GENERAL DISCUSSION 461 



different. One can see a marked surface change within a time as long as 2 months. 

 This at least seems likely at present for a surface dose of 9,000 rad, which is not 

 a dose for total necrosis of neurons, and can produce a line lesion in depth. The 

 presence of gross capillary changes in a region where neural change is not apparent 

 also suggests a possible dissociation of vascular and direct neural damage. The 

 gross discrepance between the time relations, the distribution in space in vascular 

 changes and the neuronal changes, the gross discrepance between species differences 

 and susceptibility of vessels, and the lack thereof for neurons would seem to me a 

 convincing argument that neurons are susceptible to ionizing radiation without 

 necessarily involving capillary damage. 



Horace W. Magoun (University of California, Los Angeles, California): I want 

 to ask another question related to this same program of study, which in addition 

 to its importance for central neural regeneration would seem to be a great po- 

 tential for contributing to the neurophysiology of the lamina of the cerebral cortex. 

 Here is the first time it has been possible to interfere morphologically with 

 individual cortical laminae. One of the most interesting developments that has 

 come in this area recently is the concept that one can identify in neurons two types 

 of excitation: the classic conducting all-or-none mechanism in the axon and a 

 graded response mechanism with a longer time course and excitable only to 

 chemical transmitters at the synapses on dendrites. I judge that these laminar 

 sections sever the apical dendrites, at least, of deep-lying cortical pyramidal cells; 

 and for a time before they regenerate (if that is what they do), the distal apical 

 dendrite must be destroyed. A whole category of evoked potentials recorded from 

 the surface of the cortex have been attributed to this graded response mechanism 

 of the dendrites, and they are called, perhaps loosely, "dendritic potentials." Among 

 these are the surface-negative local cortical response, the recruiting response from 

 exciting the nonspecific cortical projection, the surface chemical features of the 

 augmenting response to the repetitive excitation of specifically projecting terminal 

 cortical connections, and features of the transcollosum potential. Because each of 

 the co-authors of this remarkable contribution is a sophisticated neurophysiologist 

 as well as morphologist, I wonder if one of them would tell us what is being done 

 in the study of electrophysiology of this situation, which it seems to me is far more 

 interesting than simply the morphologic observation, and in particular what is 

 happening to what is being called today "cortical dendrite potentials." 



Leonard I. Malis (Mount Sinai Hospital, New York, New York): Again 

 as in the growth of ner\'e fibers, the story is long, and we cannot go into detailed 

 presentation. Briefly, we have prepared 75 or so cats with unilateral right striate 

 cortex laminar lesions at various layers and at various times after the irradiation 

 have studied them with surface maps and with microelectrode punctures for the 

 evoked responses in striate units above and below the laminar lesions to light flashes 

 in optic stimuli. Unfortunately, the study required a great deal of correlation with 

 the various times of the units, and with silver stains to show us the stages of fiber 

 regrowth. We are in the process of analysis now, and some of the changes have been 

 somewhat surprising. One of the most difficult to do was to get the units to stop 

 firing on top of the lesion. They have a remarkable tendency to continue this. The 

 surface maps showed almost consistently, whenever the laminary lesion was fairly 



