Blood in its passage through the brain loses
oxygen and
glucose at relatively high rates, the amount of
oxygen disappearing being approximately equivalent to the amount of
glucose consumed, calculating on the basis that the
sugar is completely oxidized. The respiratory quotient of brain in vivo is unity. These facts point to the dominance of
carbohydrate oxidation in brain respiration in vivo and are similar to those found in studies of brain in vitro.Various factors influence
glucose oxidation in brain, e.g. changes in the ionic environment of the cells,
vitamin B(1), or the presence of
narcotics. The latter bring about inhibitions of
glucose oxidation in brain tissue which may in most cases be shown to be reversible in vitro.
Glucose is not only important for the maintenance of respiration of brain but for enabling certain synthetic processes to occur. One of these is the formation of
acetylcholine whose physiological significance is now well known and whose synthesis seems to be confined to the nervous system. This synthesis depends not only on the presence of
glucose but on that of
oxygen. The influence of
glucose has been observed also in investigations on cortical potentials.An important feature of the nerve cell is its vulnerability to the lack of
oxygen. Reversibility depends on the degree and duration of the anoxaemia.During
insulin shock treatment studies of brain in vivo show lowered oxygen consumption and
glucose utilization, these depending on the degree of hypoglycaemia. In
cardiazol treatment, in vivo studies show that the
oxygen content of the blood may fall to 42%. During the convulsion there is a greatly lowered arterial and venous blood-flow through the brain and cerebral anaemia becomes a marked feature. In
narcosis treatment both in vitro and in vivo studies show a diminished ability of the brain to consume
oxygen.It is suggested that the most significant facts to be taken into account are (1) the importance of
glucose and
oxygen for the metabolism and function of the nervous system, (2) the vulnerability and varying sensitivities of nerve cells to lack of
oxygen and
glucose, (3) the occurrence of varying degrees of cerebral anoxaemia in
narcosis,
insulin and
cardiazol treatments.