Friday, November 02, 2012
  Yeah, that's just random. Right.

An interesting advance in understanding the sense of smell.
Animals use their noses to focus their sense of smell, much the same way that humans focus their eyes, new research at the University of Chicago shows. A research team studying rats found that animals adjust their sense of smell through sniffing techniques that bring scents to receptors in different parts of the nose. The sniffing patterns changed according to what kind of substance the rats were attempting to detect.
Though this is very early and tentative, it sounds similar to the ear's mechanism for distinguishing frequencies.

We're familiar with how the eyes focus: each eye rotates to center a target on the retina, and the pupil contracts to protect against excess light. We can see those actions. The eye distinguishes frequencies (colors) with separate sets of rod and cone cells.

The mammalian ear has similar functions. Though humans have lost the ability to rotate the ears for directional focus, we still have the other functions. We distinguish frequencies (pitches) by a resonant mechanism that causes a wave to stimulate different parts of the cochlea. And we protect against excess sound with a 'pupil' that tightens up the eardrum.

With soft sound the Tensor Tympani muscle is loose:


When sound gets loud, the Tensor Tympani tightens up so the eardrum doesn't move significantly more than it would with weak sounds:


There are also some protective mechanisms inside the cochlea, which are still fairly mysterious. Each hair cell has a nerve that takes its signal into the brain, as you'd expect. But each hair cell also has a nerve that brings it a return signal from the brain. As currently understood, these nerves are a classic feedback loop, serving to cut down the size of the hair cell's signal when it gets louder; but they may also stiffen the hair itself to focus the overall frequency response on a desired range.

The nasal version of the Tensor Tympani or the pupil is clearly the muscles that pinch the nostrils, plus internal constrictors that kick in when the air is cold.

The cochlear frequency response appears to have its counterpart in the nasal mechanism described above. The rats are directing airflow to different parts of the chemical sensor area, just as the cochlea's resonators direct different frequencies to different parts of the cochlea. Perhaps there are also neural suppressor loops, to suppress smells from inside the animal such as a nasal or oral infection.

Another common factor to all three senses is 'stereo-ness', obvious with vision and well-known with hearing but poorly explored with smell. Bekesy found back in 1963 that humans can reliably distinguish which nostril is receiving a specific smell. Presumably dogs and other smell-talented mammals use this ability fully, though humans seem to ignore it.

Needless to say, all three sensory systems, reflecting an identical underlying 'circuit diagram' with very different actual components, arose purely by random selection.

Mmmm-hmmmm.

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Polistra was named after the original townsite of Manhattan (the one in Kansas). When I was growing up in Manhattan, I spent a lot of time exploring by foot, bike, and car. I discovered the ruins of an old mill along Wildcat Creek, and decided (inaccurately) that it was the remains of the original site of Polistra. Accurate or not, I've always liked the name, with its echoes of Poland (an under-appreciated friend of freedom) and stars. ==== The title icon is explained here. ==== Switchover: This 2007 entry marks a sharp change in worldview from neocon to pure populist. ===== The long illustrated story of Polistra's Dream is a time-travel fable, attempting to answer the dangerous revision of New Deal history propagated by Amity Shlaes. The Dream has 8 episodes, linked in a chain from the first. This entry explains the Shlaes connection.

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