4 : 3/ The Conduction of Impulses by Nerves 77 



The neuron cell bodies are, in several ways, similar to the secretory 

 cells of endocrine glands. The dark-staining material within the nerve 

 cell body, known as Nissl substance, has been shown to be chemically 

 and morphologically identical (in electron micrographs) to cellular 

 organelles, known as ribosomes and Golgi bodies. The ribosomes are 

 associated with protein synthesis, and the Golgi bodies with secretion. 

 The nerve cell body appears to "secrete" the axon and dendrites. The 

 metabolic rate and the protein synthesis in the nerve cell body are both 

 greatest when the axon is being formed or replaced after injury and are 

 at a minimum in normal adults. By contrast, the axon appears to 

 lack ribosomes and the ability to synthesize proteins. One may con- 

 sider the nerve cell body to be an intracellular secreting cell, whereas 

 endocrine gland cells produce extracellular secretions. 



The axons do not possess the organelles necessary for protein synthesis. 

 They do contain mitochondria, which are intracellular organelles 

 associated with metabolism. No qualitative differences are known be- 

 tween the metabolism of axons and that of other animal cells. 



The axons are surrounded by myelin sheaths and satellite cells. It is 

 not known if these are important in providing the proper chemical 

 media for axon metabolism. However, it is quite well established that 

 for all axons — vertebrate and invertebrate, so-called "myelinated" and 

 "unmyelinated," large and small — for all of these, the satellite cells 

 extend out and coil around the axon forming a double lipid layer known 

 as myelin. In the larger vertebrate axons, outside of the central nervous 

 system, such as those diagrammed in Figure 3, the satellite cells, called 

 Schwann cells, have processes which are wrapped many times around 

 the axons. In these, the myelin sheath is very easily visible. 



Where the processes from two neighboring Schwann cells meet, the 

 myelin layer is much thinner and is pierced by canals about 300 A in 

 diameter. This region is known as a node of Ranvier. The so-called 

 nonmyelinated fibers appear to have similar "canals" through their 

 myelin sheaths all along the axon. Thus, their axons are in more or 

 less continuous chemical contact with the intercellular fluids. In con- 

 trast, the large myelinated fibers appear to communicate chemically 

 with the intercellular fluids only at the nodes. The possible electrical 

 significance of the myelin sheaths is discussed in the next section. 



At the end of the neural fibers, the surface layer appears in the 

 electron microscope to contain small vesicles. It is believed that these 

 are filled with the chemical carriers active in synaptic transmission. This 

 idea will be referred to again in Section 5 of this chapter. These tiny 

 vesicles are so small that they cannot be conveniently observed except 

 in the electron microscope. 



As a vertebrate, man's greatest interest has been in vertebrate nerves. 



