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.
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.
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.
Mobile Support Warning
This simulation was designed with a desktop interface in mind, and may not function correctly on smaller screens or mobile devices.
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