GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 143 



a frog is excited by a single stimulus to continue as long as the muscle remains 

 active, including the period relaxation ; in some cases it lasted from 0.05 to 

 0.06 second. Sanderson, as we have seen (see Fig. 62), tetanized injured 

 skeletal muscles of the frog, and found not only a series of negative variations 

 corresponding to the contraction processes which resulted from the separate 

 excitations, but a continuous negative variation, the diminutional effect, which 

 developed comparatively slowly and lasted after the irritant had ceased to act. 

 All these facts unite to point to the conclusion that the negative electrical 

 change which develops when a muscle is excited to action is associated with 

 the contraction process. 



4. Currents of Action in Nerves. In general, the facts which have been 

 stated with regard to the current of action in muscles apply to nerves. When 

 a normal nerve is excited a negative change is forthwith developed at the 

 stimulated point and passes thence in both directions along the nerve at the 

 same rate as the nerve impulse. This change is diphasic, first the part excited 

 and later distant parts showing the negative change. If the nerve be injured, 

 and the normal surface be compared with the dying or dead cross section, the 

 second phase is absent. If the nerve be frequently excited, each excitation 

 awakens a separate current of action. The duration of the negative change 

 caused by a single stimulus varies in different conditions from 0.007 to 0.023 

 second. The strength of the current of action likewise varies, but under 

 favorable conditions may be twice as great as the current of rest, 1 and Hering 

 has shown that it is capable of exciting another nerve to action. Nerve-cells 

 and muscles are more sensitive to nerve impulses than our instruments are to 

 the accompanying electrical changes, nevertheless a negative change may be 

 observed to accompany a nerve impulse which has been caused by the excita- 

 tion of the nerve by nerve-cells. 



Dti Bois-Reymond observed with the galvanometer a lessening (" negative 

 variation ") of the demarcation current (" current of rest ") when in strychnia- 

 poisoning the spinal motor nerve-cells were exciting the motor nerves vigor- 

 ously and causing cramp-like tetanic muscular contractions. Gotch and 

 Horsley 2 applied electrodes connected with a capillary electrometer to periph- 

 eral nerves, spinal nerve-roots, and tracts of motor fibres within the spinal 

 cord, and discovered that if the cortical brain-cells in the motor zones were 

 excited, the nerves showed currents of action corresponding in rate to the dis- 

 charge of motor impulses from these brain-cells, e. g. if the epileptiform con- 

 vulsions were occurring at the time, the capillary electrometer revealed changes 

 of potential of like rate in the nerves. 



As far as has been ascertained the nerve impulse has the same general cha- 

 racteristics in all forms of nerves, medullated and non-medullated, sensory, 

 inhibitory and motor, and except as regards strength, rhythm, etc. is the same 

 whether they be excited artificially or normally by a nerve-cell or sensory end- 

 organ. In every case the impulse appears to be accompanied by a current of 



1 Biedermann : EleJctrophysiologie, 1895, p. 666. 



2 Philosophical Transactions, 1891, vol. 182, pp. 267-526. 



