578 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



tion of a high-potassium solution like endolymph into 

 the scala tympani (fig. i6), and b') poisoning with 

 streptomycin just sufficient to cause degenerative 

 changes in the hair cells. 



Surgical injury or injections into the scala media of 

 solutions of ionic makeup substantially different from 

 endolvmph causes a depression of the cochlear micro- 

 phonic as well as the endocochlear potential, as in 

 anoxia but more slowly. The parallelism between the 

 endocochlear potential and cochlear microphonic 

 here holds in general but is not exact. 



In complete anoxia, and continuing for an hour or 

 more post mortem, a small cochlear microphonic 

 remains. This residue may depend, in part at least, 

 on oxygen that diffuses to the basal turn through the 

 round window, but perhaps it is generated in part 

 by an anaerobic mechanism. This latter might be a 

 transducer action like that of a condenser microphone. 

 The primary aerobic cochlear microphonic, however, 

 apparently represents an amplifier action in which 

 energy from a pre-existing ' biological pool of energy," 

 such as is suggested by the endocochlear potential, 

 is 'valved' or modulated by the mechanical bending 

 of the hairs. In any case the sustained changes in the 

 endocochlear potential noted above certainly repre- 

 sent a modulation of a biological source of energy and 

 not simplv a passive physical transducer mechanism. 



The summating potential, as usually seen, is a dis- 

 placement of the base line of the oscilloscopic record 

 on which the cochlear microphonic is superimposed. 

 It is a paradoxical fact that, with mild anoxia, ab- 

 normal ionic concentrations, etc., as the cochlear 

 microphonic diminishes the negative summating 

 potential increases (see fig. i6). The diphasic effect 

 of anoxia, etc., is best explained by assuming a) that 

 there is a positive as well as a negative summating 

 potential, generated by a different set of sensory cells, 

 and ft) that the positive response is more sensitive to 

 anoxia than the negative. The negative summating 

 potential seems to be stronger, although higher in 

 'threshold' (of detection); only under more severe 

 anoxia or ionic injury does it weaken and finally 

 disappear. The positive summating potential is 

 attributed to the inner hair cells which are known to 

 be the more sensitive to anoxia. "^ With moderate 

 stimuli in a fresh preparation the positive summating 



'° On the basis of more recent evidence (6), the negative 

 summating potential is attributed to the internal hair cells and 

 the cochlear microphonic and the positive summating potential 

 are attributed to the external hair cells. 



ENDO-TYRODE IN SCALA TYMPANI 



Cochlear 



MlCROPHONIcl 

 AND .1 



Summating/ I 

 Potential 



Action / 

 Potential 



STIMULUS 

 4 MSEC 

 9450 GPS 

 I MSEC 



RISE TIME 



-."^J^^fNUlC^ 



BEFORE 

 INJECTION 



10 SEC 

 AFTER 



90 SEC 

 AFTER 



DC REMAINED CONSTANT at 75 MV 



FIG. 1 6. A solution with high potassium and low sodium 

 concentration like endolymph injected into the scala tympani 

 depresses cochlear microphonic, summating potential and ac- 

 tion potential before affecting the endocochlear potentizd in 

 the scala media. Note the transient increase in summating 

 potential at lo sec. Downward deflections indicate the scala 

 vestibuli to be more negative relative to the scala tympani 

 {top tines^ or the cochlea more negative relative to the neck 

 (lower lines'). [From H. Davis, unpublished observations.] 



Positive SP 



Positive and Negative SP 





0\^^r^ 



AP 



TONE BURST 



AP 



8600 GPS 



+ 20 DB 



FIG. 17. Action potentials, cochlear micophonics and sum 

 mating potentials from the basal turn. An increase of 20 db in 

 stimulus intensity causes the negative summating potential 

 nearly to obscure the smaller positive response. Note Ni, No, 

 and Nt in the action potential response to the stronger tone 

 burst. [From H. Davis, unpublished observations.] 



potential may dominate (fig. i 7) The reduction in 

 positive summating potential causes the apparent 

 increase in the opposing negative summating poten- 

 tial. The full sequence of changes may be even more 

 complicated and depends in its details on the initial 

 condition of the organ of Corti, the location of the 

 recording electrodes and the frequency of the tone 

 bursts used to elicit the responses. 



