SIGNS OF ACTIVITY IN MUSCLE AND NERVE 



433 



The action current does not represent an artificial product, but is a process 

 intimately connected with the process of excitation, for it is produced by all 

 kinds of stimuli ; it is propagated at the same rate of speed 

 as the excitation and varies in strength to a certain ex- 

 tent with the strength of stimulation. 



If the nerve or the muscle be led off to the galvanometer, 

 not from two points on the longitudinal surface, but from 

 the longitudinal surface and a cross section, the second 

 phase of the action current no longer appears, but the cur- 

 rent is now directed from the longitudinal surface over to 

 the cross section. It was in this form that the action cur- 

 rent was first discovered. Since it runs in the opposite 

 direction from the current of rest (see page 48) it was 

 designated by Du Bois-Reymond as the negative variation 

 of the current of rest. 



The action current is the only functional change 

 which we have thus far been able to observe in living 

 nerves. It is of great importance also for the reason 

 that it permits us to determine the nature of any mus- 

 cular contraction. 



FIG. 170. Schema 

 illustrating spread 

 of an excitation 

 causing an action 

 current. 



We have already become acquainted with an example of 

 this in studying the heart. The action current there showed 



us that, notwithstanding its long duration, the contraction of the heart is in 

 reality a simple muscular twitch (cf. page 179). 



There are other kinds of contractions, like tetanus and voluntary contrac- 

 tions, which as we have seen are apparently continuous, but which the action 

 current proves to be discontinuous. If by the use of the rheotome, a muscle be 

 stimulated often enough to produce complete tetanus, the excursions of the gal- 

 vanometer will show that each separate stimulus produces a special action cur- 

 rent of its own i. e., every excitation causes a molecular change in the muscle, 

 although the change may not be apparent in the mechanical behavior of the 

 muscle. 



The action current of muscle as well as of nerve is strong enough to have 

 a stimulating action of its own (Matteucci). If the nerve of one muscle, B, be 

 laid across the belly of another muscle, A, and the second muscle be then stimu- 

 lated through its own nerve, with 

 each contraction of A, B also con- 

 tracts, and this even in case A is 

 so tense that it no longer changes 

 its form. The contractions of B 

 agree minutely in number, strength 

 and sequence with those of A. If 

 A is tetanized, B also is tetanized. 

 These phenomena are called sec- 

 ondary contractions, secondary tet- 

 FIG. 171. Illustrating the theory of electrotonic anus, etc. 



currents, after Hermann. 2. Electrotonic Currents. 



When an electric current is con- 

 ducted through a certain length of a medullated nerve and another portion 

 of the nerve outside of this length is connected with the galvanometer, an 



