IDENTIFICATION AND ANALYSIS OF SINGLE UNIT ACTIVITY IN CENTRAL NERVOUS SYSTEM 



269 



extrinsic potentials inside a volume conductor such as 

 the spinal cord or brain are small and do not ap- 

 preciably distort the potentials recorded by micro- 

 electrodes inserted in active neural elements except in 

 the presence of large synchronous volleys in a limited 

 volume conductor. 



Fi?;ure 6.4 is a section of the lumbar region of a cat's 

 spinal cord showing cell bodies of neurons as black 

 dots. A line drawn across such a section indicates the 

 structures which may be encountered by a micro- 

 electrode as it is advanced through the tissue. A very 

 few motoneuron somata but many small cells and in- 

 numerable fibers will be in the path of the electrode. 

 Apparently penetration of fibers occurs only when 

 very fine micropipettes are used. With coarser elec- 

 trodes the majority of the elements which can be im- 

 paled behave as if they were cell somata. Figure 6B 

 shows the relative sizes of a cat's motoneuron and a 

 typical glass micropipette. 



The potentials recorded from a microelectrode as 

 it is moved through a cat's spinal cord are indicated 



A 



v.. ^%^- 





. \ 



FIG. 6.^4. Section of cat's spinal cord at L6. Thionin stain to 

 show cell bodies. B. Methylene-blue stain of unfixed slice of 

 spinal cord showing KCl filled micropipette penetrating a 

 motoneuron near the surface of the slice. [From Frank & Fuortes 

 (26).] 



in figure 7. The upper extreme of this potential is re- 

 peatedly recorded and is taken to indicate the poten- 

 tial in the extracellular spaces since it is close to the 

 potential recorded from the fluid conductor on the 

 cord surface. The negative deflections are presumed to 

 indicate penetration or destruction of cellular mem- 

 branes, on analogy with peripheral findings. While 

 some of the negative potentials recorded must be 

 from neural elements since they are correlated with 

 spikes like action potentials, others may be from 

 nonneural elements such as glia cells. 



If the electrode is allowed to remain in a position 

 where it records a steady negative potential, spikes or 

 action potentials can generally be seen occurring 

 either spontaneously or in response to stimulation 

 (figs. ']A and 5). The amplitude of these action po- 





r^ 



4,t 



v_4 



FIG. 7. Simultaneous records taken during penetration of a 

 cat's spinal cord with a KCl filled micropipette. / : Carotid 

 blood pressure. 2: Movement of the electrode. The limit of 

 deflection of the instrument was reached by a movement of 200 

 M- After this the pen jumped back and began recording further 

 movement in the same way. Upward deflection indicates in- 

 creased penetration. 3: Signals from shutters of the cameras 

 used for making records of inserts A and B. ./ : Record of elec- 

 trode potential relative to reference electrode on vertebral 

 column. Note potential fluctuations when electrode is moved 

 and steady negativity when it is, presumably, inside the mem- 

 brane of a unit. Insert A shows responses of the penetrated unit 

 to stimulation of a ventral root, as photographed by a single 

 frame camera. Insert B shows a strip of record taken by a 

 moving film camera at the time indicated by the two arrows in 

 ^. Calibrations: /, 50 to 150 mm Hg; 2, 200 m; inserts A and B 

 and 4, 50 mv. Time; 60 sec. for /, 2, 3 and 4; i msec, for A. 

 [From Frank & Fuortes (26).] 



