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

Abstract

SUMMARY1. 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 Fm.3. Fibre cross-section appears to become circular with stretch to sarcomere lengths exceeding 4 hm. 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.

Muscle fiber conduction velocity changes with length

“…Conversely, the increase in velocity across sarconiere in shortened fibers can be expected to be associated with considerably smaller (or possibly no) increase in uctuul velocity measured across a fixed distance between two recording electrodes, such as two surfaces of a SFEMG electrode. The above studies are thus not in contradiction with the results reported here; however, it should be added that the two performed on isolated single muscle fibers'&, 15 did not explore changes in velocity along contracted fibers.…”

Estimation of the muscle fibre semi-length under varying joint positions on the basis of non-invasively extracted motor unit action potentials

“…Such a relative estimate is more appropriate also because we, in fact, estimate changes in the length of the distal portion of the muscle only. A length-independent CV is in line with the results obtained in situ by Gydikov and Kosarov [15] and by Brown et al [3], as well as with observation on amphibian muscle fibres in vitro [17,25,27] and with theoretical conclusion of Hodgkin [18]. They are, however, in contrast with the results obtained in situ by Kossev et al [20,35], for m. biceps brachii, by Morimoto [26] for m. vastus medialis, and by Arendt-Nielsen et al [2] for vastus lateralis muscle.…”

“…Amplitude, frequency characteristics and estimates of muscle fibre conduction velocity (CV) used to assess the muscleÕs functional state, are sensitive to the electrode position with respect to active muscle fibres [4,[11][12][13]18,21,23,28,33] and to changes in fibresÕ lengths [5,14,21,27,28]. A variation of the joint angle is accompanied by a change in muscle length and as a result a shift of the muscle with respect to the skin and electrodes [28].…”

Myoplasmic calcium transients in intact frog skeletal muscle fibers monitored with the fluorescent indicator furaptra.

“…Somewhat larger changes may apply to the half-width of A [Ca~+], which was increased by ~20-30% at 3.8 Ixm and by ~35---45% at 4.3 p,m. However, the precise magnitude of these latter changes was more difficult to ascertain because of possible contamination, at the shorter sarcomere lengths, of the falling phase of A associated with differences in the conduction velocity of the action potential, which shows almost no variation with sarcomere length (Oetliker and Schumperli, 1982).…”