NOTES ON YOST Chapter 7: Structure of the inner ear and its mechanical response pp 81-104

(11 February 2002)

It is of course in the inner ear that the mechanical energy of the sound wave is changed into bioelectrical energy. The almost faithful reproduction of the incoming sound wave is done substantially by mechanical filtering, but also by mechanical and electrical filtering at the level of the individual haircell. Apparently this is accomplished in terms of its electrical resonance characteristics produced by the kinetics of calcium and potassium channels which are genetically controlled as given in a recent Science paper: K. Ramanathan et al. (1999). A molecular mechanism for electrical tuning of cochlear hair cells. Science, 283, 8 January, pp 215 — 217.

STRUCTURE OF THE INNER EAR

The inner ear has three major segments, the semicircular canals, the vestibule, and the cochlea. The perilymph is continuous through the entire labyrinth and the vibrations in one place occur in all places. However the necessary frequency of vibration seems quite different for the different parts of the ear. For example, I do not think that we "hear" head movements – for that is what affects the semicircular canals and the two receptor surfaces in the vestibule, the saccule and the utricle, because the frequency is too low for the auditory system. Similarly, the vestibular orientation system does not orient to sounds because the frequencies are too high.

Yost does not cover the vestibular system at all in this book but it is in fact very similar to the auditory system. The receptors are hair cells embedded in gelatin. This structure is sensitive to the passage of fluids that move the gelatinous substrate, in the semicircular canals because swings in the position of the head start the fluid flowing past this substrate and in the patches of hair cells in the utrical and the saccule by virtue of their varying relationship to the pull of gravity as the head is tilted. They send messages through the 8th nerve, but the vestibular branch rather than the acoustic branch. And they go to the vestibular nuclei rather than the adjacent cochlear nuclei in the brain stem. The oval window is actually in the vestibule rather than the cochlea, which contains the receptors for hearing. The several parts of the cochlear are given in Figure 7.1, and all of these parts are worthy of note. Figure 7.2, another of Hunter-Duvar’s great photographs is more than a little confusing, and we should go through it in detail in class. Figure 7.3 is the standard schematic and most of the terms should become familiar to us. The stria vascularis looks like it is just the wall of the scala media, but in fact it is a critically important structure that is responsible for the metabolism of the ear, and maintains the very high voltage differentials that are necessary for hearing. The voltage differential across the hair cell membrane is much higher than is found in any other neuron. The dimensions of the basilar membrane turn out to be very important, particularly as seen in Figure 7.4a, that it widens as it proceeds to the apex of the cochlea. As it widens it becomes more massive and less stiff: and hence its resonance frequency changes systematically from base to apex. Figure 7.5a is a "real" picture, in contrast to Figure 7.3, and Figure 7.5b shows the actual arrangement of the hair cells and the cilia as they emerge from the reticular lamina. Note on page 88 how the hair cells are held in support cells, and the great photomicrographs in Figures 7.6 and 7.7.

Most of this chapter is taken up with these great photos. Particularly impressive is Figure 7.9 (a and b). Imagine that these tiny processes are in fact the mechanical arms that open ion channels and lead to depolarization and neurotransmitter release, as can be seen in Figure 7.11. Figure 7.12 is the famous traveling wave, in various schematic forms in 7.13 and 7.14. Figure 7.15 is another schematic, but possibly more realistic as it comes from a model based on cadavers, as does von Bekesy’s data in Figures 7.16 a and b. The main point to be made here is that the traveling wave is enormously asymmetric, and this probably accounts for all sorts of effects, particularly masking. Note how phase changes with frequency and distance along the basilar membrane. There is an interesting theory from Shanna that relates the changing wave length of the traveling wave along the basilar membrane to phase differences and lateral inhibition in the cochlear nucleus, in the support of combined place/temporal coding theory of tonality (pitch perception). Figure 7.17 is a classic, showing how sharp the basilar membrane is tuned, in terms of the amplitude of a sound necessary to maintain a constant displacement across frequency. Again, note its asymmetry. These are "real data" taken from a living animal, while the earlier data are based on measures taken in cadavers at very high sound levels. In the third edition Yost showed another "real data" graph of basilar membrane displacement showing distortion products at high stimulus levels. Here instead he shows how the input/output functions for two non-simultaneous tones, one at the best frequency where the function is non-linear, and another for an off-best frequency tone which is linear.

This chapter is all about biomechanical responses in the inner ear; and the question raised at its end is if the neural response is the same or different from this mechanical response. The answer is going to be that it is the same. The statistics of the inner ear described in the supplementary section are many and probably too many to keep in mind save for trivia questions. But do note how very few receptors we have for audition. Each ear has about 3,000 inner haircells along the basilar membrane, but in comparison, there are 50,000 haircells in the saccule and utricle alone responding to orientation in space. We do have about 12,000 outer hair cells, but their sensory function is largely to amplify the vibration of the basilar membrane, and they have few afferent connections to the central auditory system. In comparison, each eye has about 120,000,000 photoreceptors. So the ratio of photic receptors to acoustic receptors is about 40,000 to 1.