IDENTIFICATION AND ANALYSIS OF SINGLE UNIT ACTIVITY IN CENTRAL NERVOUS SYSTEM 



267 



FIG. 4. Approximate equivalent circuit for an intracellular 

 micropipette. E, cell potential to be recorded; Re, resistance 

 of microelectrodc tip plus preparation, Ce, capacity between 

 electrolyte inside micropipette and grounded volume conductor 

 in which it is immersed; V, potential recorded by amplifier. 



J. W., & K. S. Cole, manuscript in preparation; 

 Lettvin, J. Y., & B. Howland, manuscript in prepara- 

 tion; Bak, A. F., manuscript in preparation. This type 

 of preamplifier, by utilizing positive feedback to the 

 input grid, in effect adds a controlled negative 

 capacitance in parallel with the positive input capaci- 

 tance. By minimizing the sum of these capacities, a 

 considerable improvement can be made in the record- 

 ing time constant. Used with micropipettes like those 

 described above, these circuits can reach an effective 

 input capacitance of 0.5 /i^uf or less. 



recorded voltage will be 



V = E - 



dV 

 dt"" 



The error in recorded voltage is thus proportional to 

 the electrode resistance, to its capacity to ground and 

 to the first derivative of the recorded voltage. When 

 the input voltage is a sine wave, the frequency at 

 which the recorded voltage is reduced to 0.7 E is 

 given by 



f = 



27rRp. Cp 



Amplifiers. Amplifiers used for recording from micro- 

 pipettes must have special features to minimize the 

 effects of the inherent shortcomings of the electrodes 

 described above. Ideally, the amplifier must have an 

 input resistance which is high in comparison with the 

 electrode resistance, a low enough input grid current 

 so that its effects at the tip of the electrode can be 

 neglected and a negligible effectixe capacity between 

 input and ground. These requirements of the ampli- 

 fier do not include voltage gain which can be accom- 

 plished in a following amplifier. Thus the preamplifier, 

 as it is usually called, is actually an impedance trans- 

 former intended to isolate the source of potential being 

 measured from the loading effects of the conventional 

 voltage amplifier. 



A number of practical preamplifiers have Ijeen de- 

 signed to meet these special requirements with varying 

 degrees of success. The simplest is the cathode fol- 

 lower circuit of which a good example is that de- 

 scribed by Nastuk & Hodgkin (47). This circuit gave 

 an overall recording time constant of 70 /z sec. when 

 tested with a 22 Mli pipette, showing an effective 

 input capacitance of 3.2 /i^if. An improved circuit 

 called a negative capacity amplifier has been used in 

 various forms by several authors: Solms el al. (51); 

 Woodbury (58); Wagner & MacNichol (57); Moore, 



IDENTIFICATION OF SINGLE UNITS 



Position 



One of the more diHicult problems in the use of 

 micropipettes is the determination of their positions. 

 Identification of the structure or structures generating 

 the various potentials recorded by the micropipette 

 requires some knowledge of the relative positions of 

 the pipette and the structures which might be respon- 

 sible for the potentials. Knowledge of which neuron 

 the pipette is in or near is of less interest than the kind 

 of neuron and the position of the pipette relative to 

 the various parts of such a neuron. Information of 

 this kind has been obtained by direct microscopic ob- 

 servation, by marking techniques and by inferences 

 drawn mostly from the nature of the potentials 

 recorded. 



Direct observation of the microelectrodc position is 

 limited to those structures which can be dissected 

 free of opaque or translucent surrounding tissues. This 

 technique has been used in recording from muscle 

 fibers [see bibliograph\- in Jenerick & Gerard (39)]; 

 peripheral nerve fibers (Chapter III, 54); inverte- 

 brate heart ganglion cells (12); eel electroplaques (5); 

 dorsal root ganglion cells (53); photoreceptor cells 

 C.33); and stretch receptor cells (41). Even in struc- 

 tures where this technique is possible, there are 

 severe limitations. The tips of micropipettes are often 

 submicroscopic or close to the limit of resolution 

 with visible light, thus requiring ideal conditions of 

 lighting, numerical aperture, color contrast and 

 contrast of indices of refraction. Pressure from the 

 pipette often distorts the ti.ssue, and even under the 

 best conditions it may be difficult for example to de- 

 termine optically whether the pipette is inside or 

 outside of a particular cell wall. However, a large 

 part of the present body of knowledge of the electro- 

 physiology of single cells has been acquired by 

 studies u.sing this technique. 



