ON REACTIOX-TIMES AND THE VELOCITY OF THE NEIUOUS IMPULSE.' 



By J. JU Keen Cattkix, Ph. D., and Chakles S. Dolley, M. 1). 



The object of this icseiucli is to detenniue the coiulitioiis which artect the length of the 

 reaction-time on dermal stimuli, and to study the application of the reaction-time to the measure- 

 ment of the velocity of the nervous impulse in motor and sensory nerves and in motor and sensory 

 tracts of the spinal cord. 



Since \()ii Hehiiholtz tirst measured the velocity of the nervous imi)ulse in 1850 much work 

 has been directed to the subject,*but the results are not accordant. The experiments on the 

 nerve-muscle preparation of the frog are the most easily carried out, and these are usually 

 regarded as valid for the motor and sensory nerves of man. It does not, however, follow that 

 the effects of electrical stimulation on the excised and dying nerve of the frog are the same as the 

 effects of cerebral discharge iu the living animal, nor that these effects (could they be determined) 

 would hold for nuxn. 



Dcterminatious made by electrically stimulating the living nerve of the lower mammals and 

 of man arc of more value for huuum ])hysiology than those on the excised nerve of a frog. They 

 are, however, less accordant. We are ignorant of the relations between electrical stiuuilatiou 

 and nervous discharge, and do not know what happens iu the motor nerve and muscle when the 

 skin is stimulated by electricity. It seems evident that the velocity of the normal nervous impulse 

 cau not be determined iu this way, owing to the great variation in results, which must be due to 

 the method of stimulatiou and not to the velocity of the normal impulse. Thus von Helmholtz 

 obtained times twice as long iu winter as iu summer, and supposes this to be due to differences 

 in the conductivity of the nerve. This is not, however, the case, as we flud that the reaction- 

 time, in which the time of transition along the uerves is a large factor, is the same iu winter as 

 iu summer. It is further evident that such experiments api)ly (udy to the uu)tor nerve. The time 

 of transmission may be the same as iu the sensory nerve, but to assume this would be arbitrary. 



So long as we can not record the progress of the nervous impulse along the nerve nor the 

 instant at which it reaches or leaves the brain, the rate of transmission of the normal sensory or 

 motor impulse can only be determined indirectly. In the case of motor nerves it is necessary to 

 make movements with muscles at varying distances from the brain following as quickly as possible 

 on the same stimulus. In the case of sensory uerves the stimulation must be given at varying 

 distances from the brain, aud the arrival must be followed by a movement or directly judged by 

 consciousuess. 



In these experiments the results are obtained by measuring the time of a complex process — 

 the reacticm. The reaction-time is the interval elai)sing before a predetermined movement follows 

 on a jiredetermined stimulus. During this interval a series of physiological processes takes 

 place. (1) The stimulus is converted into a nervous impulse; (2) the nervous impulse travels 

 along the sensory nerve and, it may be, the spinal cord to the brain; (3) through sensory tracts of 

 the brain to a sensory center; (4) changes occur iu this center; (5) these changes are followed by 



Presenteil by I'rof. (i. F. Harkc r betViic tlie meeting oC the Xatioual Academy of Sciences, Albany, 1893. 



393 



