How do we measure brain consumption

Brain metabolism

Brain metabolism m, Brain metabolism, brain metabolism, brain metabolism, E.brain metabolism, Term for all (bio) chemical reactions (metabolism) taking place in the brain. For these reactions the brain needs about a fifth of the total oxygen taken up by the organism, although it only makes up one fiftieth of the body weight. The energy expenditure of an adult's brain is around 20 watts, for which 7 mmol of ATP (adenosine triphosphate) have to be synthesized per minute. In order to provide the energy-rich phosphates, the brain needs a constant supply of glucose via the blood circulation. A stop of the blood flow leads to unconsciousness within a few seconds due to the lack of oxygen and glucose. This shows that the brain's glycogen stores are low; theoretically they would be able to maintain its function for less than 5 minutes. Key enzymes in glycolysis (glucose, small print) in the brain are hexokinase and phosphofructokinase, which regulate glucose metabolism. When the end product of glycolysis, pyruvate, is introduced into the citric acid cycle (glucose, small print), pyruvate dehydrogenase plays a decisive regulating role. The citric acid cycle not only provides energy, but also various amino acids such as glutamate (glutamic acid), aspartate (aspartic acid) and gamma-aminobutyrate (GABA), which act as excitatory (glutamate, aspartate) and inhibitory (GABA) neurotransmitters are. - The brain's normal oxygen consumption is 3 ml of O2 per 100 g brain tissue and minute, the blood flow 54 ml / 100 g brain tissue and minute and the glucose consumption 31 μmol / 100 g brain tissue and minute. The almost exclusive dependence of the brain metabolism on the substrate glucose becomes clear when the glucose concentration in the blood plasma is reduced. At a acute Reduction of the plasma concentration of glucose from the normal value (70-100 mg / 100 ml plasma) to 30-40 mg / 100 ml plasma leads to confusion and then unconsciousness (coma ). Chronic Decreases in plasma glucose concentration occur with prolonged fasting. In addition to glucose, the brain can metabolize other substrates, mainly ketone bodies (especially beta-hydroxybutyrate and acetoacetate). These can replace up to half of the glucose intake. The prerequisite for this is an increase in the plasma concentration of the ketone bodies. Elevated ketone bodies in the plasma are also present in newborns because of the high fat content of breast milk. Here, too, ketone bodies are consumed as substrates for brain metabolism. - The obligatory substrate of brain metabolism, glucose, and the optional substrates, the ketone bodies, are absorbed from the blood plasma into the nerve cells. For this purpose, they are transported across the blood-brain barrier in the endothelium of the brain capillaries with the help of carriers (special glycoproteins). The transport of glucose through the brain capillaries takes place via carriers called Glut1. The brain capillaries are surrounded by appendages of astrocytes. The transport of glucose through the astrocyte processes is based on the presence of carriers with a different relative molecular mass, but which also belong to the group of Glut1 carriers. Finally, glucose is transported through the membranes of the nerve cells with the help of the carrier Glut3 into the cells, where it is then metabolized. Oxygen, on the other hand, reaches the nerve cells without a carrier, only by diffusion along a gradient of the oxygen partial pressure from the brain capillaries into the nerve cells. - The energy released in the metabolism is used for active ion transport, mainly through the Na+K+-ATPase (sodium-potassium pump), required. In this way, the membrane potential of nerve cells and glial cells is maintained and the electrical excitability of the nerve cells is guaranteed. In contrast, the metabolic expenditure for neurotransmitter synthesis and turnover as well as for the synthesis of structural proteins (e.g. microtubules and actin filaments, which are involved in axonal transport) is very low. - The close connection between function and metabolism in the brain has made it possible to obtain essential information about functional changes in the brain based on measurements of the local metabolism and the associated local blood flow. While the metabolism and blood flow to the brain are only slightly changed when there are functional changes, activations of individual core areas of the brain lead to increased local metabolism and blood flow values. From these local changes, conclusions can be drawn about which brain regions are activated under which conditions. For each sensory system, local metabolic and blood flow increases in the afferent sensory nerve pathways could be detected when activated, for example in the structures of the visual pathway during visual stimuli. The glucose analog is used for local metabolic measurement 2-deoxy-glucose (Deoxyglucose method) is used, which is radioactively marked and, when used appropriately, provides quantitative values ​​of the brain metabolism. functional imaging, hypoglycemia.

W.K.