4 : 3/ The Conduction of Impulses by Nerves 



75 



"Myelin Sheath" 



Schwann Cell 



Node of Ranvier 



Fiber Endings 



Figure 3. Diagram of a 

 large myelinated axon. 

 Dark-staining material in 

 cell body is called Nissl 

 substance. In some neu- 

 rons, the axon is off to one 

 side of the cell body; in 

 others the cell body is in 

 the middle of the axon. 

 In both of these types the 

 dendrites are attached 

 directly to the axon; that 

 is, both extreme ends of 

 the axon appear similar. 

 After A. A. Maximow and 

 W. Bloom, Textbook of 

 Histology (Philadelphia : 

 W. B.Saunders Company, 

 1957). 



dimensions. For example, the neurons that 

 control the muscles in the finger have their cell 

 bodies in the spinal cord, whereas their axons 

 run the entire length of the arm. In the elephant 

 and giraffe, the lengths of some of the axons are 

 as great as 3 or 4 meters. 



The axons are grouped together in bundles 

 called nerves. Each nerve consists of a large 

 number of axons of various sizes carrying im- 

 pulsesintheirrespective physiologically important 

 direction. Just as the axons are grouped in 

 nerves, the nerve cell bodies usually occur in 

 compact clusters known as ganglia. In verte- 

 brates, the ganglia within the central nervous 

 system are referred to as nuclei (which is a confusing 

 term). The axon bundles are called fiber tracts 

 within the central nervous system. 



Connections between two neurons are called 

 synapses. These occur between the branched ends 

 of axons, dendrites, and collaterals of different 

 neurons. Other axons end at receptors, such 

 as the hair cells of the organ of Corti in the ear. 

 Still others terminate at controlled targets, as the 

 motor end plates of a muscle fiber, for example. 



In vertebrates, the nervous system is organized 

 into a central nervous system and peripheral 

 nerves and ganglia. Throughout the entire 

 system, the basic element is the neuron. There 

 is nothing inherent in the axon to control the 

 direction in which it conducts. This is a property 

 of the synapse only. In general, a peripheral 

 nerve will contain both sensory (or afferent) 

 axons which conduct toward the central nervous 

 system, and motor (or efferent) axons which 

 conduct away from it. 



The sensory axons conduct impulses from the 

 receptors to the nerve cell bodies. Along verte- 

 brate, afferent pathways extending from receptors 

 to the cerebral cortex, the axons conduct toward 

 the cell bodies. In these, there is always at 

 least one cell body in a ganglion outside of the 

 central nervous system. For example, the sen- 

 sory pathways entering the spinal cord have their 



