A thermal component of excitation in the lateral eye of Limulus

Abstract

SUMMARY1. The temperature dependence of the relative spectral sensitivity of the excised lateral eye of Limnulus was examined using its electrical response to light.2. At wave-lengths longer than 625 m,u lowering the temperature from 27 to 70 C reduced the relative spectral sensitivity, while no effect was measurable at shorter wave-lengths.3. The reduction in relative sensitivity increased linearly with decreasing wave number.4. The observations support the hypothesis that a critical amount of energy (activation energy) must be supplied to the photopigment molecule in order that it excite the photoreceptor. Quanta with energy lower than the activation energy are effective only if the thermal energy of the molecule can supply the deficit.5. The lower limit of the activation energy for the photoreceptor of the lateral eye of Limulus determined from the results of this studv is 44 kcal mole-1.

The thermal origin of spontaneous activity in the Limulus photoreceptor

“…Fig. 1 (open circles) show the values of a(A) for the extracted photopigment of the lateral eye of Limulus calculated from the data of Hubbard & Wald (1960), and curve A shows S(A) for the lateral eye of Limulus (Srebro, 1966). There is excellent agreement between the values of a (A) and S(A), and this suggests that /Z(A) = , a constant with respect to wave-length.…”

“…The absorption spectrum is an important physical chemical property of visual pigment, and it largely determines the relative sensitivity of the photoreceptor to lights of different wave-lengths. In the Limulus lateral eye, the relative sensitivity depends on the temperature of the cell as well as the wave-length of the incident light (Srebro, 1966). In this study we show that the shape of the relative sensitivity curve, and the effect of temperature on this curve can be used to predict the relationship of spontaneous discrete wave frequency to temperature.…”

“…Secondly,eqn. (10) SPONTANEOUS PHOTORECEPTOR ACTIVITY the change in S(A) when the temperature of the eye was changed from 7 to 270 C (calculated from the data of Srebro, 1966). At wave-lengths shorter than about 600 nm there is no significant effect of temperature on S(A), but there is an increasingly strong effect of temperature as A increases from 600 nm.…”

“…(7) Srebro, 1966. Open circles, quantum relative absorption difference spectrum for extracted photopigment of Limulus lateral eye, from Hubbard & Wald, 1960. Curve B, filled circles and continuous line, change of quantum relative spectral sensitivity when the temperature was changed from 7 to 270 C, from Srebro (1966).…”

“…Eqn. (12) was fit to the observed curves of S(A) for the Limulus lateral eye at 7 and 270 C given by Srebro (1966) and the values of AO and m were estimated as Ao = 575 nm, and m = 4.…”