any of these motor neurons will only result in the muscle producing a small amount of force (appropriate for lifting a small weight). Other motor neurons control a larger number of muscle fibres and activating any of these motor neurons will result in the muscle producing more force (appropriate for lifting a very large weight).
Thus the brain "recruits" motor neurons to become active in a very systematic order. First, it activates the small motor neurons that produce a small amount of force only as they each control only a small number of muscle fibres. Then, if that amount of force is not enough to do the task, the brain activates the intermediate sized motor neurons that produce a moderate amount of force as they each control only a moderate number of muscle fibres. Finally the brain activates the large motor neurons that produce a large amount of force as they each control a large number of muscle fibres.
In this simulation you will study the process of recruitment, by increasing the voltage that you will apply to the bundle of motor neurons going to a muscle, activating small motor neurons first and then progressively activating moderate and large motor neurons as you increase the voltage systematically.
0:00 One way to change the amount of force that a muscle generates, is to change the number of muscle fibres that are active. You will recall that an individual muscle consists of a large number of muscle fibres.
0:13 We can then bring into play only the number of muscle fibres that are needed to do the task at hand. For example, if you wanted to lift a pen like this, you may bring into play only a few muscle fibres. If you want to then lift a much heavier weight, you may bring into play more and more muscle fibres.
0:31 That's a process called Recruitment, because as the name implies, you're recruiting or bringing into play more and more players, or in this case, muscle fibres.
0:41 You're going to do this in the virtual lab, by increasing the amount of current that you will apply. When you apply a small amount of current, you will activate the muscles close to it. When you increase the current, you will activate more and more muscle fibres, recruiting more of these.
1:07 It's important to recognise one thing though. This is not the way in which recruitment occurs in real life. In real life, recruitment does occur, yes we can modulate the number of muscle fibres, but we don't do this by simpling increasing the amount of current the motor neuron delivers to the muscle.
1:28 Instead, each muscle is contacted by a number of motor neurons. Each motor neuron does not contact all the muscle fibres, each motor neuron only contacts a small number of fibres. That's called a motor unit. One motor neuron, and a number of muscle fibres that it contacts.
1:50 What's neat about the body is the fact that the size of the motor unit varies quite systematically. In some cases, you have small motor neurons. One motor neuron contacting a small number of muscle fibres. That means, when that motor neuron is active, only a small amount of tension will be generated.
2:10 You can also have large motor units, where you have a motor neuron contacting a large number of muscle fibres. If that motor neuron becomes active, a much larger number of muscle fibres becomes active, and a much larger amount of tension is generated.
2:25 It turns out that for any muscle there's a range in size for the motor units. We have small motor units, intermediate size motor units, large motor units.
2:37 That tells you that there is another way in which you can recruit the number of muscle fibres that are active. If you want to do a small amount of work, you may activate only the small motor units, each of which only contact a small number of muscle fibres, so you produce a small amount of force.
2:55 As the amount of work you need to do increases, you activate the small motor units, AND the intermediate-sized motor units, bring into play larger numbers of muscle fibres.
3:10 Finally, if you've got a large amount of work to do, you bring into play the small motor units, the intermediate-sized motor units, and then the very large motor units. This is what's known as the size principle, and you would have heard about this in lectures.
3:25 The size principle essentially says when we need to do work with a muscle, we start off by activating the small motor units, as we need to do more we bring into play the intermediate-sized, and finally the large motor units.
3:41 There's another very important reason for following this order. You will learn in lectures that the small motor units, once they contact a small number of muscle fibres, it's a different set of fibres that are contacted than in the case of the large motor neurons, which contact large numbers of fibres.
3:59 Small motor units also tend to contact the muscle fibre type which can generate small amounts of force, but for longer periods of time. Large motor units contact large muscle fibres, which are generally a large amount of force but can't do so for very long periods of time.
4:18 You know that from your own experience every time you go to the gym, something I don't do myself so I'll have to depend upon you to know that from your own personal experience.
4:27 So this screengrab of the muscle contractions shows you four responses. There is the flatline response, and then you can see four successively increasing muscle contractions. The first flatline response was obtained when we applied a very low voltage. When we applied the very low voltage to the nerve, it was not enough to activate the neurons, so there was no muscle contraction, hence the flatline response.
4:59 When we increase the voltage to 50 millivolts, we see the first muscle contraction there, and it's relatively small in size. As we increase the voltage thereafter to 70, 80 and 100 millivolts, you can see that at 70 the response is bigger than that at 50, but when you get to 80, 90 and 100 millivolts there's no further difference in the amount of force that the muscle is developing.
5:26 What we've demonstrated here is the process of recruitment. As you increased the amount of current you activated motor neurons in the systematic order that we talked about, the size principle, and thereby increased the number of muscle fibres that have become activated, until eventually at 80 millivolts, we've activated all the fibres in this particular muscle.
The above video outlines some important physiology relating to muscle fibre recruitment and the results to expect.
In this simulation you will record the muscle contraction response to a single electrical pulse applied to the nerve supplying a muscle.
0:00 In this part of the practical, you'll be looking at the effects of recruitment. Recruitment is where you activate progressively larger motor neurons, each in turn bringing in more active muscle fibres, to increase the amount of muscle tension.
0:18 You will do this by systematically varying the voltage that you will apply, and the voltage is up here. You can select the voltage that you want, then hit the button "Stimulate", record the response. Do this until you first get a response, and for that we suggest you start at something like 0.2.
0:36 Hit stimulate, notice an absense of responses, go up in increments of 0.05, so go to 0.25. Again, no change, try 0.3, no response.
0:50 Now let's go up to 0.35, and now we start to see an increase in muscle tension. Go on systematically increasing these voltages, though I will only be increasing a few for the sake of this demonstration.
1:05 And notice that the muscle force is increasing as you do so.
1:12 Continue doing so until you start to see that the increase in muscle force is beginning to plateau. So now you will notice that going from 1 to 1.3, there was only a very small change. If I were to go up another 0.05, it's in fact a very marginal change. At 1.4 there's no further increase.
1:33 Confirm this by trying the next two highest levels, and essentially you can see that you have saturated in the amount of muscle force. This is the process of recruitment, and we would like you to conduct this experiment.
1:48 Once you've done so, and you're able to record the data, which you can see down here, you'll notice that you're able to record data. Then go back up there, and press "Clear Data" before you go on to the next experiment.
Please note that although this video demonstrates an older version of the simulation, it should function the same.
- Apply a single pulse to the nerve, at a stimulus voltage of 0.2 V.
- If the stimulus voltage of 0.2 V used in the first step does not evoke a response, then increase the stimulus voltage by 0.1 V to 0.3 V, and again stimulate.
- Repeat this process until you first get a response.
- Carry on this protocol by increasing the voltage by 0.2 V steps until you see that there is no further increase in the size of the contraction paired with an increase in voltage.
- Continue this until you get three successive recordings where you have increased voltage with no increase in muscle twitch size.
Once you have finished, look at the second graph which automatically plots your data, so you can more readily observe the effects of stimuli voltage on muscle fibre recruitment and hence force of contraction.
Full instructions can be found on the previous tab. In short:
- Voltage can be selected from the drop down box on the left.
- First, stimulate the muscle's nerve at 0.20 V. Then, systematically stimulate the nerve voltage increments.
- Observe how the force of contraction increases as you increase the voltage - but only to a certain point.
- You will reach a plateau where an increase in voltage will not lead to an increase in force of contraction.
- Active tension
- Passive tension
- Total tension