36 



E. F. MACNICHOL, JR. 



nmnmu^ 



Fig. 1. Response of single Limulus photoreceptor unit to prolonged illumination. 

 The black band above the bottom line of each record indicates the duration of illu- 

 mination. Time marks are fifths of a second. The intensity of illumination for each 

 record is 10 times that for the one immediately underneath. (Figure courtesy of 

 H. K. Hartline.) 



an engineer would do with a photo-cell circuit or a microphone, many qualita- 

 tive relationships have been established (Hartline et al. 1952). These show 

 what the receptor must do but give no indication of how it does it. 



More recently we have tried to get inside the "black box" and learn some- 

 thing about the mechanism. By penetrating the sensory structures with micro- 

 pipette electrodes we have been able to follow changes in the electrical polari- 

 zation of the receptor cells themselves. 4 



Starting with the work of Graham and Gerard (1946) and Gerard's associates, 

 many workers have successfully used the micropipet te technique to study cell 

 potentials. Wagner and I merely followed their lead and concocted an amplifier 

 which combines a rapid response with extremely low grid current (MacNichol 

 and Wagner, 1954). Then, using saline filled pipettes of less than half micron 

 external tip diameter, we were able to lead off potentials from the interior of 

 the receptor cells with little apparent damage either to the cells or to the po- 

 tentials. Fig. 2 is a simultaneous recording of potentials obtained from a 

 micropipette electrode and from a pair of electrodes placed on the nerve fibers 

 coming from the sensory structure. Except for the time delay required for the 



4 This work was supported in its initial phases by contract Nonr-248(ll) between 

 the Johns Hopkins University and the Office of Naval Research. It is currently being 

 supported by Grant G-922 to the Johns Hopkins University from the National Science 

 Foundation. 



