How we hear

As part of publishing my book, I of course got to know the people at Wiley to a certain extent. They invited me to review a book proposal, in return for which I was offered a credit against anything Wiley published. I browsed around and decided to indulge myself with a copy of Principles of Human Anatomy (Tortora and Nielsen), a topic that has always interested me.

It’s about a thousand pages, lushly illustrated. I have been working through it for many weeks now, and learning a lot of interesting things. Prior to my Achilles surgery, of course, I went over the foot and leg part of the book, an exercise during which it became very apparent that a thousand pages is only enough for a cursory overview. Although the drawings are very well done, I also found that dissection videos [not for the squeamish!] on YouTube gave me a much better understanding of the three-dimensional arrangements of these things.

Most recently I have been reading about eyes and ears. Lots of really interesting things about eyes, but this post is about ears.

To my nasty, suspicious engineer’s mind, the cochlear ducts with the organ of Corti look a lot like a filter. My thought was that forward- and counter-propagating waves might create highly localized resonance peaks that could help account for our ability to discriminate fine pitch. So I did some research.

Simple filtering doesn’t quite tell the story, but a lot is known. There are three outer rows of hair cells, which are mostly enervated by motor neurons (!), and a single inner row of hair cells, which receives about 95% of the efferent (sensory) neurons. The outer hair cells appear to act as an active amplifier (Gold, as summarized by Bell), and probably as a filter.

The ear actually produces sound — I didn’t know that! — otoacoustic emissions. I was disappointed not to find a recording of such an emission on the web. At any rate, none of this is new to those in the business.

I did find some interesting videos that explained how all this works in more detail:

The first one is a good introduction to the process of hearing, and in particular to the arrangement of the scala vestibuli, the scala tympani and the scala media (between them, as you would expect). The organ of Corti is the amplifier, filter and detector. It exists in the scala media, atop the basilar membrane, which separates the scala tympani from the scala media. A tongue-like membrane in the scala media, the tectorial membrane, overlaps and touches the tops of the hair cells. The simple idea is that motion between the membranes causes the hair cells to flex, ultimately resulting in the generation of a neural impulse and the sensation of hearing.

Hair cells are equipped in four rows, three rows of outer hair cells and one row of inner hair cells. 95% of the sensory neurons go to the single row of inner hair cells: this is where signal detection occurs. The outer three rows receive efferent (motor) neurons, and act to filter and amplify pressure waves before unleashing them onto the inner row.

All this is really cool, except for one small detail. In the part of this video that shows how auditory sensation is generated, the animation zooms in on an outer hair cell. Well, no, I don’t think so. Not unless there’s a whole lot of the story they’re not telling us!

The second video covers some of the same ground, but has some excellent electron microscopy of actual hair cells, and even shows a hair cell in action. Definitely worth the watching.

In the discussion about hearing loss, however, this video also talks about  loss of hairs on the outer cells, not the inner ones. Now, we would expect that loss of outer cell hairs would result in less amplification, perhaps, and poorer pitch resolution, but those are surely secondary effects. The primary loss would surely be loss of hairs on the inner layer cells.

I checked out several other models and animations, as well as learned papers on the web, and they all of them are missing what seems to me to be another important point.

Above (in the usual drawing) the scala media is the scala vestibuli, separated from the scala media by the vestibular membrane, also known as Reissner’s membrane. I found only one explanation of its purpose, namely to assist in maintaining chemical and nutrient balance for the part that really matters.

No, no, no. This is like hanging a dog on the wall in act I and then wondering why it did not bark in the night. If there were no functionality associated with the scala vestibuli, it wouldn’t be there, and for sure it wouldn’t have a flexible membrane coupling it in with the organ of Corti.

There has to be something there, and I return, with modifications, to my conjecture from the beginning that the difference between the pressure wave from the two outer scalae is related to delay evaluation and helps with stereolocation of sound sources.

Further, the hairs on at least the outer cells are of different lengths, and could therefore plausibly be understood to respond to compression, rather than just horizontal displacement. That surely has to be part of the story, too.

Well, I am a complete amateur on all of this, and probably don’t even know what questions to ask, much less how to recognize a good answer. But I have also learned that the frontier of our knowledge is closer than we sometimes think, and a thoughtful question can sometimes be productive.

Tags: cochlea, hair cells, Hearing