IDENTIFICATION AND ANALYSIS OF SINGLE UNIT ACTIVITY IN CENTRAL NERVOUS SYSTEM 



26s 



The conductivities of several electrolytes which have 

 been used are given in table i. The rate of diffusion 

 from the tips varies widely, of course, but the magni- 

 tude of this effect can be seen from an example given 

 by Nastuk & Hodgkin (47). They report a diffusion of 

 KCl from a 0.5 m micropipette filled with 3 m KCl of 

 6 X to"'* M per sec. If a micropipette maintaining 

 this flow is introduced into an infinite liquid space, 

 the concentration of KCl at equilibrium can be de- 

 termined from the relation 



C\ = 



F 



4ir.vZ) 



+ c. 



where C'^ is the uniform concentration ol KCll in the 

 space before introducing the pipette, a is the dis- 

 tance from the pipette tip, F is the rate of flow of KCl 

 from the tip and D is the difTusion coefficient. Apply- 

 ing reasonable values for the electrode described 

 above indicates that the order of magnitude of the 

 increase in concentration of KCl at a distance of 10 

 H from the tip is 3 X io~* m per 1. This may be com- 

 pared to the figure 1.5 X 10"^ m per 1. taken by 

 Coombs et al. (15) as the concentration of K+ in the 

 intercellular spaces of the cat's spinal cord. 



When a micropipette carries an electric current, 

 there is a selective migration of ions through the tip 

 superimposed on the movement by diffusion just dis- 

 cussed. If the ionic concentration in the pipette is 

 much greater than that outside the tip then, regardless 

 of its direction, the current will be carried largely by 

 movement of ions from inside to outside the tip, by 

 anions if the electrode is negative and by cations if it 



TABLE I . Coiuhutivity of Solutions Used in Microelectrodes* 



* In reciprocal ohms (mhos) per cm. 



Composition of solutions: KCl 3 m solution: 224 gm KCl 

 dissolved and diluted to i liter. NaCl 1*^0: 0.5 gm NaCl dis- 

 solved and diluted to 50 cc. NaCl 2 m : 5.85 gm NaCl dissolved 

 and diluted to 50 cc. K.2SO4 0.6 m: 5.2 gm KoSOj dissolved 

 and diluted to 50 cc. AgNoj saturated: 122 gra AgNos dis- 

 solved in 100 cc water. CuClj saturated: no gm CuClj dis- 

 solved in 100 cc water. FeClj 20%: 20 gm FeCh dissolved and 

 diluted to 100 cc. Trypan Red saturated: about i gm/ioo 

 cc; excess filtered off. 



is positive. When the mobilities of the ion species are 

 different, the electrical conductivity of the pipette 

 will change with the direction of current carried, 

 and the electrode will show rectification. These 

 properties of micropipettes have been used to ad- 

 vantage both for excitation of membranes and for de- 

 termining the effects of specific ions on the behavior 

 of single cells (15). 



ELECTRic-^L PROPERTIES OF MICROPIPETTES. Resistance. 

 The electrical resistance of a micropipette may be 

 thought of as the sum of the resistance of a truncated 

 cone of the inside electrolyte and the resistance of the 

 voluine conductor around the tip. If the tip diameter 

 is less than i n, more than 90 per cent of the resistance 

 lies in the last 10 /i of the tip. In actual practice these 

 electrodes generally range from a few to several 

 hundred Mti. 



The resistance of a pipette is also dependent on the 

 direction, amplitude and sometimes on the dura- 

 tion of the current it is carrying. For very small cur- 

 rents of brief duration, the electrode usually behaves 

 either like a pure resistance or a simple rectifier. The 

 rectifier action of pipettes has not been studied sys- 

 tematically for a large number of electrolytes. When 

 even small currents are maintained for a long time, 

 the resistance may increase. This has been inter- 

 preted by Taylor in the Appendix to Jenerick & 

 Gerard C39) as a movement of low-conductivity ex- 

 ternal electrolyte into the tip due to bound surface 

 charges on the glass, and is a reversible phenomenon. 

 Larger currents delivered in pulses at constant voltage 

 may cause sudden erratic changes in resistance, and 

 the amplitude of current pulses at which these changes 

 begin generally differs with polarity. Sustained ap- 

 plication of several volts across a micropipette will 

 often increase its resistance irreversibly to more than 

 10^0. Some form of clogging in the extreme tip is sug- 

 gested, and it is usually possible to break the fine tip 

 by gently bumping the pipette under a microscope, 

 thus reducing its resistance to a usable value. A pipette 

 of the dimensions described above might have a re- 

 sistance of 20 Mi2 for a current of up to about io~' 

 amp. in either direction and pass this current without 

 markedly departing from a pure resistance. Tasaki 

 (personal communication) has been able to select 

 equally fine, hand-drawn inicropipettes carrying up to 

 io~^ amp. provided the external volume conductor 

 was sufficiently acid. 



When used with a good preamplifier (see page 267), 

 the ability of a micropipette to carry current is rela- 

 tively unimportant if it is to be used only for measur- 



