58o 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



nerve impulses, derived originally from studies of the 

 whole-nerve action potentials, have now been con- 

 firmed or extended by studies of single fiber activity. 



The auditory axons are 2.5 to 4.0 fi in diameter. 

 Tasaki succeeded in inserting hyperfine electrodes 

 into individual axons in the modiolus near the internal 

 auditory meatus while delivering brief tone bursts or 

 steady tones to the guinea pig (fig. 18). The spike 

 responses resembled those from myelinated fibers of 

 similar size elsewhere. Injury discharge was some- 

 times seen and also responses clearly related to the 

 auditory stimuli. The response to a brief burst or 

 click was often repetitive as shown in figure i g, some- 

 times outlasting the stimulus by 20 to 30 msec. The 

 minimum interval between impulses in such discharges 

 was I msec. 'Spontaneous' impulses, i.e. not correlated 

 with acoustic stimuli, were often seen in the same fibers 

 that also gave clear responses to sounds. No inhibition 

 of spontaneous impulses by acoustic stimuli was ever 

 seen. 



Some fibers regularly tended to give single, others 

 repetitive responses. Some fibers had low thresholds; 

 others high. Most fibers were partially selective with 

 respect to frequency. Each showed a very sharp and 

 very stable cut-off frequency above which it failed to 

 respond even at high intensities of stimulation. At a 

 frequency only slightly below the cut-ofF the fiber was 

 most sensitive, but the rise in threshold with further 

 reduction in frecjuency was very gradual (fig. 19). 

 Nearly all fibers encountered had cut-ofFs above 1000 



Response Area of a single auditory nerve fiber 



- 



> 



2 -20- 



3 



S -40 



-60 



2345 6 7 8KC 



Frequency 



FIG. 19. Repetitive responses of a single auditory fiber to 

 tone pips of different frequencies and intensities. Doited line 

 shows boundary of response area' of this fiber. [From Tasaki 

 (■4)-] 



cps but in a few fibers a cut-off as low as too cps was 

 found. The 'response areas' mapped out by Tasaki 

 are much like those described earlier by Galambos 

 and Davis for units now known to be second order 

 (cochlear nucleus) neurons; but the high-frequency 

 cut-off is rather sharper, the low-frequency decline is 

 more gradual and inhibition of acoustic responses was 

 never observed. 



During continued tonal stimulation an apparently 

 irregular discharge continued but at a gradually 

 diminishing rate. This is the phenomenon of adapta- 

 tion, for which there is also good psychoacoustic 

 evidence. The continuing discharge was superficially 

 irregular but aciually, except for a few (presumably 

 'spontaneous') impulses, all the impulses from a given 

 fiber fell in approximately the same phase relation to 

 the tonal stimulus and the cochlear microphonic as 

 explained above. 



Concerning recovery from adaptation, fatigue, or 

 both, the information from psychoacoustics has con- 

 siderably outrun that from physiology. The recovery 

 curve of Ni of the composite nerve response is mono- 

 tonic, unlike the recovery curve of psychoacoustic 

 threshold. The partial depression of a second nerve 

 response depends both on the intensity of the first 

 click and on the duration of the interval following it, 

 and it outlasts by 10 msec, the refractory period of the 

 fillers. 



The action potential threshold for clicks in guinea 

 pig or cat may be within an order of magnitude of 

 the human auditory threshold. With increasing power 

 (in decibels), Ni increases along a sigmoid curve, 

 reaches a nearly flat plateau and then, with fairly 

 strong stimuli, rises much more rapidly again. The 

 tendency of single units to group into high -threshold 

 and low-threshold classes may explain this nonlinear 

 behavior of Ni in the whole-nerve response. The maxi- 

 mum of response is uncertain, due to the onset of 

 'fatigue' or 'incipient acoustic trauma.' 



Efferent Inhibitory Aetion'- 



Stimulation in the medulla of the olivocochlear 

 tract of Rasmussen produces an inhibitory effect on 

 the action potential response to clicks. Nj is clearly 

 reduced, but the cochlear microphonic is not affected. 

 The effect is very specific with respect to the location 

 of the stimulating electrodes, and the middle ear with 

 its tympanic muscles is definitely not involved. The 

 reduction appears some 20 to 30 msec, after stimula- 



'^ See especially the paper by Galambos (7). 



