I just did a lab where we dissected the gastrocnemius muscle of a frog and then stimulated the sciatic nerve and measured the force of response (the resulting muscle twitch). As we increased the voltage of the stimuli, the corresponding twitch response increased in force until it reached a max value and then proceeded to fall as we continued to increase the stim voltage. My question is why the did the response correspondingly increase with the stimulus voltage. Is it because the sciatic nerve is a bunch of nerves and it started to "recruit" more muscle fibers in the muscle as voltage increased? Or, does it have to do with the amount of neurotransmitter at the NMJ? thanks.