The mammalian ear features three tiny bones that transmit air compression waves from the eardrum to the cochlea, a snailshell-shaped organ in the inner ear. The cochlea features two fluid-filled chambers separated by the basilar membrane, which supports numerous hair cells, which feature cilia which are bundled into a shape which, under an electron microscope, looks like a rugose cone:
Here's a good animation of a "straightened out" basilar membrane, and its response to sounds of different frequencies:
The cilia of the "hair cells" have ion channels at the tip which are activated by the motion of the cilia bundle, and change the electrical potential of the hair cell, resulting in a nerve impulse (I am grossly oversimplifying in the interest of brevity).
Human hearing is distinguished by relatively low amplitude discrimination (a difference of 10% amplitude is fairly hard to distinguish), but very well developed frequency discrimination.
Tetrapod hearing is characterized by an "active process", which can be summed up succinctly:
The hearing of tetrapods including humans is enhanced by an active process that amplifies the mechanical inputs associated with sound, sharpens frequency selectivity, and compresses the range of responsiveness. The most striking manifestation of the active process is spontaneous otoacoustic emission, the unprovoked emergence of sound from an ear.
Yes, the ears generate sound, as well as discern it.
The lecture was accompanied by demonstrations- human hearing is very sensitive on a horizontal plane, one can typically determine the source of a sound within one degree horizontally, but the vertical perception of sound is not as sensitive (within about ten degrees)- the superior olivary complex plays a role in this discriminatory ability. He also demonstrated phantom tones (especially note the clip labeled "beats at 10Hz-35Hz"- far out!). He also briefly touched on the ability of zebrafish to regenerate hair cells in their lateral lines (similar to those in the tetrapod cochlea), and possible therapeutic implications of hair cell regeneration (humans don't regenerate hair cells, and many of us have hearing degradation and loss as we age, or due to exposure to loud rock and/or roll music... not that I'm singling anybody out here... Finally, Dr Hudspeth discussed cochlear implants, and their use to treat hearing loss.
In the Q&A, some bastard asked Dr Hudspeth if arthropods had a similar hearing mechanism. While insects do not possess hair cells, they have analogous cells. Interestingly enough, insects also have an "active process" similar to that of vertebrates. After the lecture, this bastard also had the temerity to discuss the controversy in the Deaf (note the capital D, denoting a culture) community regarding cochlear implants. The controversy has simmered down somewhat (Dr Hudspeth likened the situation to that of other "bilingual" communities, and recommended the film Sound and Fury and its follow-up.
All told, another fantastic lecture. I'll write about the pre-lecture entertainment in another post.
POSTSCRIPT: The fact that the hair cells in the cochlea and the colour-sensitive cells of the retina are both cone-shaped has got me thinking... maybe the