Spreading Depression of Leao

PRELIMINARY CONSIDERATIONS The spreading depression of Lefio has been well estabhshed as a reproducible reaction m the hssencephahc rabbat (Lefio 1944a b 1947 1945) The reaction can be initiated by weak mechamcal or electrical stamulat~on apphed to the surface of the pm-arachno~d membranes or ~t can also be mttmted by mtense neuronal activity It conststs of a slowly expanding wave of depression of electracal act~x~ty moving through the cortex at a rate of 2 to 3 mm per man which persists at any point for from 2 to 6 m m, and from whach complete recovery sometimes reqmres I0-15 mm It is accompamed by a strlkmq varmtaon of a D C potentml After recovery the cortex exhab~ts approximately the same react~wty as ~t d~d prewous to the depression wave This Laboratory has corroborated man~ points m the above reports on 19 rabbats anesthetazed with Dial However we found m 3 experaments on cats that the reaction was capraclous m that ammal (Marshall 1949) These experiments were done an December with the entire dorsal a~pect of one hemasphere inadequately protected from room a~r Exammataon of spectf~c sensory responses often sho~x ed evidence of ~rreqular spread and decremental transmission of the reaction In contrast w~th the hssencephahc rabb~t the ~rregulartty of the reaction was s tnkmq an all of these experiments done on the convoluted cortex of the cat There are lmpress~ve stmdar~tms between the suppressor reactaons and spreadm0 corhcal depress,ons wh,ch are presented an detad by Sloan and Jasper (1950) There Is some indication (Garol 1942 Gellhorn 1947 Marshall Woo l sey and Bard 1937) that a common factor m these phenomena is the exposure of larqe areas of the cortex to room aw This constderataon suggests that the p~aarachnoM mechamsms are crucmlly m~olved the low velocity of propagation the relatively long tame during which the depressaon persists the vascular ddataon which ~s reported to accompany at and other characteristics suqgest a neurochemaca] reaction or a reaction complex revolving at least one such step W h e n the subarachnoM functions are ~mpa~red by exposure to room a~r normal chemacal processes are altered and the reaction of Lefio can take place Accord |ngly two s~mple propos~ttons were postulated for experimental test~ The hr~t ts that the mteqrlty of the pm-arachnoad system is more ~mportant than has heretofore been suspected If adequately protected from exposnre to room air no spreadmq depressaons should be obserxed followmq any kind of stamulat~on Secondly adequate anternal dehydrataon of the brain should make it possable to secure regular, repeatable depression cycles an the convoluted cortex of the cat