Biological complexity: smell nerve plasticity (Introduction)

by David Turell , Thursday, March 03, 2016, 14:35 (3187 days ago) @ David Turell

We are born knowing how to interpret smells and odors. This must be learned and develops into adulthood. The exact mechanism of how the neurons In the olfactory bulb learn to do this is not exactly known, but specific neurons develop and axons attach, resulting in smell discrimination that can pick up differences of two atoms in molecules!:-http://www.evolutionnews.org/2016/03/more_news_on_th102651.html-"Last time we focused on the olfactory epithelium, the tissue that receives the odor molecules. We saw how it is organized into a hierarchical pattern that provides the best possible reception for different kinds of odorants. Each nostril's epithelium contains half a million olfactory sensory neurons (OSNs), long cells with cilia at one end and an axon at the other end. The cilia are where the odor molecules make contact with olfactory receptors (ORs). When a molecule "fits" just right, the receptor responds, triggering a cascade of activity. -***-"A team from Italy, publishing in Scientific Reports, found evidence for discrimination between molecules with identical shapes but different vibrations. Four pairs of odorant molecules were carefully designed to be identical except that some hydrogen atoms were replaced with deuterium (heavy hydrogen, containing an extra neutron). The slight mass difference in these "isotopomers" ("same topology") alters the vibration frequency of the molecule. These same-shaped odorants were wafted into the noses of honeybees while the scientists monitored their brains in real time.-"Sure enough, the bees appeared able to discriminate them, showing very different responses to the same-shaped pairs.-***-"The debate [as to how it works] will continue, undoubtedly, but more to our interest, it illustrates the complexity of the olfactory sense and its extreme precision that has baffled scientists for decades. Imagine a honeybee, fruit fly, or salmon being able to discriminate twin molecules that differ only by one or two atomic mass units. Design doesn't get better than that.-***-"Meanwhile, a recent paper in Nature Communications takes us down the other end of the olfactory neuron to the tip of the axon. As shown in the Illustra animation, the nerve endings of a million OSNs converge on a remarkable organ, the olfactory bulb (OB), which is studded with connection points called glomeruli. In an amazing example of preprogrammed networking, these axons "know" during development somehow which glomerulus to attach to, depending on the type of odorant receptor they express (and there are hundreds of those). Axons for one receptor might grow toward a glomerulus on top of the bulb; axons for another to the backside. Between top-bottom, front-back, and left-right, the OB has three axes by which to discriminate connections coming from different classes of receptors. This is the first stage of sorting and classifying odorant types. -***-"We've discussed "plasticity" before as a challenge to Darwinism. Why would a blind evolutionary process create "plasticity potential" that can be "harnessed when needed" in case of an altered experience? Darwinian evolution has no foresight. Plasticity makes perfect sense, though, from a design-based perspective on biology. There's no better example than right there in a salmon's nose, where the olfactory system will be encountering numerous new environments over a period of years. The scientists' expectations of roles for synaptic plasticity were confirmed in their conclusions.-Comment: That last paragraph is the nub of the issue.