CHAPTER X 

 GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM 



Conduction within Single Neurons. Since the nervous sys- 

 tem, whose function as a whole is the conduction of impulses 

 from sensory regions to motor ones, is made up of individual 

 neurons, the study of its physiology can best be begun by con- 

 sidering the phenomenon of conduction as exhibited in single 

 neurons, passing later to conduction as it involves more neurons 

 than one. 



The passage of an impulse along a nerve is attended by no 

 visible alteration of the nerve itself; it is impossible to tell by 

 looking at a nerve whether it is carrying impulses or not. For 

 this reason nerve impulses can only be studied indirectly. If 

 as the result of stimulating a motor nerve at some point along 

 its course the muscle in which it terminates is thrown into con- 

 traction the obvious conclusion is that nerve impulses are passing 

 from the point of stimulation to the muscle. When the prick of a 

 finger gives rise within the brain to a conscious sensation of pain 

 we know that a nerve impulse must have passed between the 

 finger and the brain, although we would be unable to detect any 

 sign of its passage if the nerve were visible throughout its length. 

 In addition to these methods of studying nerve impulses, in which 

 the passage of the impulse is made known through its effect on 

 some other part of the Body, we have a method which depends 

 upon the fact that activity of nerve, like activity of muscle or 

 any other living tissue, is accompanied by changes of electrical 

 potential which may give rise to action currents. Every time an 

 impulse passes along a nerve it is accompanied by this electrical 

 alteration. Sensitive electrometers applied to nerves will indi- 

 cate the passage of impulses under their points of contact. 



By the use of these methods of studying nerve impulses we have 

 learned many things about them, although much more remains 

 unknown. 



How Nerve Impulses Are Aroused. We know that nerve im- 



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