Like other tissues, the brain requires oxygen and food nutrients to supply its metabolic needs. However, brain metabolism features special peculiarities 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 percent of the total metabolism in the body, even though the mass of the brain is only 2 percent of the total body mass. Therefore, under resting conditions, brain metabolism per unit mass of tissue is about 7.5 times the average metabolism in non-nervous system tissues.
Most of this 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 mem brane and potassium ions to the interior. Each time a neuron conducts an action potential, these ions move through the membranes, increasing the need for additional membrane transport to restore proper ionic concentration differences across the neuron mem branes. Therefore, during high levels of brain activity, neuronal metabolism can increase as much as 100 to 150 percent.
Special Requirement of the Brain for Oxygen—Lack of Significant Anaerobic Metabolism. Most tissues of the body can live without 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 metabolism, which means release of energy by partially breaking down glucose and glycogen but without combining these with oxygen. This process 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 metabolism. One of the reasons for this is the high metabolic rate of the neurons, so most neuronal activity depends on second-by-second delivery of oxygen from the blood. Putting these factors together, one can understand why sudden cessation of blood flow to the brain or sudden total lack of oxygen in the blood can cause unconscious ness within 5 to 10 seconds.
Under Normal Conditions, Most Brain Energy Is Supplied by Glucose. Under normal conditions, almost all the energy used by the brain cells is supplied by glucose derived from the blood. As is true for oxygen, most of this glucose 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 mem brane is not dependent on insulin, even though insulin is required for glucose transport into most other body cells. Therefore, in patients who have serious diabetes with essentially zero secretion of insulin, glucose still diffuses readily into the neurons, which is most fortunate in pre venting loss of mental function in persons with diabetes. Yet when a diabetic patient is overtreated with insulin, the blood glucose concentration can fall to an extremely low level because the excess insulin causes almost all the glucose in the blood to be transported 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 properly and mental function becomes seriously deranged, leading sometimes to coma and even more often to mental imbalances and psychotic disturbances—all caused by overtreatment with insulin.
