'454 



II \M)HIM 'k Ml PHYSIOLOGY 



NEUROPHYSIOLOGY III 



From future experiments new kinds of information 

 are needed to fill in the gaps which now exist. We 

 need in particular to know more about: a) patterns 

 of neural activity elicited In peripheral stimulation 

 and b) effects of systematically varying patterns of 

 nerve pathway or nerve center activity on dis- 

 criminatory behavior. 



INTENSm DISCRIMINATION. In the very first experi- 

 ments in which electrical signs of activity in sensory 

 nerves were recorded successfully during stimulation 

 of the sense organs supplied by the nerves, it was 

 apparent that increase in intensity of stimulation 

 leads to increase in rate of firing of nerve impulses. 

 It was also noted that as intensity is increased, more 

 nerve fibers are activated (3, 5). 



Through the use of techniques designed to isolate 

 single sensory nerve fibers and through the use of 

 better methods of electrical recording, analysis of 

 the responses in single first-order fibers of the visual 

 (87), auditory (203), tactual (5, 6), kinesthetic (140- 

 142), vestibular (133, 134) and taste (166) systems 

 has now been accomplished. In all instances, a 

 systematic relationship between strength of stimulus 

 and frequency of nerve impulses has been found. The 

 picture has been complicated, however, by the 

 further finding that in all of these systems there are 

 many fibers in the first order neurons in which a 

 'spontaneous 1 discharge occurs, i.e. a regular firing 

 of nerve impulses occurs even when the end organ 

 is not being stimulated. This phenomenon was 

 noted by Adrian in his 1926 report (3) and again by 

 Adrian & Zotterman (5); but they could not rule 

 out the possibility that in the receptors of resting 

 muscle or in tactual receptors not being intention. ilk 

 stimulated, a mild degree of stimulation was taking 

 place due to such factors as muscle tonus or skin 

 tension. In other systems, e.g. vestibular, visual and 

 auditory, it appears less likely that the spontaneous 

 firim; which has been observed (88, 133, 134, 203) 

 can be accounted for in terms of mild external 

 stimulation ol receptor organs. A more likerj ex- 

 planation is that metabolic processes within the end 

 organ give rise to the spontaneous discharges. 



In addition to spontaneous discharge and the in- 

 e in frequency ol discharge with increase in 

 intensity ol stimulation, another phenomenon has 

 been ui sei ved in first ok lei neurons ol the \ estibular 

 and taste systems. Lowenstein & Sand (133, 1 34) 

 have isolated single libers from the ampullae ol the 

 semi, in nl. 11 canals ol the ra\ and have found that 

 these fibers fire spontaneous!) when the end organ is 



at rest, fire at an increased rate when the head is 

 rotated towards the side of the semicircular canal 

 from which a recording is being made, and fire at a 

 rate below that of spontaneous discharge when the 

 head is rotated in a direction away from the side of 

 the canal being tested. 



Pfaffman (167) has observed inhibition of spon- 

 taneous discharge in single fibers of the chorda 

 tympani of the rabbit. The inhibition occurs when 

 taste receptors are stimulated by weak solutions of 

 sodium chloride; other substances such as potassium 

 chloride and hydrochloric acid produce increased 

 discharge. 



While similar findings to those reported by Lowen- 

 stein & Sand for the vestibular system and by Pfaff- 

 man for taste have not been observed for first-order 

 fibers of the other sensory systems, observations ob- 

 tained from microelectrode recording in peripheral 

 neurons one or more synapses removed from the end- 

 organ receptors reveal a number of phenomena 

 which must be taken into consideration in relating 

 neural discharge rate to peripheral stimulus in- 

 tensity. 



In the optic nerve of the frog, Hartline (88) has 

 described three types of nerve fibers, classified in 

 terms of their responses to peripheral stimulation. 

 Fibers of one type respond to onset of stimulations, 

 after an initial burst of rapid activity, these fibers 

 maintain a somewhat slower steady rate of response 

 during the period of stimulation. A second type 

 responds with a burst of impulses at onset and at 

 cessation of stimulation but remains silent in between. 

 Fibers of a third type fire only at cessation of stimu- 

 lation. 



Recording from cell bodies of second order neurons 

 of the auditory nerve of the cat, Galambos & Davis 

 (69, 70) have discovered single units which fire 

 spontaneously in the absence of end-organ stimula- 

 tion, at an increased rate during stimulation by tones 

 of some frequencies, and at a decreased rate during 

 stimulation bv tones in a different part of the fre- 

 quency range. Other fibers of the auditor) system 



show only increased rate of discharge when the end 

 organ is stimulated. No 'off responses were reported 

 by Galambos & Davis [asaki & Davis (205), re- 

 cording with microelectrodes from second order 

 auditor) libers in the guinea pig, failed to find fibers 

 in which spontaneous discharge was inhibited b) 

 tonal stimulation. Tas.iki (204) suggests that the 

 difference between the results obtained bv him and 

 Davis and those reported earlier bv Galambos & 



I ).iv is 1 1, 1 the cat ma) be due in part to difference in 



