Analysis of the potential‐dependent changes in optical retardation in the squid giant axon

Abstract

SUMMARY1. An analysis has been made of the change in optical retardation of the membrane elicited by the application of voltage-clamp pulses in squid giant axons.2. The retardation response consists of three separate voltage-dependent components. For freshly mounted axons, defined as being in state 1, hyperpolarizing pulses give a rapid increase in the light intensity measured with crossed polarizers which has been termed the fast phase. This is followed by a rather slow return towards the base line termed the rebound. On treatment of the axon with certain agents that include tetrodotoxin, high calcium and terbium, the rebound disappears and the fast phase slows down, increases in size, and has a new slow component added to it. This transition from state 1 to a second state, 2, appears to be irreversible.3. In state 1, the time constant of the fast phase is 20-40 tsec at 13°C; it has a very large negative temperature coefficient (Q10 = Ca.j).The size of the retardation change is independent of temperature and * National Science Foundation post-doctoral fellow, 1966-68. 6. A tenfold increase in external calcium concentration had no discernible effect on the fast and slow phases, but reversibly reduced the amplitude of the rebound nearly to half.7. In experiments on perfused axons, the retardation response was not measurably altered by any of the modifications made to the composition of the perfusing fluid.8. There was some indication of the possible existence of a small current-or conductance-dependent component of the retardation response. 9. These phenomena seem likely to originate either from molecular relaxation processes analogous with the Kerr effect, or from changes in membrane thickness under the influence of the pressure exerted by the electric field. However, the specific molecules involved in the retardation response cannot yet be identified.

Mechanical, thermal, and optical changes of the nerve membrane associated with excitation.

“…The relationship is quadratic and shows a maximum at about 60 mV of the membrane potential. The curve is very similar to that between membrane potential and birefringence change [16]. This gives rise to the very interesting speculation that the mechanical change and the birefringence change might share a common cause.…”

“…The term "potential dependence" means that the shape of the optical signal should be expressed by a single-valued function of the membrane potential. In fact, the birefringence of the squid glant axon may not be regarded as strictly potentialdependent, as is carefully pointed out by COHEN et al [16]. Especially, the birefringence signal corresponding to the phase of after-hyperpolarization (which will be called the second phase) tends to be more accentuated than the birefringence signal corresponding to the spike of the action potential (which will be called the initial phase).…”

“…12). A similar discrepancy exists also between membrane potential change and birefringence change [16]. TASAKI and IWASA [62] showed that when tetraethylammonium was injected intracellularly to a squid giant axon so that the action potential formed a plateau, the corresponding mechanical change was not a reproduction of the time course of the membrane potential change.…”

“…In terms of the ratio between the response and the background light intensity, the birefringence signal is much larger than the scattering signal; the size sometimes is 10-3 of the background light intensity in the crab nerve, and 10 -5 in the squid giant axon [15]. A detailed analysis of the birefringence signal could therefore be performed using the squid giant fiber [16]. The signal closely follows the time course of the action potential with a small delay which varies with the temperature.…”

Birefringence signals from surface and t‐system membranes of frog single muscle fibres.

“…function of potential (Cohen, Hille, Keynes, Landowne & Rojas, 1971 (Sandow, 1966). This in turn could produce a change in form birefringence detectable as a change in retardation.…”