Unlike other organs, the energy requirements of the brain are satisfied mainly due to aerobic glucose catabolism. The brain is practically not capable of synthesizing glucose from proteins and fats (gluconeogenesis). The passage of glucose through the blood-brain barrier (BBB), that is, the barrier between the blood and the brain, and its entry into neurons and glia is associated with a transport protein system that is formed in ontogenesis. Passive diffusion accounts for no more than 5% of total glucose transport.
To carry out its activities, the brain needs a significant influx of energy. Although brain weight is about 2% of the adult body weight, it consumes more than 20% of the total body consumption e nergii . There is a wide range of data on the use of glucose delivered to the brain by ozone . According to G. Siebert , about 7% of glucose is converted to lactate as a result of glycolysis and enters the bloodstream in this form, 30% of glucose undergoes aerobic oxidation in the Krebs cycle, and 60% of glucose is used for the synthesis of amino acids, neurotransmitters, glycolipids and glycoproteins. According to other data , about 85-90% of glucose is completely oxidized to CO 2 and Н 2 О, about 5% is consumed in glycolysis reactions, and only 5-7% is used for the synthesis of amino acids, neurotransmitters, etc.
The intensity of respiratory processes is maximum in the cerebral cortex , somewhat less in the cerebellum and diencephalon. When moving to phylogenetically older parts of the brain – the middle, oblong and spinal cord – its further neck decrease is observed .
Using labeled carbon, glucose intake has been shown to be associated with increased activity of brain structures. Thus, during light stimulation of glucose metabolism in primary and association visual cortex was increased from 12 to 59% depending on the experimental conditions, with auditory and tactile stimulation of glucose metabolism in growth detected corresponding guides projection region astyah . Energy production is significantly increased with epileptic seizures and decreases with phenobarbital anesthesia .
With an increase in energy needs (for example, with stress), glycolysis increases in the nervous tissue. Anaerobic oxidation is also enhanced with certain types of pathology, in particular with hypoxia and ischemia, which is accompanied by the accumulation of lactate. This can also occur after severe traumatic brain injury, with Alzheimer’s disease and others .
If the supply of glucose to the brain decreases, the products of fatty acid oxidation, ketone bodies, can also be used as an energy source. Increased utilization of ketone bodies is observed in the fetus and in early childhood. The decrease in the brain’s use of ketone bodies for energy metabolism as they grow older is due to a decrease in their concentration in the blood, a decrease in the BBB permeability for these substances, and a change in the activity of key enzymes that limit the rate of this process . When using ketone bodies as an energy source, acidification of the nervous tissue (acidosis) occurs. This type of metabolism in an adult is observed with stress and some types of pathology, for example, with diabetes mellitus, hyperthyroidism. But even in these cases, due to the oxidation of free fatty acids and ketone bodies, not more than 20% of the brain’s energy needs are covered.
The brain can also use amino acids as an energy source. In this case, the decay is carried out along the metabolic pathway, called the gamma-shunt
aminobutyric acid (GABA). Using a GABA shunt for glucose deficiency can lead to an increase in GABA levels in the event of an increase in energy expenditure. Since GABA is a inhibitory mediator, an increase in its level decreases the functional activity of certain parts of the brain. This process can be considered as regulation by the negative feedback mechanism: high cerebral activity causes a deficiency of glucose, it triggers the GABA shunt, as a result of which an amino acid that inhibits brain activity accumulates. Normally, the use of the GABA shunt is limited.