CHANGES ASSOCIATED WITH FOREBRAIN EXCITATION PROCESSES 



325 



elusion that SD and the spread of convulsive discharge 

 may be closely related. 



By recording the d.c. change at successive cortical 

 depths Leao (27) was able to show that the negative 

 shift appeared later at an intracortical electrode than 

 at one directly above it on the surface. Also, when an 

 electrode upon the pial surface or in the superficial 

 cortex recorded a significant negative variation, a 

 deeper cortical electrode showed a positive variation. 

 From these observations Leao concluded that SD 

 starts in the superficial cortex and is propagated down- 

 ward to involve the entire cortical thickness. Freygang 

 & Landau (9) reached the same conclusion. 



A negative voltage variation similar to the one 

 which accompanies SD has been observed with 

 cortical anemia, anexia and asphyxia (11, 17, 26, 43), 

 a fact suggesting neuronal depolarization as an im- 

 portant contributory factor in all. Extrapolating from 

 results obtained during depolarization in peripheral 

 nerve (6, 7), one would expect decreased cortical 

 impedance to result from both SD and cortical anoxia. 

 However, several workers (9, 28, 41) have reported 

 the opposite occurrence. Freygang & Landau (9) 

 have suggested that swelling of the neuronal and glial 

 elements of cortex may account for the increased 

 cortical impedance which accompanies SD and cor- 

 tical anoxia, arguing that cellular swelling would 

 increase the resistance of the extracellular current 

 shunt and thus the tissue resistance, van Harreveld 

 & Ochs (41) have agreed with this view. However, 

 recent studies of nervous tissue with the electron 

 microscope (31) have failed to reveal the existence 

 of an extracellular space in the cortex, thus .suggesting 

 that other explanations need also be entertained. On 

 the other hand van Harreveld (39) has reported 

 direct confirmation of swelling in the superficial 

 dendritic plexus during SD, using histological sections 

 prepared after freezing. 



Grafstein's (21) recent observations with micro- 

 electrodes are also important. She noted increa.sed 

 firing of single units at the start of SD, sugs;esting some 

 other explanation for the depression of EGG con- 

 ventionally recorded with macroelectrodes. Asyn- 

 chronous firing of single units leading to cancellation 

 of opposing effects could, however, reconcile macro- 

 and microelectrode results. Grafstein's observations 

 have also led her to implicate depolarization resulting 

 from the liberation of pota.ssium as the cau.se of SD. 

 Thus, the increased neuronal discharge shown by 

 microelectrode studies could result in decreased cell 

 permeability (depolarization) and the liberation of 



potassium. The latter could chemicalh- stimulate 

 adjoining cells, the process spreading to involve the 

 entire cortex. 



Relalion oj Polarity oj Evoked Traiisiiml to Polarity 

 of SP Change Also Consequent to Stimulation 



Much indirect evidence has been presented here 

 indicating; that the same neuronal activity occasions 

 both the transient of the conventional EGG and the 

 SP changes described. Direct proof is also needed and 

 is possible to obtain in an experimental situation 

 which provides a layer of neurons synapticallv ac- 

 tivated from one surface with impulses conducted 

 away from the other. The lateral geniculate nucleus 

 of the cat was studied by Bishop & O'Leary (2) who 

 showed that the postsynaptic spike recorded from a 

 critical electrode in the optic radiation over the genic- 

 ulate cell layers has a positive polarity, and that, as 

 the critical electrode enters the cell layers, the polarity 

 reverses to negative. They concluded that, with regard 

 to evoked potentials of the lateral geniculate, the cell 

 body during activity becomes negative to its own con- 

 ducting axon. Vastola (46) undertook to repeat this 

 experiment, determining the reversal point of the 

 evoked transient from the same electrode used to 

 record the summated SP shift which accompanies 

 rapid repetitive stimulation. For this purpose he used 

 glass capillary tube electrodes having a tip diameter of 

 250 ;u and led from calomel cells. The critical electrode 

 was passed from the optic radiation through the cell 

 layers, the reference electrode being situated in the 

 central white matter anterior to the lateral geniculate 

 body. He deterinined the maximal evoked response 

 transient obtainable from a single shock applied to 

 the contralateral optic nerve, and then proceeded to 

 study, at different strengths of stimulation between 

 threshold and maximum, the SP shift which occurs 

 concurrently with stimulation to 150 per sec. Dorsal 

 to the cell layers SP became positively shifted during 

 repetitive stimulation; at 0.5 mm dorsal to the cell 

 layers the SP shift accompanying repetitive stimula- 

 tion reversed polarity; with increasing depth of the 

 critical electrode the negative shift increased further 

 until the electrode tip was in the middle of the first 

 layer of the nucleus. Then it gradually decreased as 

 the electrode passed through the remaining cell 

 layers and into the thalamus ventral to the nucleus. 

 The polarity of the SP shift coincided with the polarity 

 of the postsynaptic wave which he recorded by the 

 conventional single shock method. As an added pre- 



