324 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



slowly outwards from the site of a weak mechanical, 

 electrical or chemical stimulus to the cortex. This 

 reaction was elicited more readily from the rostral 

 pole of the hemisphere, and from there a wave of 

 depression could envelop almost the whole of the con- 

 vexity, propagating at a velocity of only 2 to 5 mm 

 per min. Neither .evoked potentials nor motor re- 

 sponses to cortical stimulation could be observed when 

 the front of the depression reached the sensorimotor 

 cortex. Leao also observed that the propagation of the 

 SD was accompanied by dilation of pial vessels (25), 

 an observation confirmed by van Harreveld & Ochs 

 (42) who also held that in the rabbit the vasodila- 

 tation is preceded by a smaller wave of vasocon- 

 striction. Species differences in cortical su.sceptibility 

 to SD have been noted. It is produced more easily 

 in the rabbit than in the cat and is seen only occasion- 

 ally in the monkey (33, 36, 45). Cortical maturity 

 may also play a role: Bures (3) was unable to obtain 

 SD during the first days of life in the rat, although 

 he could elicit it readily in the newborn guinea pig. 

 There is substantial evidence indicating that SD 

 is an abnormal reaction which results from exposure 

 of the brain to unphysiological conditions. Marshall 

 and co-workers (32, 34, 35, 36) have shown that SD 

 appears consistently only if the brain has been de- 

 hydrated, exposed to the atmosphere for long periods, 

 cooled, or bathed in Ringer's .solution having ten 



times the usual concentration of potassium. In the 

 absence of one or another of the above conditions 

 they were unable to elicit SD in the cat or monkey at 

 all, although occasionally it could still be developed 

 in the rabbit. Because they were able to record SD 

 through the intact dura of the rabbit, van Harreveld 

 rt al. (45) disagreed with the view that SD is an ab- 

 normal reaction. However, later, utilizing the various 

 conditions described by Marshall and co-workers, 

 van Harreveld & Bogen (40) obtained SD in the area 

 retrosplenialis granularis dorsalis, a region into which 

 SD does not propagate under usual recording con- 

 ditions in the rabljit. 



The d.c. variation which accompanies SD has a 

 duration of 4 to 6 min. (26). With the critical record- 

 ing electrode placed upon the pial surface an involved 

 cortical region becomes negative for i to 2 min. with 

 respect to a subcortical or an extracortical reference 

 electrode. Within that time the surface-negativity 

 reaches a maximum of 8 to 15 mv, thereafter decreas- 

 ing with somewhat greater rapidity. The involved 

 region then becomes 3 to 8 mv surface-positive but 

 returns to the predepression base line in 3 to 5 min. 

 (fig. 12). As the d.c. potential changes from surface- 

 negative to positive, large amplitude (2 to 3 mv) 

 negative slow waves or the repetitive spikes of con- 

 vulsive discharge may occur. The occurrence of these 

 transients or repetitive spikes led Leao to the con- 



8 MIN. 9 



FIG. 12. A representative experiment on the slow voltage variation accompanying the spreading 

 depression of activity. The curve was drawn from voltage readings taken with hve sec. intervals. In 

 this and the other two figures, an upward deflection denotes negativity of the corte.x with respect to 

 the extracortical reference electrode. Electrodes arranged as shown in the inset (s: stimulating 

 electrodes). Stimulation, 5 sec. of 'tetanizing' current from an induction coil, delivered at the time 

 marked S. In this representative curve there is indicated the time of occurrence (from .\ to B) of the 

 specific electrical activity which often develops during the depression of the spontaneous pat- 

 terns.' [From Leao (26).] 



