306 ANIMAL BIOCHEMISTRY 



ponent on ;i weight basis, with brain tissue containing as much as 80 

 per cent. Of the solids present, the lipide fraction is of interest since 

 it contains particularly large quantities of the phospholipides lecithin, 

 cephalin, and sphingomyelin. Glycolipides (cerebrosides), cholesterol, 

 and sulfolipides comprise the remaining portions of the lipide fraction. 

 Triglycerides do not occur in quantity in nerve tissues. The lipide 

 fraction of the brain accounts for 30 to 40 per cent of the dry weight. 



Proteins constitute about 50 per cent of the dry weight of nervous 

 tissue, including small quantities of albumin, several globulins, and 

 nucleoprotein. The major component is a keratin-like protein called 

 neurokeratin, containing from 25 to 50 per cent lipides. The protein 

 and lipide components form alternate concentric layers in the nerve 

 fiber, so that a cross section of the nerve shows rings similar to those 

 observed in a cross section of a tree trunk. 



Nerve tissue also contains appreciable quantities of calcium, magne- 

 sium, and sodium and is exceptionally rich in potassium. When the 

 nerve is stimulated, potassium diffuses into the surrounding fluid; 

 during rest it diffuses back into the fiber. 



Nervous tissue contains the usual extractives associated with metabol- 

 ically active tissue. Creatine, lactic acid, purines, inositol, and other 

 vitamins and cofactors have been isolated. However, only small quan- 

 tities of glycogen are found in the brain. Therefore, nerve tissues 

 must depend on a continual supply of blood glucose as a source of 

 energy. In conditions where blood sugar drops, as, for example, during 

 insulin hypoglycemia, patients become dizzy and confused, and in 

 severe cases lapse into coma. 



Nerve Action 



Nerve impulses arc transmitted from the site of origin to the region 

 of action through a series of neurons. The neuron consists of the 

 nerve cell, dendrites, and the axon (Figure 13-3). Dendrites are the 

 small terminal branches of the nerve fiber which pick up the original 

 impulse i&r transmission through the axon to the site of action. Motor 

 neurons conduct impulses from the brain to areas of contraction and 

 secretion. Sensory neurons receive the stimuli from the sense organs 

 for transmission to the brain. Motor neurons and sensory neurons 

 are interconnected by the interlacing of the dendrites with the fine 

 branches of the sensory fibers. These connections are called synapses. 



Although our knowledge of chemical reactions involved in nerve 

 transmission is quite deficient, there is evidence showing the involve- 

 ment of acetylcholine in the parasympathetic nerve system. This 

 substance accumulates in nervous tissue during rest periods and 



