Another important way to grade the amount of force that a muscle can produce is to activate each individual muscle fibre (muscle cell) of the muscle more rapidly. When you activate them closer and closer in time (i.e., each successive stimulus to the muscle fibre occurs closer and closer in time), then the contraction due to one stimulus can sum with the contraction due to the previous stimulus.
This is the process of summation, and you will be able to explore that here, in this simulation.
The above video outlines some important physiology relating to summation of muscle fibre contraction.
In this simulation you will be looking at the phenomena of summation. You do this by delivering two electrical pulses to the nerve supplying a muscle, with different time intervals between the two pulses.
Please note that although this video demonstrates an older version of the simulation, it should function the same.
- First, you need to set the voltage to be used in this experiment. Use the voltage you figured out in the recruitment simulation which is the lowest voltage to give a maximum contraction (approx 1.2 V).
- Record the response to two electrical pulses by applying two stimuli with an inter-pulse interval of 400 ms.
- Systematically decrease the inter-pulse interval and record muscle twitches by changing the inter-pulse interval to 200 ms, 100 ms, 80 ms, 60 ms, 40 ms, and 20 ms.
- At each inter-pulse interval apply the stimulus, and record the response.
After collecting your data, look at the second graph which has peak muscle contraction plotted against interval between stimuli. What trends do you observe?
Full instructions can be found on the previous tab. In short:
- Set the voltage to the lowest voltage which elicited a maximum response in the recruitment simulation.
- Record the response to two electrical stimuli while varying the time between stimuli in each recording.
- Record data for the following intervals: 400 ms, 200 ms, 100 ms, 80 ms, 60 ms, 40 ms, and 20 ms.
- Active tension
- Passive tension
- Total tension