282 



im INIIRNAL LNVIRONMLNT OI- THL BODY 



Part III 



Ccrvtrol 

 System. 



Axon. 

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Axon. 

 (m.yclirwi.tad) 



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 otrtd. coven^d VilK 

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Fig. 16.3. Diagram of a multipolar nerve cell, i.e., one with more than one 

 dendrite (process that conducts an impulse into the cell body). Most of the nerve 

 cells in the human body are multipolar. The processes of nerve cells have contacts 

 or synapses; branches of the axon of one cell in contact with the dendrites of 

 another or with the cell body of another or with both. Such contacts are essential 

 for the coordination of the body and for memory. (Courtesy, Ham: Histology, 

 ed. 2. Philadelphia, J. B. Lippincott Co., 1953.) 



main distinction between dendrites and axons is a purely functional one: the 

 dendrites conduct impulses toward the body of the nerve cell and the axons 

 away from it. 



Axons are usually long, often several feet, since in man many of them ex- 

 tend from the spinal cord to the toes. The axon ends in a brush of short 

 branches which in the case of muscle may actually pierce the cell membranes. 

 The axon of the spinal ganglion cell gives off branches (collaterals) along its 

 course in the spinal cord, e.g., one at the level of the fourth and another 

 at the fifth rib, ending in a synapse with the dendrites of other nerve cells. 

 Many muscle cells are stimulated by impulses from one or a few nerve 

 cells (Fig. 16.4). One pinprick in the back starts impulses speeding over 



