GENERAL PHYSIOLOGY OF NERVE-TISSUE. 117 



Nature of the Nerve Impulse. As to the nature of the nerve 

 impulse generated by any of the foregoing stimuli, either general or 

 special, but little is known. It has been supposed to partake of the 

 nature of a molecular disturbance, a combination of physical and 

 chemic processes attended by the liberation of energy, which propa- 

 gates itself from molecule to molecule. The passage of the nerve 

 impulse is accompanied by changes of electric tension, the extent of 

 which is an indication of the intensity of the molecular disturbance. 

 Judging from the deflections of the galvanometer needle it is probable 

 that when the nerve impulse makes its appearance at any given point 

 it is at first feeble, but soon reaches a maximum development, after 

 which it speedily declines and disappears. It may, therefore, be 

 graphically represented as a wave-like movement with a definite 

 length and time duration. Under strictly physiologic conditions the 

 nerve impulse passes in one^ direclTorTohly ; in eflerenrrierves* f r om 

 the center to the periphery, in afferent nerves from the periphery to 

 the center. Experimentally, however, it can be demonstrated that 

 when a nerve impulse is aroused in the course of a nerve by an ade- 

 quate stimulus it travejteequally well in both directions from the point 

 of stimulation. When once started, the impulse is confined to the 

 single fiber and does not diffuse itself to fibers adjacent to it in the 

 same nerve-trunk. 



Rapidity of Conduction of the Nerve Impulse. The passage 

 of a nerve impulse, either from the brain to the periphery or in 

 reverse direction, requires an appreciable period of time. 

 velocity with which the impulse travels in human sensory nerves has <4fl 

 been estimated at about 50 meters a second, and for motor nerves at 

 from 28 to 33 meters a second.l\The rate of movement is, however, 

 somewhat modified by temperature, cold lessening and heat increas- 

 ing the rapidity; it is also modified by electric conditions, by the 

 action of drugs, the strength of the stimulus, etc. The rate of trans- 

 mission through the spinal cord is considerably slower than in nerves, 

 the average velocity for voluntary motor impulses being only 1 1 meters 

 a second, for sensory impulses 12 meters, and for tactile impulses 40 

 meters a second. /fU>0 J 



Nerve Fatigue.- Inasmuch as nerves are parts of living cells, 

 the seat of nutritive cEanges, it might be supposed that the passage 

 of nerve impulses would be attended by the disruption of energy- 

 holding compounds, the production of waste products, the liberation 

 of heat, and in time by the phenomena of fatigue. ^Though it is 

 probable that changes of this character occur, yet no reliable 

 experimental data have been obtained which afford a clue as to the 

 nature or extent of any such changes. Stimulation of motor nerves * 

 with the induced electric current for four hours appears to be without 

 influence either on the intensity of the nerve impulse or the rate of its 

 conduction. 



