Influence of sarcomere length, tonicity, and external sodium concentration on conduction velocity in frog muscle fibres.

1. Using an optical technique, conduction velocity in isolated frog muscle fibres has been measured at different sarcomere lengths and in solutions of altered tonicity and Na content. 2. Conduction velocity (in m/s) in normal Ringer solution is found to be independent of sarcomere length in the range of 2-5 microns. 3. Fibre cross-section appears to become circular with stretch to sarcomere lengths exceeding 4 microns. The data on fibre diameter and length are in agreement with the assumption that constant fibre volume is maintained during passive length changes. 4. In Na-deficient solutions, conduction velocity is reduced, in agreement with predictions based on action potential parameters. 5. In solutions of half or twice the normal tonicity, the conduction velocity is proportional to the square root of the measured fibre diameter. After correcting the bias involved in estimating fibre cross-section from only one measurement of fibre diameter, the data suggest an increase in specific internal resistance (Ri) by about 8% in twice hypertonic solution and a decrease by about 5% in half normal tonicity. 6. Releasing and stretching a fibre in hypertonic solution has no effect on conduction velocity as long as the initial sarcomere length is not exceeded. Stretching the fibre beyond the sarcomere length at which it was transferred to hypertonic solution reversibly increases conduction velocity.