3i6 



HANDBOOK OF PHYSIOLOGY -^ NEUROPHYSIOLOGY I 



voltage convulsoid discharge. Later Leao (26, 27) 

 proved that a marked d.c. change also accompanies 

 a wave of spreading depression as it propagates across 

 the cortex. This phenomenon will ije discussed in 

 detail under experimental findings. 



One may assume with Libet & Gerard (30) that a 

 significant component of the pia-\entricular potential 

 arises from an end-to-end polarization of the cortical 

 pyramids, and that the transcortical potential over an 

 area of corte.x represents chiefly the average of the 

 polar charges of the contained neurons. However, 

 the potential difference along an individual pyramidal 

 neuron has relative rather than absolute pertinence, 

 for it can be recorded not only when one end of the 

 cell is depolarized, but when, for any reason, the two 

 ends become unequally depolarized. The polarization 

 of both ends might, for example, be lowered un- 

 equally, raised unequally, or one raised and one 

 lowered. With externally applied polarization as 

 studied by Bishop & O'Leary (2) it is almost certainly 

 the latter which occurs. Surface-positixe polarization 

 applied to an area of cortex accentuates the surface- 

 negative component of evoked potential transients 

 recorded therefrom; accentuation of the surface- 

 positive component occurs during applied surface- 

 negative polarization. 



Reviewed herein are: a) the requisites for reliable 

 d.c. recording together with an assay of difficulties 

 which, if unrecognized, may render experimental data 

 unreliable; h} the transient d.c. alterations which 

 have been shown to accompany or follow spontaneous 

 or induced changes in the pattern of the usual ECG, 

 such as changes in spontaneous activity, evoked re- 

 sponses, recruiting waves, barbiturate spindles, strych- 

 nine and veratrine spikes; c) changes associated with 

 the occurrence of high voltage convulsoid activity; 

 rf) d.c. change accompanying spreading depression; 

 i) evidence linking the polarity of usually recorded 

 evoked potential phenomena with those of accom- 

 panying d.c. change; /) d.c. cortical changes pro- 

 duced by repetitive stimulation at sites distant from 

 the recording electrodes and requiring transmission 

 along paths containing intervening synapses; and g) 

 injury potential effects. 



SUGGESTED TECHNiqUE FOR CORTICAL D.C. RECORDING 



For many experimental studies it is important to 

 monitor the potential continuously. Minor displace- 

 ments of the recording electrodes or injury (either of 

 which mav ije due to movement of the animal), an 



obstruction in the airway, or periodic excesses of 

 stimulation, can unstabilize a preparation temporarily 

 or permanently. Then, swings of several millivolts 

 from one polarity to the other may occur. Movement 

 of the electrodes alone may occasion swings in po- 

 tential in either direction, whereas with the other 

 conditions mentioned the changes are characteristic; 

 these will be discussed later. In the rabbit (less often 

 in our experience in the cat) SP swings occur which 

 accompany either spreading depression (Leao) or 

 the appearance of high voltage convulsoid activity. 

 Li the rabbit the latter is a common enough accom- 

 paniment of excesses of electrical stimulation. 

 L nidirectional drifts may also occur during systemic 

 deterioration, or with oxygen lack or deepening 

 anesthesia. These necessitate quick recognition if the 

 preparation is to continue to provide reliable data. 



To measure SP one needs nonpolarizable elec- 

 trodes. Continuous monitoring during experiments 

 lasting for se\eral hours indicates the need for stable 

 electrodes. We have used calomel half-cells having a 

 difference of potential in Tyrode's of 0.5 to i.o mv. A 

 flexible pipette may be led from one member of a 

 pair to the cortical surface of the selected region; 

 that from the other can be introduced either into the 

 \entricle or the subcortical white matter. In the 

 rabbit the former (ventricular) position to the surface 

 gives an individual diff'erence of potential of surface- 

 positive polarity amounting to i to 4 mv. If the deep 

 electrode tip is introduced into the subcortical white 

 matter the potential difference may be larger due to 

 the addition of injury potential. Monopolar recording 

 from surface cortex to a point upon the periosteum 

 gi\es a change of the same polarity as that evident 

 with transcortical recording. The diff"erence of po- 

 tential is greater than the pia-\entricular potential 

 ijut less than that led between surface and white 

 matter. Transcortical (or pia-ventricular) d.c. re- 

 cording is ijclieved to off'er greater promise for estab- 

 lishing correlates with spontaneous and evoked ac- 

 tivity of the usual ECG because of the more localized 

 leading. 



A conventional condensor-coupled electroencepha- 

 lograph as well as a d.c. amplifier ma\- be used both 

 for monitoring and for the study of d.c. changes. It 

 is only necessary to short-circuit the input 4 to 8 

 times per sec. Each short circuit discharges such 

 potentials as have accumulated upon the input con- 

 densors because their time course has been too long 

 to permit passage into the amplifier. The discharge 

 causes the pens to return momentarily to the baseline 

 of the amplifier. When a chopper is u.sed, if the record- 



