Nerve Cells and Neuroglia
There are said to be 27,000,000,000 nerve cells in the human brain and there are about ten times as many neuroglial cells. On the whole, nerve cells are large and glial cells are small, so that their weights are about equal. The cytology of nerve cells does not provide any really startling information. There is a nucleus and a nucleolus complete with sex chromatin, and in the cytoplasm there is Nissl substance formed of RNA and endoplasmic reticulum, mitochondria carrying respiratory enzymes, and lipochromes. Fairly certainly the RNA is concerned with the synthesis of proteins which move out along the axon, and the rate of protein synthesis is increased during axon regeneration. The rate of oxygen uptake is very high, and weight for weight is said to be higher than that in any other cell. Its measurement presents great difficulties, as oxygen uptake of nervous tissue in vivo gives values 5 to 100 times those obtained in vitro. The metabolic activity of nerve cells is believed to increase about five times during and after nervous activity. At the same time, the amounts of cytochrome oxidase and RNA present increase, and there is an increased ammonia production suggesting proteolysis. Excessive activity is said to cause a decrease in cytoplasmic protein and a decrease in RNA. There is a vast literature on cytological changes in cells in overaction, exhaustion and after deprivation of sleep, but there cannot be said to be a coherent body of observed facts, let alone a satisfactory biochemical interpretation of the facts.
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