The Sense of Smell

I began the research for this paper with several questions about the sense of smell. How are smells identified? How and why are they remembered so vividly, and why are they so emotionally charged? What does it mean that cells in the olfactory system are the only neurons to regenerate - what is retained and what is lost in this process? And what does it mean that o lfactory neurons are the only sensory neurons to synapse directly in the brain?

What I know about my own sense of smell is scant and sketchy. It is clear that smell must be functionally very different from sight or sound, because it is bas ed on the recognition of actual objects, of molecules in the gaseous phase. Unlike differences in light or sound, which can both be plotted as a range on the continuum of a single feature (wavelength for color and frequency for pitch), the range of odo rs we perceive cannot be plotted as a wave function. The movement of odorous molecules depends on diffusion, and the gathering and funneling of air that my own nostrils do. Ones nose is involved in a sort of constant sampling of a randomly selected pop ulation of airborne particles, and with every breath it performs a battery of tests for the presence of the molecules in its repertoire. But what is the nature of this testing? And how is the leap made from this molecular interaction to the identifica tion of smells, and from there to response?

At the level of receptors, the perception of color depends on only three types of cells, and from the ratio of activity of these three types of photoreceptors, the brain infers color. Is there a n analogous system at work in the perception of odor? An odiferously active molecule wafts into the nose and into the proximity of the nasal epithelium, the bed of tissue at the top of the nose where all of the chemoreceptive neurons are clustered. Th e molecule, by lovely blind chance, bumps into a receptor protein, with whom it does a little tango. Said tango leaves our protein somewhat bent out of shape, and the permeability of the neuron is altered: it fires. Several questions arise form this e ncounter. The first is, what is the nature of the receptor? Will it promiscuously respond to a wide range of molecules, or is it specific?