Cortical Potentials and Pyramidal Cells—: a Theoretical Discussion

CORTICAL POTENTIALS AND PYRAMIDAL CELLSA THEORETICAL DISCUSSION SIDNEY OCHS* I. Introduction As a former student, it is a pleasure for me to contribute to this Festschrift in honor ofProfessor Gerard's sixty-fifth birthday. Gerard [i] had postulated that differences in polarization of apical dendrites and soma ofthe vertically oriented pyramidal cells in the cortex could be the basis for the steady potential (SP) which was recorded across the pia-ventrical thickness ofthe cortex [2]. Furthermore, these potential gradients could be related to the briefer "convulsive" and evoked responses recorded from the cortex. In an attempt to analyze cortical responses , direct electrical stimulation ofthe exposed surface ofthe cortex was first studied by Adrian [3]. The electrical response obtained from a nearby recording electrode was called by Adrian the superficial response; by others a local response, dendritic response, etc., and it will be referred to here as a direct cortical response (DCR) [4]. However, DCRs can differ in form depending, for example, on the strength of stimulation and the cortical site recorded from, as would be expected from the complex neuronal connectivities present in the cortex. With microelectrodes which give a localized stimulation, a study of laminar stimulation at different depths within the cortex showed that several types of DCRs could be distinguished in regular fashion [5]. These are shown in Figure I in response to stimulation of the sensory (visual-somesthetic) region ofthe rabbit cortex. A simple appearing slow negative wave DCR (N wave DCR) is elicited upon surface stimulation. Stimulationjust belowthe surface givesriseto a surface positive wave with long latency. This is similar to the "after-potential" usually found follow- * Department of Physiology, Indiana University School of Medicine, Indianapolis, Indiana. Supported by Public Health Service Grant Nos. NB-01993-06 and MH-04815-03. 126 Sidney Ochs · Cortical Potentials and Pyramidal Cells Perspectives in Biology and Medicine · Autumn 1963 ing the N wave DCR. Stimulation ofthe middle cortical layers gives rise to a positive-negative DCR (P-N DCR) sequence which often has a small fast wave present at its onset and several more fast waves continuing along the slow positive phase ofthe response. From the deepest cortical layers, an N wave DCR is excited which is similar in form to the N wave DCR excited from the surface, but differs in having a longer latency and one or two small fast waves preceding it. In cortical areas which are not primary sensory regions (such as cinguFig . i.—Responsesto laminar stimulationin the rabbit visual cortex atthedepths in mm. indicated at theleft ofeach trace. Surface at o. Negative deflection upward and calibrations 2 msec, 2 mv. [5]. late cortex) a P-N DCR is not seen when stimulating the middle cortical layers. This P-N sequence ofslow waves is similar in form to the evoked response given by sensory stimulation. In chronic islands where sensory afférents have been cut through and degenerated, the P-N DCR is either eliminated or much diminished, showing that this response is in fact related to elements in the cortex giving rise to the sensorily evoked responses [5]. By keeping the stimulus strength low when the upper cortical surface is stimulated, the N wave DCR can be excited in a relatively isolated fashion. Three problems are considered with respect to this N wave DCR. First, 127 what elements are excited and conducting the impulses laterally in the cortex? Second, what cellular elements generate this response at the recording site? Third, what hypothesis can be constructed regarding the mechanism ofthis response and the role it may play with respect to other cortical functions? II. Transmission ofthe N Wave DCR The elements conducting impulses laterally from the site of electrical stimulation are located in the uppermost layer, i.e., the molecular layer of the cortex. This was shown by undercutting the cortex between stimulating and responding sites, severing all lateral connections but those present in the molecular layer. The result was that the N wave DCR was not blocked in such molecular layer preparations, even when only the uppermost part ofthe molecular layer was left uncut [4, 6]. This experiment is the converse ofthat ofBurns and Grafstein [7], who found that shallow cuts made down into...