44 



E. F. MACNICHOL, JR. 



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Fig. 10. Effects of electrical polarization of a Limulus photoreceptor through the 

 same micropipette electrode used to record the discharge. The cell was firing spon- 

 taneously when the current was applied. Upward deflection indicates that the pipette 

 became more positive. D.C. amplification; impulses approximately 40 MV. in height; 

 electrode resistance approximately 20 megohms. Resistor through which the current 

 was applied 10 10 ohms. 



Only about ICh 10 ampere was required to produce a very noticeable effect. 

 Here I should like to insert a word of warning to electrophysiologists since 

 many physiological amplifiers have grid currents of this magnitude. This experi- 

 ment indicates that the grid current may alter the properties of some excitable 

 cells. Besides illumination and electric current several other factors have been 

 shown to influence the spike discharge frequency in the Limulus photoreceptors. 

 These are: The state of light or dark adaptation, changes in calcium and potas- 

 sium ion concentration, and activity of adjacent units. 



Fig. 12 shows the effects of addition of calcium and potassium ions to the 

 fluid in which the receptors were bathed (MacNichol, 1952). The recording 

 here was taken back in the nerve so that we were not able to follow the slow 

 potential; just the spike activity. We have not yet repeated these experiments 

 using micropipet tes. The eye was placed in a flowing control solution and al- 

 lowed to dark adapt. It was then subjected to hundredth second flashes of light 

 at regular intervals. The intensity of the flashes was increased in small steps 

 and the number of responses to each test flash recorded. After the most intense 

 flash a new bathing solution was applied and the eye allowed to equilibrate in 

 the dark for about a half hour after which an identical test run was made. This 



