D-C Potentials of the Brain

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Abstract
This review is concerned largely with those d-c phenomena which can be assigned an origin in a specific locus of nervous tissue and interpreted in terms of widely recognized neural processes that take place simultaneously. The potentials referred to are potentials so slow as to require d-c recording to produce undistorted records and consider potentials that have a purely resting as well as action connotations. The d-c potentials of the central nervous system surveyed include slow components and other aftereffects of usual evoked potentials. When repetitively activated some such slow aftermaths fuse, while others sum temporally to occasion base-line shifts which may outlast the stimulus period. These have been called steady potential (SP) shifts. The class also includes sustained potential changes produced by more enduring depolarization such as arises in states of injury or anoxia or as a result of applying known depolarizing agents. Other long lasting changes occur incident to anesthesia or administration of a variety of pharmacological agents and also as an accompaniment of ictal or postictal states, activation, sleep, and in spreading depression. The best means of studying the SP shifts were found to be direct cortical and recruiting responses. It was found that surface-positive polarization accentuates the amplitude of the negative potential of the evoked response, whereas surface-negative polarization accentuates the positive ones. It has been shown that surface-positive polarization accelerates unit firing, which conversely, is depressed by surface-negative polarization. This observation has been extended to the cell membrane to show that local anelectrotonus applied through a micro-electrode to a single cell produces its activation, whereas catelectrotonus brings about inhibition. The threshold for motor cortex also is lower for positive than for negative stimuli. The negative d-c shifts accompanying repetitively evoked responses indicate the same neuronal membrane which produces the transitory depolarization can remain depolarized under continuing synaptic bombardment.