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1. An analysis has been made of the change in optical retradation 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 µsec at 13° C; it has a very large negative temperature coefficient (Q₁₀ = Ca.). The size of the retardation change is independent of temperature and varies as the square of the applied voltage, but the voltage—retardation curve is symmetrical about a point well beyond zero membrane potential, at an internal potential of around + 70 mV. In state 2, the time constant is about five times larger, and varies much less markedly with temperature; the apex of the voltage—retardation curve is shifted to + 200 mV.

4. The rebound has a time constant of the order of 20 msec at 13° C. A 10° rise in temperature more than halves the time constant and roughly doubles the amplitude of the rebound. The voltage dependence of the rebound differed from that of the fast phase.

5. The slow component of state 2 has a time constant of about 2 msec which does not change noticeably between 10 and 25° C. The size of this component seems to be linearly dependent on the applied voltage, rather than obeying a square law.

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.

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