Cornwall M C, Gorman A L
Cell Mol Neurobiol. 1983 Dec;3(4):311-28. doi: 10.1007/BF00734713.
Intracellular recordings were made from distal photoreceptor cells of the file clam Lima scabra in order to examine the ionic and spectral mechanisms which underly the response to light decrement. These receptors are primary sensory neurons that generate nerve impulses in the optic nerve upon light termination without benefit of synaptic interconnections between photoreceptor cells. Microelectrode measurements were made on these cells. Membrane conductance changes were assessed by measuring membrane voltage changes elicited under different conditions while passing constant-current pulses through the microelectrode from an active bridge amplifier. Responses of membrane potential in light and darkness in different concentrations of external potassium ions were fitted to a simplified form of the constant field equation. This analysis allowed an estimation of internal potassium activity (281 mM) as well as changes in PNa/PK in darkness and light. PNa/PK changed from 0.09 in darkness to 0.03 at the peak of the light response. A persistent decrease in membrane conductance at the termination of light is associated with a depolarization that overshoots the dark resting membrane potential. This transient depolarization is dependent on the intensity and duration of the preceding period of light. The amplitude of the dark-dependent depolarization is related to the absorbance of light during the preceding period of light by a long wavelength intermediate of rhodopsin bleaching (metarhodopsin). The frequency of discharge of action potentials with rapid kinetics which occurs following light is proportional to the amplitude of the after depolarizing response. The delay to onset of the discharge is inversely proportional to the amplitude of the after depolarizing response. The sensitivity (response/photon) of distal cells can be modified by background light which passes through a screening pigment found in cells that surround the eye. These data, taken together, provide an explanation for the persistent discharge of action potentials which occurs on termination of light in these cells as well as the visual cells of other gastropod mollusks.
为了研究光强度降低反应背后的离子和光谱机制，对锉蛤（Lima scabra）的远端光感受器细胞进行了细胞内记录。这些感受器是初级感觉神经元，在光终止时能在视神经中产生神经冲动，而无需光感受器细胞之间的突触连接。对这些细胞进行了微电极测量。通过测量在不同条件下，当从有源桥式放大器通过微电极施加恒定电流脉冲时引起的膜电压变化，来评估膜电导的变化。在不同浓度的外部钾离子存在下，对光和黑暗中膜电位的反应拟合到恒定场方程的简化形式。该分析使得能够估计内部钾离子活性（281 mM）以及黑暗和光照条件下PNa/PK的变化。PNa/PK从黑暗中的0.09变化到光反应峰值时的0.03。光终止时膜电导的持续降低与超射黑暗静息膜电位的去极化有关。这种短暂的去极化取决于先前光照的强度和持续时间。黑暗依赖性去极化的幅度与视紫红质漂白（变视紫红质）的长波长中间体在前一光照期间的光吸收率有关。光照后快速动力学的动作电位发放频率与去极化后反应的幅度成正比。发放开始的延迟与去极化后反应的幅度成反比。远端细胞的敏感度（反应/光子）可被背景光改变，背景光穿过眼睛周围细胞中发现的一种屏蔽色素。综合这些数据，为这些细胞以及其他腹足纲软体动物的视觉细胞在光终止时发生的动作电位持续发放提供了解释。
