Larry M

Results from simulation of isolated frog muscle stimulation

1. Voltage stimulation versus force generated (shows how additional muscle fibers/cells are recruited to increase the total force the muscle can generate).

Here is our set-up with the frog muscle clamped at each end to a device that can measure how much pulling or contraction force the muscle generates. Electrodes are placed into the muscle to simulat the effect of the action potentials of the motor nerves arriving to trigger the muscle cells to undergo their own action potential which, in turn, stimulates a "twitch" or "firing" or "contraction" of the muscle cells.

On the above screen, we've stimulated the muscle with increasing voltages and the trace of the curves shows the force that the muscle generates when it is stimulated. Higher voltages increase the number of muscle cells that are triggered to fire resulting in a larger force. Eventually, all the muscle cells are being stimulated and additional voltage does not increase the amount of force generated.

In this plot, taken from the force curves above, we can see that as voltage increases, the force of the muscle (representing the number of cells that are firing) increases, but eventually reaches a point where all cells are firing and additional voltage does not increase the whole muscle force that is generated.

2. Length of the muscle and position of the actin/myosin proteins (thin and thick filaments) relative to the muscle force that is generated.

In this experiment, we stimulated the muscle with maximum voltage and changed the length of the muscle. Remember, this was like our bucket demonstration where we found an ideal middle muscle length where maximum force is generated due to ideal actin/myosin overlap. The traces on the screen show the force generated at different muscle distances, always stimulating with maximum voltage (thus stimulating all the muscle cells to fire).

Then, when we plot muscle length against force generated, we see that at very small lengths, due to too much actin/myosin overlap, low force is generated and at very long lengths, not enough cross-bridges between the actin and myosin are in place so, again, we have low overall muscle force. At medium lengths, actin-myosin overlap is just right to generate maximum force. When we did the bucket experiment, the 90 degree elbow bend produced maximum biceps force generation (or ability to sustain weight) compared to 45 degree extended muscle and a 120 degree bunched-up muscle.

Larry M Frolich, Ph.D. ∞ Miami Dade College ∞ Wolfson Campus ∞ Natural Sciences ∞ Miami, FL 33132 ∞ Office 1504 ∞ (305) 237-7589 ∞ e-mail