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Gastrocnemius muscle recording arrangement The gastrocnemius muscle, together with its nerve, is shown earlier, in Fig. 13. The exsected preparation is clamped in the bath, firmly held (at S in Fig. 31 below) by the stump of the femur and of the tibio-fibula. An elastic thread is tied to the tendon of Achilles and to the electrode wire at R, and the muscle is drawn through the hole in the rubber dam at the end of tube R. Longitudinal currents are detected as voltage drop in the region of high resistance where the muscle passes through the barrier. The tube can be moved (A) relative to the muscle, so the record can be taken at any point along the muscle.
The muscle can be activated by stimulation delivered directly to the nerve (N). Alternatively, the proximal tendon of the muscle (at S), can be tied (having been freed of the bone stumps) and drawn into a gap in a barrier, similar to the arrangement at R. A stimulus can then be applied across the barrier, at S. The stimulus current is directed into the muscle and into the nerve fibers in the vicinity, but the nerve fibers are preferentially stimulated, because the nerve threshold is lower than that of the muscle. (Consider why.)

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