Electrical stimulation of rats: Part 2-The effect of electrical parameters on muscle tension and post-Mortem...

Anaesthetized rats were used as a model to determine the effect of changes in electrical stimulation parameters on the pH fall of the M. longissimus dorsi and M. triceps surae and tension development by the M. triceps surae. Stimulation 5 min before decapitation resulted in a reduced post-stimulation fall in pH. Tension development resulting from stimulation of the sciatic nerve decreased rapidly 5 min after decapitation and ceased 30 min after decapitation, whereas a current pathway through the muscle was still effective. Post-stimulation pH fall was less in the leg not in the stimulation pathway, indicating a lack of crossover effect from one leg to the other; but the difference diminished as stimulation voltage increased. For direct stimulation, 20 V produced a maximal response in the stimulated leg; at least twice that voltage was required for a maximal pH fall in the leg not in the stimulation pathway. The total resistance of the rat with a needle electrode at the nape of the neck and crocodile clips attached to the unskinned legs was 3400 ω, whereas with skinned hindlegs the resistance was 860ω. With this high skin resistance at least 40 V was needed for effective stimulation whereas, for low voltage stimulation of much larger animals, only twice this voltage is used. High contact resistance and unknown current density factors make simple correlations of animal size and voltage effects impossible. The optimal stimulation frequency for rats was 20-30 pulses a second. If the cranial electrode was positive, about 15% more tension was developed than when the cranial electrode was negative. With alternating-polarity pulses, there was a high-low tension response, which was more pronounced at low voltages but evened out as the muscles became exhausted. Alternating and cranial-positive pulses produced similar pH falls but cranial-negative pulses were inferior.