HAT.LOWELL DAVIS 



63 



also shows that the less well known 'summating potential' appears in the scala 

 media. Both of these potentials occur in response to stimulation by sound. 



Let us first dismiss the summating potential. This response is best described as 

 a change in the resting DC potential of the scala media. In its amplitude and 

 time course it looks like a full-wave rectification and running integration of the 

 acoustic stimulus. It appears only at relatively high sound levels and it con- 

 tinues to increase in amplitude at levels well above that at which the cochlear 

 microphonic reaches its maximum. The summating potential is thus a high- 

 intensity resi)onse. It may have an origin and function broadly similar to the 



Endolymphatic 



Potential 

 Resting DC 



Summating 

 Potential 

 DC Response 



Action Potential 

 Transient response 



BULLA 



Fig. 3. Position of electrodes and orientation of potentials in the first turn of the guinea 

 pig cochlea. The small droplets of cement on the electrodes prevent the wires from slipping 

 too far through the holes drilled through the bony wall. The pipette is introduced through 

 the spiral ligament by means of a micromanipulator. 



cochlear microphonic, but, from the point of view of trigger action, it is less 

 interesting than the microphonic and we shall not consider it further. 



Neither shall we be concerned with the way in which the middle ear conducts 

 sound to the inner ear or with how the inner ear performs its mechanical 

 acoustic analysis. The result of this acoustic 'analysis' is that low-frequency 

 sounds agitate the whole length of the basilar membrane of the cochlea, and 

 particularly its apical end, while high-frequency sounds cause movement only 

 in the basal turn. The movements are, however, fundamentally the same regard- 

 less of the part of the cochlea in which they occur. It is with these typical move- 



