IDENTIFICATION AND ANALYSIS OF SINGLE UNIT ACTIVITY IN CENTRAL NERVOUS SYSTEM 



263 



tion; and 36). Electrode polarization causes a varia- 

 tion in sensitivity of the electrode-recording device 

 combination with the frequency of the recorded po- 

 tential. Lower frequencies are more attenuated than 

 higher frequencies, and slow changes in potential tend 

 to be lost. In addition there is sometimes a fluctuation 

 in contact potential between metals and electrolytes. 

 Because of these shortcomings, metal microelectrodes 

 have been used more for recording extracellular 

 transient potential changes and patterns of unitary 



activity than for studying the exact form of the po- 

 tential waves or lasting potential changes. 



.viiCROPiPETTES. The problems inherent in recording 

 from a metal surface of very small area are in part 

 avoided by using a glass pipette filled with an electro- 

 lyte. With such an electrode the electrolyte-metal 

 boundary is moved back from the tip to the shank 

 where a long wire provides a large surface area (fig. 

 2.4). This electrode, while not suffering from a limited 





■••III 



I 



FIG. I. A. Silver-filled glass micropipettcs a and c show glass-insulated microelectrodes with tips 

 containing no metal, h and d are the same electrodes after electrolytic filling with rhodium. Scale: 

 10 M [From Svaetichin (52).] B. Stages in preparation of an indium-filled micropipctte: a, capillary 

 tubing half filled with low melting point metal; b, the capillary after tip is drawn but before metal is 

 pushed to fill it completely; c, electrode tip showing platinum black deposit. Calibrations: i mm for 

 a and A ; 10 /u for c. [From Dowben & Rose ( 1 6).] C Lacquered timgsten microelectrodes sharpened by 

 electropolishing: a, electronmicrograph of uncoated wire; b, optical photograph of three coated 

 electrodes immersed in water to show normal variation in coating. [From Hubel C36).] 



Glass with Pf Plating 



Glass 1 Stainless Steel 



40 35 30 25 20 



MILLIMETERS 



^M 



FIG. 2.-4. (/(■//) KCl filled microelectrode used for intracellular recording showing one method of 

 mounting. Platinum lead from amplifier contacts inner mercury droplet as outer shield is 

 clamped. If shield is driven electrically by negative capacity amplifier, platinum coating is usually 

 omitted. [From Frank & Fuortes (26).] B. (right) Electronmicrograph of the tip of a similar pipette. 

 [From Nastuk & Hodgkin (47).] 



