Analysis of the origin of SCP when registering electrical activity from the brain

According to experimental data obtained on animals, the surface of the cortex is positively charged (1.5 – 20 mV) with respect to the electrically indifferent points, for which skull bones were most often chosen . In adult rats, the SCP between the intact surface of the cortex and its coagulated portion is approximately 20 mV. In young animals in the early stages of postnatal ontogenesis, this indicator is lower. If the reference electrode was in the ventricles of the brain, the difference in constant potentials varied from 0.2 to 5.5 mV. The greatest potential difference in the registration of SCP between the cortex and the bones of the skull did not exceed 13 mV in rabbits. In cats, these values ​​reached 25 mV. However, in the experiments of L.L. Klimenko (1986) revealed that in rats under nembutal anesthesia, the potential difference between the brain and the reference electrode located on the tail is negative. In humans, the transcortically recorded potential difference varied .

In some works performed on animals, the recording of SCP was performed in parallel with the registration of the membrane potentials of nerve cells. In these studies, a close relationship was found between the dynamics of brain SCP and changes in the membrane potentials of neurons . With asphyxiation, SCP was correlated with the membrane potentials of nerve cells, while there was no such connection with the potentials of glial cells. These and other studies were supplemented by work on the layer-by-layer registration of SCP, on the basis of which it was suggested that the positive pole is at the level of either apical dendrites or neuron bodies, and the negative pole is at the level of myelinated axons.

In a number of works, on the contrary, a high correlation was found between SCP and membrane potentials of glial cells . Most modern authors believe that in reality the membrane potentials of both neurons and glia make a significant contribution to the genesis of SCP, which is diverted from the brain. The fact is that during the depolarization of neurons a large amount of potassium ions is released into the interstitial space. In this case, glia behaves like a potassium electrode: it changes its potential depending on the concentration of potassium in the environment. This phenomenon is clearly seen with a change in SCP under the influence of sensory stimulation and with spreading depression (RD).

Thus, with visual and sound stimulation in rabbits, a negative shift in SCP of up to 1 mV was recorded in the parietal cortex . Parallel to this shift, an increase in extracellular potassium was observed, associated with depolarization of cortical cells, and a corresponding change in the membrane potentials of glial cells. Furthermore SCP shifts induced sensory stimulation, such changes constant potentials arise during electrical stimulation of specific and non-specific thalamic nuclei and other Connectivity Orc formations , wherein when a negative shift SCP to increased extracellular concentration Alia .

The most studied electrophysiological phenomenon associated with brain SCP is spreading depression (RD), the discovery of which in the 40s. ХХв . associated with the name and which attracts the attention of researchers at the present time. RD is a nonspecific response of the cortex to local effects of a different nature: electrical, chemical, mechanical, etc. In RD, the inhibition of all types of electrical activity of the cortex that has arisen in any of its areas begins to spread sequentially, capturing more and more new departments. It has been hypothesized that RD occurs with brain injuries, with migraine attacks and ischemia. During RD, a negative shift in SCP is observed directly on the surface of the cerebral cortex by 10-30 mV, followed by a positive shift of 5-15 mV. The speed of wave propagation RD – 2-5 mm per min. At each point, its duration is 2-3 minutes. In RD there is a significant increase in the concentration of extracellular potassium, which is associated with the observed parallel depolarization of glia and neurons. With the passage of the RD wave in the brain tissue, the ATP content decreases. At the present stage, RD is studying a number of scientific teams, of which the school of self is the most famous . In Russia, fundamental work in this area has been carried out , comprehensively exploring this phenomenon.

A large number of experimental studies performed on animals are also devoted to the study of the dynamics of AMR in the epileptic focus created by the surface application of penicillin, strychnine or other substances. A negative change in SCP in the epileptic focus area is 0.2–0.3 mV, according to the data . With the intravenous administration of convulsants (metrazole, pentazole, picrotoxin, thiocarbohydrazide, etc.), the negative shift reaches 4 mV or more. The shift of SCP is accompanied by the accumulation of extracellular potassium due to the depolarization of neurons and glia. On the periphery of the focus and in the contralateral hemisphere, the surface displacement of the SCP is positive .

Many authors have studied changes in AMR during brain death. S. Goldring, JL O’Leary (1951b) found that in rabbits, when the active electrode is located on the brain, and the reference electrode on the bones of the skull, tracheal compression first causes a positive shift of the SCP by 1-3 mV, followed by a significant negative shift of 10 mV. Complete flattening of the EEG occurred during the positive phase, while cardiac arrest was noted at the beginning of the negative shift. When the cats brain cooled to 23-26 C, the development of a positive shift and a decrease in SCP were stretched for 5-12 minutes. Such a delay was regarded by the authors as an indicator of the brain’s resistance to asphyxiation. G.N. Sorokhtin showed that when mice died, regardless of whether death was associated with asphyxia or poisoning, an initial increase in SCP was observed by an average of 4 mV, lasting from 1 to 10 minutes, which was replaced by a decrease in SCP by several tens of millivolts. The detected primary increase in SCP in anoxia is associated with hyperpolarization of nerve cell membranes in its initial period, and a sharp decrease in SCP in the future is associated with an indiscriminate increase in membrane permeability, leading to a rapid exchange of ions between extracellular and intracellular spaces .

With local ischemia, changes in SCP in the corresponding region of the brain are generally similar in nature to the disorders that occur during brain death. In a dog, when a disturbance of cerebral circulation was experimentally caused by occlusion of the middle cerebral artery, a significant negative shift in the SCP occurred during the weakening of the ligature. In animals with large changes in SCP, a more pronounced neurological deficiency was also observed . In cats, with occlusion of the middle cerebral artery, the AMP in the affected area shifted in the negative direction by 9.1 +0.7 mV .

Thus, numerous experimental data indicate that when registering SCPs directly from the surface of the brain, the dynamics of SCPs with various effects on the brain is somehow related to changes in the membrane potentials of nerve and glial cells. As a rule, depolarization of the membranes of neurons and glia is accompanied by a decrease in SCP, and hyperpolarization is accompanied by an increase in SCP.

local_offerevent_note September 17, 2019

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