Diagram of the formation of membrane potential in the case of selective membrane permeability for hydrogen ions

A similar mechanism for the appearance of a potential difference is also observed in the case when solutions of different concentrations with unequal mobility of anions and cations are separated in any way that complicates the alignment of concentration gradients, but does not impede the free exchange of ions. Due to the different diffusion rates, separation of ions with different mobilities occurs, accompanied by the appearance of a diffusion potential. Diffusion potential is also described using the Nernst equation. There are no sharp differences between the membrane and diffusion potentials. In some cases, different concentrations of substances are separated by a membrane, which, although it passes anions and cations, is a big obstacle, for example, for anions. Such a situation has a place about for called these potentials membrane-diffusion. A detailed description of the membrane theory and diffusion potentials is available in many textbooks , and in this paper only those elements of the membrane theory that are directly related to the material presented are presented.

Another question: why do cells need to generate electrical reactions, i.e. Is some information transmitted through electrical processes in nerve cells, or are these processes an epiphenomenon – a by-product of cell activity? It turned out that both phenomena take place in the brain. Neurons transmit information reflected in a certain sequence of electrical impulses, and at the same time there are processes in the brain that lead to the appearance of a difference in ion concentrations on both sides of the membranes and, as a result, to the appearance of a potential difference that is not directly related to the exchange of information, although it can perform other useful functions. The first include the potentials of action of nerve cells, as well as exciting and inhibitory postsynaptic potentials (EPSP, TPPS), and the second – the potentials of the BBB.

A necessary condition for the normal operation of neurons is the presence of a membrane potential. This potential difference is created, first of all, due to the difference in the concentrations of potassium ions, as well as some other ions outside and inside the cells, and at rest is about 60 mV, while the outer surface of the neurons is positively charged with respect to the inner one. Excitation of a nerve cell is accompanied by its depolarization, and inhibition

– hyperpolarization. When the depolarization of a neuron reaches a critical level, a propagating nerve impulse (action potential) occurs. The transition of excitation from neuron to neuron occurs through synaptic contacts, and EPSP and TPPS are generated on the postsynaptic membrane, the summation of which can lead to the occurrence or inhibition of nerve impulses.

The most studied type of integrated electrical activity recorded from the surface of the head is the electroencephalogram (EEG) and evoked potentials (EP). EEG is a variable electrical activity of the brain in the frequency range from about 0.5 to 50 Hz and with an amplitude not exceeding hundreds of microvolts. EEG, according to modern concepts, integrally reflects EPSP and TPPS of a large number of neurons. The spectral characteristics of an EEG significantly change with a change in synaptic activity, for example, with sensory stimulation, a change in a functional state, etc. EP is an electrical reaction of the brain to afferent stimulation of various modalities. VPs consist of a series of electrical vibrations or waves. The latent period, i.e., the time from the moment the stimulus is applied to the occurrence of the wave, for the early components of the EP is from units to tens of milliseconds, for later – from 100 ms to 1 s or more. Late components of long duration (up to a second) are called constant components of the VP. The amplitude of the constant component does not exceed 0.5 – 1 mV. These components are most noticeable in the so-called evoked potentials associated with events. Some researchers believe that the genesis of the later components of this kind of constant potentials associated with the activity of glial cells etok .

To maintain membrane potentials, the generation of EPSP, TPPS and nerve impulses, almost continuous energy supply of nerve cells is required. The correlation between neural activity, EEG, EP parameters with local cerebral blood flow and the level of glucose consumption by the brain is shown. Nevertheless, in a healthy person, EEG is to some extent independent of energy metabolism. For example, EEG changes do not always correspond to the degree of chronic cerebrovascular insufficiency. Of course, with significant changes in brain energy metabolism, the ability of the brain to generate EEG noticeably changes. This also applies to VP.

It is clear that neither EEG nor EP can be used as an indicator reflecting the energy metabolism of the brain. For this, it would be desirable to register potentials whose value is more directly related to the main indicators of energy metabolism, as is the case when using modern technologies for studying metabolism.

The previous section described modern high-tech methods for visualizing cerebral energy metabolism, as well as some electrophysiological methods used to evaluate processes that are somehow related to the energy supply to the brain. There is reason to believe that some bioelectric brain reactions can be directly related (due to their genesis) to cerebral energy processes and, therefore, are sufficiently informative to evaluate brain energy metabolism. First of all, we are talking about the level of constant potentials (SCP), recorded on the surface of the head, the genesis of which is closely related to cerebral energy metabolism.

By SCP, we mean the stable potential difference of the millivolt range recorded between the brain (or extracerebral structures) and reference regions using direct current amplifiers . With a stable functional state, this potential difference is stable within 1 mV for tens of seconds. In Russian literature, such potentials are often described as super slow electrical activity, a quasi-stable potential difference, or omega potentials. In Western literature, these potentials are called dirrect current potentials , i.e., DC potentials.

SCP is less sensitive to sensory stimulation than EEG and EP, but changes actively when exposed to various parts of energy metabolism. The genesis of SCP is rather complicated, many components are involved in it, the quantity and composition of which mainly depends on the method and conditions for recording potentials. These conditions will always be indicated hereinafter. An initial review of works devoted to constant potentials should begin with a short historical digression, which will be useful primarily for understanding the circle of ideas that was associated with the SCP phenomenon.

local_offerevent_note September 19, 2019

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