Now Hear This (Part II)

We have all heard of the “One-Hit Wonder”; when a seemingly inspired artist is able to create a masterpiece but only once in their career. The work is often played and remembered as a pop sensation, but the remaining body of work is totally underwhelming in comparison or even non-existent in the case that the artist ceases to write new pieces following the success of the acclaimed song.

Alfonso Corti

Alfonso Corti

But in science? The same phenomenon can occur. A brilliant mind publishing field-defining work promptly disappearing from the major journals following the famed discovery? This is certainly the case for Alfonso Corti, the man who nearly single-handedly uncovered the mysterious cellular basis for our understanding of sound. The diligent, detailed study of the mammalian cochlea is unrivaled in its precision. His studies on the topic may have only lasted one season in the spring of 1850 at Wurzburg, but the body of work is so complete that one could hardly argue against his proposed mechanism of sound transduction. However, less than a year after the publication of his studies on the cochlea, Conti’s inherited estate following his father’s death forced him into the busy career path of a nineteenth century European aristocrat and he never returned to his careful studies.

What exactly is the focus of Conti’s season of intense observation? In part one we looked at the physical logic of sound and the overarching themes of the cochlea’s ability to dissect sounds. Now we will examine the cellular basis for this phenomenon in the Organ of Conti, a structure within the cochlea.

Recall that a cross-section of the cochlea reveals three separate compartments and the wavelike movements of the separating membranes create stress on small structures called hair cells. The hair cells are organized in a row of single cells closer to the attachment point of the basilar membrane called “Inner Hair Cells”, and a triple row further out appropriately named the “Outer Hair Cells”. The hair cells, just like Merkel cells, are specialized skin cells, not neurons, with the specific purpose of signaling changes in mechanical force. These cells are easily distinguished by a tuft of hair-like structures, called Stereocilia, arranged on the top layer in order shortest to tallest (kind of like the signal bars on any cell phone).corti2

When stress is created between the two cochlear membranes, the tallest hair on each hair cell, tethered to the tectorial (top) membrane is pulled away from the bundle of other cilia. However, the other cilia are then yanked in the same directions by a thin connection called the Tip Link connecting one end of each cilium to the sidewall of the next tallest one. It is believed that the pulling of these tip links in response to stress from sound waves opens gated ion channels to flood these cells with charged particles necessary for signaling the underlying neurons.

These responses are extremely fast (occurring in under a millisecond) and it is due to this responsiveness that our cochlea is able to so accurately discriminate between different sounds. The same process occurs every time you tune in to listen to the one-hit wonders of today.