LASTING CHANGES IN SYNAPTIC ORGANIZATION 



PRODUCED BY CONTINUOUS NEURONAL 



BOMBARDMENT' 



Frank Morrell 



I should like to present some experimental observations which have led 

 us to suggest an analogical model for cellular learning. The model is 

 interesting in two respects, (i) Although the process is a pathological one, 

 it is presumed to utilize the same neural pathways available to physio- 

 logical stimulations. (2) Changes in synaptic organization, cellular 

 excitability and cellular chemistry have been demonstrated on a time scale 

 comparable to behavioural learning. 



For the past few years our laboratory has been conccrnexl with a study 

 of chronic epileptogenic lesions produced in animals by local application 

 of an ethyl chloride spray to a small (2 mm.) area of cortex (Morrell and 

 Florenz, 1958; Morrell, 1959). Such animals develop paroxysmal electro- 

 graphic discharges at the periphery of the lesion within a few hours. 

 Within a few days a secondary focus of paroxysmal discharge may be 

 recorded in the contralateral homologous cortex. Fig. i illustrates such a 

 development. In the tirst 24 hours after application of the ethyl chloride 

 (Fig. 1 A), the electrical record disclosed paroxysmal activity limited to the 

 area around the primary lesion. There was little or no evidence of ab- 

 normal discharge in an electrode (electrode 4) placed symmetrically over 

 the cortical region opposite the lesion. Three days later, however, in the 

 same animal, epileptiform spikes appeared quite prominently in the 

 symmetrically placed contralateral electrode (Fig. iB). 



Such secondary foci have been observed in patients with convulsive 

 disorders (Gastaut, 1959) and in animal preparations by Erickson (1940), 

 Rosenblueth and Cannon (1942), Speransky (1935), Nims, Marshall and 

 Nielsen (1941), Morrell (1959), Kopeloff cf nl. (1954, 1955), Pope, Morris, 

 Jasper, Elliott and Penficld (1947) and Roger (1954). In the terminology of 

 the electroencephalographer, the area of secondary discharge has been 

 called the 'mirror' focus. In the early stages of its development, the 

 secondary discharge represents a 'reflection' of the primary discharge. It is 

 dependent upon activity in the primary lesion in the sense that it subsides 



1 This work was supported by the Tcagle Foundation and by USPHS Grant 1185. 



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