THE FUNCTION OF NERVE FIBER 533 



THE FUNCTION OF NERVE FIBER. 



The Nerve Impulse. The motor nerve fibers of the muscle-nerve 

 preparation are of the medullated type described on page 66. But the es- 

 sential structure, possessed by all fibers, is the axis cylinder. The peculiar 

 function of the nerve fiber, i.e , of the axis cylinder, is its power to conduct 

 a physiological change along its extent, a phenomenon known as a nerve 

 impulse. A normal nerve impulse in a motor nerve has its origin in the 

 motor cell of the central nervous system of which the fiber is an outgrowth. 

 The manner in which such discharge from the cell takes place will be dis- 

 cussed later. But nerve impulses may be aroused by various artificial 

 means, they are influenced by certain conditions in the environment, and 

 possess certain other properties that may be discussed at this point. 



Nerve Stimuli. Nerve fibers like skeletal muscle require stimulation 

 before they can manifest any of their properties, since they have no power 

 of themselves of originating nerve impulses. The stimuli which are capable 

 of exciting nerves to action are, as in the case of muscle, very diverse. The 

 mechanical, chemical, thermal, and electrical stimuli which may be used 

 in the case of muscles are also, with certain differences in the methods em- 

 ployed, efficacious in stimulating the nerve. The chemical stimuli are chiefly 

 these: withdrawal of water as by drying; strong solutions of neutral salts of 

 potassium, sodium, etc.; free inorganic acids, except phosphoric; and some 

 organic acids. The electrical stimuli employed are the induction and con- 

 tinuous currents concerning which the observations in reference to muscular 

 irritability should be consulted. Galvanic currents stimulate nerves only at 

 the moment of turning on the current and of turning it off. Weaker electrical 

 stimuli will excite nerves than will excite muscles; the nerve impulse appears 

 to gain strength as it descends, and a weaker stimulus applied far from the 

 muscle will have the same effect as a slightly stronger one applied to the 

 nerve near the muscle. 



Characteristics of the Nerve Impulse. When a nerve impulse is 

 aroused in a motor nerve, as by stimulating a nerve in its course by an in- 

 duced current of medium strength, it is propagated along the axis cylinder 

 to the muscle where it arouses a contraction of the muscle fiber. In the con- 

 traction of the muscle we have indirect but conclusive evidence of the passage 

 of the nerve impulse, for it can be readily proven that the electrical current 

 does not escape to the muscle. In this instance it can be shown that there 

 is a nerve impulse passing from the point of stimulation in the direction away 

 from the muscle; i.e.j the artificially aroused nerve impulse passes over the 

 entire extent of the fiber stimulated. In fact, a nerve impulse is known to 

 travel from its point of origin over the entire neurone affected. This anti- 

 dromal nerve impulse, of course, does not exist in the normal case, since the 



