Like other tissues, the brain requires oxygen and food nutrients to supply its metabolic needs. However, there are special peculiarities of brain metabolism that require mention.
Total Brain Metabolic Rate and Metabolic Rate of Neurons.
Under resting but awake conditions, the metabolism of the brain accounts for about 15 per cent of the total metabolism in the body, even though the mass of the brain is only 2 per cent of the total body mass. There-fore, under resting conditions, brain metabolism per unit mass of tissue is about 7.5 times the average metabo-lism in non–nervous system tissues.
Most of this excess metabolism of the brain occurs in the neurons, not in the glial supportive tissues. The major need for metabolism in the neurons is to pump ions through their membranes, mainly to transport sodium and calcium ions to the outside of the neuronal membrane and potassium ions to the interior. Each time a neuron conducts an action potential, these ions move through the membranes, increasing the need for addi-tional membrane transport to restore proper ionic con-centration differences across the neuron membranes. Therefore, during excessive brain activity, neuronal metabolism can increase as much as 100 to 150 per cent.
Special Requirement of the Brain for Oxygen—Lack of Significant Anaerobic Metabolism. Most tissues of the body can livewithout oxygen for several minutes and some for as long as 30 minutes. During this time, the tissue cells obtain their energy through processes of anaerobic metab-olism, which means release of energy by partially breaking down glucose and glycogen but without com-bining these with oxygen. This delivers energy only at the expense of consuming tremendous amounts of glucose and glycogen. However, it does keep the tissues alive.
The brain is not capable of much anaerobic metabo-lism. One of the reasons for this is the high metabolic rate of the neurons, so that most neuronal activity depends on second-by-second delivery of oxygen from the blood. Putting these factors together, one can under-stand why sudden cessation of blood flow to the brain or sudden total lack of oxygen in the blood can cause unconsciousness within 5 to 10 seconds.
Under Normal Conditions Most Brain Energy Is Supplied by Glucose. Under normal conditions, almost all the energyused by the brain cells is supplied by glucose derived from the blood. As is true for oxygen, most of this is derived minute by minute and second by second from the capillary blood, with a total of only about a 2-minute supply of glucose normally stored as glycogen in the neurons at any given time.
A special feature of glucose delivery to the neurons is that its transport into the neurons through the cell membrane is not dependent on insulin, even though insulin is required for glucose transport into most other body cells. Therefore, in patients who have serious dia-betes with essentially zero secretion of insulin, glucose still diffuses readily into the neurons—which is most fortunate in preventing loss of mental function in diabetic patients. Yet, when a diabetic patient is overtreated with insulin, the blood glucose concentra-tion can fall extremely low because the excess insulin causes almost all the glucose in the blood to be trans-ported rapidly into the vast numbers of insulin-sensitive non-neural cells throughout the body, especially into muscle and liver cells. When this happens, not enough glucose is left in the blood to supply the neurons prop-erly, and mental function does then become seriously deranged, leading sometimes to coma and even more often to mental imbalances and psychotic distur-bances—all caused by overtreatment with insulin.
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