RECEPTION, TRANSMISSION, AND DISCHARGE 127 



ms 





course, much slower than electrical transmission through metals 

 or air. That an electric current occurs simultaneously with the 

 transmission of an impulse has been clearly established, although it 

 must be granted that that does not prove they are one and the 

 same. It may well be that the process is dependent upon both 

 chemical and physical factors. 



An interesting feature of transmission is that the direction of 

 conduction in the fiber is not reversible. Afferent fibers always 

 carry impulses toward the central nervous 

 system; efferent fibers always carry im- 

 pulses away from the central s}- stem. No 

 fundamental difference in the morphol- 

 ogy of these fibers has been found. It 

 was stated above that it was impossible 

 to fatigue a nerve trunk, that is, to 

 diminish its capacity to transmit impulses. 

 However, fatigue occurs when the im- 

 pulse is permitted to pass over the entire 

 reflex arc and produce an effect. Con- 

 duction through the central nervous sj-s- 



tem includes transmission through ,, __ ., , 



° I'lG. 70. — Neurorimseular 



synapses that connect neurones. The junction of a medullated 



evidence indicates that changes occurring nerve and striated muscle. 



in the synapses are responsible for nerve Note that the medullary 



fatigue. 



When the impulse reaches the effector . ^ . ^ • .• . 



11, that come mto mtnnate 



cell. It passes through the neuromuscular ^^^^^^^ ^^^^,^ ^,^^ ^^^^,^^.1^ 



junction, or end organ, to the protoplasm cell. 



(Fig. 70). This is the discharge of the rns, medullary sheath; n. 



impulse, and little is known of its nature. °^"'"''^™'"='; "^- "'^'"^e fiber. 



. (From Schafer, "Essentials of 



It has been mentioned before that the same Histology," copyright, loie, by 

 response can be obtained in an effector Loi^gmans. Green and Co.. re- 



. • I- Trv i printed by permission.) 



by impulses coming from many different 



types of receptors. Nervous control may be said to be of two 

 kinds, excitation and inhibition. The effector reacts as a result 

 of excitation. Its reaction ceases as the result of inhibition. Inhi- 

 bition might be brought about by blocking the transmission of the 

 exciting impulse, by altering the mechanism of reaction in the 

 protoplasm of the effector, or by making the neuromuscular junc- 

 tion non-functional. Some evidence indicates that inhibition 



sheath does not cover the 

 branches of the nerve fiber 

 come 



