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PHYSIOLOGICAL TRIGGERS 



The cochlea represents the most sensitive of the general class of mechano- 

 receptors. At the same time, its anatomy makes it accessible, although with 

 some difficulty, for the study of its mechanical, electrical and neural events. In 

 the guinea pig we are particularly fortunate in finding the cochlea of the inner 

 ear projecting into a large tympanic bulla with only a thin covering of bone, like 

 a snail's shell, between the empty space of the bulla and the inner sense organ 

 itself. Figure i shows how holes may be drilled through this bony shell. One or 

 more pairs of electrodes in the form of very fine nichrome steel wires, insulated 

 except at the tips, may be introduced into the various parts of the inner ear 



Fig. 2. Mid-modiolar section of a guinea pig cochlea, showing the auditory nerve. In this 

 particular specimen the nerve cells of the spiral ganglion and the organ of Corti in Turn I 

 have degenerated. 



Larger holes can also be drilled for the insertion of micropipet tes with which to 

 withdraw or inject fluids or to measure DC potentials. It is even possible to 

 introduce hyperfine pipette-electrodes into the cells of the sense organ them- 

 selves, and record their electrical potentials with very little damage to the 

 tissue. Figure 2 shows an actual cross section of the cochlea, and figure 3 shows 

 diagrammatically the introduction of two wire electrodes and one pipette elec- 

 trode into the scala vestibuli, the scala tympani and scala media, respectively, 

 of the first turn and the placement of a reference electrode on the tissues of the 

 neck. It also shows how the AC potential known as the 'cochlear microphonic' 

 appears across the organ of Corti between scala media and scala tympani, and 



