A large birefringence signal preceding contraction in single twitch fibres of the frog.

Abstract

1. When a single muscle fibre was externally stimulated to give a propagated action potential, a large decrease in light intensity was measured with the fibre positioned between crossed‐polarizers oriented at +/‐ 45 degrees with respect to the fibre axis. This large optical signal begins just after stimulation and its man phase precedes the development of positive tension. 2. The peak (or plateau) amplitude of the signal, expressed as the peak (plateau) change in light intensity, delta I, divided by the resting light intensity, I, was typically (minus) 1 to 3x10‐3 and the time‐to‐peak (plateau) was 4 to 6 ms (20 degrees C). 3. When mechanical activity was minimized by stretch or Ringer replacement with D2O or hypertonic solution, the peak tension response was reduced in far greater proportion than the peak optical response, suggesting that the early optical signal is not due to changes in tension or gross fibre movement. 4. The magnitude of the optical response was increased by nitrate and double‐shock stimulatin, procedures which potentiate the twitch response. 5. The optical signal was shown to propagate along the fibre length with a conduction velocity appropriate to the surface action potential. 6. The above results suggest that the large, early birefringence signal reflects a step or steps in the sequence of events leading to contractile activation.

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

“…It has been shown that the early large birefringence signal in field stimulated fibres follows the action potential with a constant delay (Oetliker &WSchfimperli, 1980) and that it propagates with the speed expected for the action potential in point stimulated fibres (Baylor & Oetliker, 1977 a).…”

“…Wilska & Varjoranta, 1939;Martin, 1954). In this report, a signal is recorded which is assumed to represent a subsequent step in excitation-contraction coupling (Baylor & Oetliker, 1977 a). The exact nature of this process is still controversial.…”

“…Isolated single muscle fibres in a large volume of solution were used and the propagation of excitation was measured with an optical method (Baylor & Oetliker, 1977 a) which is far less sensitive to stimulus artifacts than recording of extracellular action currents. This permitted measurements close to the stimulus electrodes in a region of the muscle fibre not yet disturbed by tension generation in the first contracting sarcomeres.…”

Conduction pattern of excitation in the amphibian atrium assessed by multiple-site optical recording of action potentials.

“…When we optically monitor action potentials from contractile tissues using voltage-sensitive dyes, optical changes generally include light-scattering changes due to mechanical movement (BAYLOR and OETLIKER, 1977;VERGARA and BEZANILLA, 1976;MORAD and SALAMA, 1979;SAWANOBORI et al, 1981;KASS, 1981;HIROTA et al, 1985a). Accordingly, in a strongly beating heart stained with voltagesensitive dyes, action potential-related optical signals are often covered by a larger mechanical movement artifact.…”

Proceedings of the Physiological Society, 18‐19 April 1986, Sheffield Meeting: Communications

“…In single fibres at room temperature illuminated with plane polarized light oriented at 450 to the fibre axis a transient decrease in light intensity is measured if an analyser, orientated at -45°, is placed between fibre and photodetector. This signal was named 'early large birefringence signal' (Baylor & Oetliker, 1977). Based on indirect measurements using isolated muscle components and a fluorescent probe it was suggested to reflect changes in free myoplasmic calcium by reflecting conformational changes of the sarcoplasmic reticulum (Oetliker, 1982).…”