TRANSMISSION OF IMPULSES THROUGH THE NERVE CELL 507 



Specific Energy of the Nerve Impulses. We have already discussed 

 the fact that a nerve fiber, also its cell body, is irritable to various forms of 

 mechanical, electrical, etc., stimuli. In the complex differentiation of the 

 nervous system it is found that whatever the form of the external stimulus 

 applied to a nerve the resulting nerve impulse produces the same effects in the 

 central nervous system. This idea has been called the specific energy of the 

 nerve impulse, and was first advanced by Johannes Miiller. 



Transmission of Nerve Impulses through the Nerve Cell. The the- 

 ory has been advanced that in the nerve cell the primary function of some 

 processes is to carry nerve impulses toward the cell body, and of other processes 

 to. carry nerve impulses away from the cell body. At the present time this 

 view is advocated by perhaps the ablest living anatomists and neurologists. 

 The dendrites conduct toward the cell body, and the axones away from it. 

 That is, the former are cellulipetal, the latter cellulifugal. 



Impressions made upon the terminations or upon the trunk of a cellulifugal 

 nerve may cause, a, pain or some other kind of general sensation; b, special 

 sensation; c, reflex action of some kind; or d, inhibition or restraint of action. 

 Similarly impressions made upon a cellulipetal nerve may cause, a, contraction 

 of muscle (motor nerve); b, it may influence nutrition (trophic nerve); 

 c, it may influence secretion (secretory nerve); or d, inhibit, augment, or stop 

 any other efferent action. 



By artificial stimulation nerve impulses can be made to pass in both di- 

 rections in all classes of nerve processes. That is to say, if a motor axone is 

 stimulated in the middle of its course it will not only convey a nerve impulse 

 to its distribution, but also a nerve impulse will pass back over the fiber to 

 the cell body and out over the dendrites. Normally, in the complex of the 

 body it is probable that such a neurone will be stimulated only at its points 

 of contact with other neurones chiefly through its dendrites, and especially 

 by means of the sensory cells. The dendrites will therefore receive the nerve 

 stimulus, carry it through the cell body to the axone and its distribution- 

 In such cells there is isolated, or uninterrupted, conduction throughout the 

 extent of the neurone. The nerve impulse is able to pass from a given 

 neurone to adjacent ones only at the termination of the axone or its branches, 

 which may be considered as special organs for the transference of the nerve 

 impulses. This activity involves isolated conduction in nerve fibers bound in 

 a common nerve trunk. It has been supposed that the myelin sheath of a 

 medullated nerve acts as an insulator of the axis-cylinder, but this can be 

 only relatively true, for the reason that non-medullated nerves do not possess 

 the myelin sheath. In non-medullated nerves we must suppose that the 

 primitive sheath is sufficient to give insulated conduction, or that it is an in- 

 herent property of the axis-cylinder itself to carry the nerve impulse without 

 transmission to adjacent fibers. 



We have already, page 470, discussed the rate of transmission of the nerve 



