For some, more than sight, sound, or touch, smell triggers a
very powerful emotional response. A freshly baked apple pie, an ocean breeze,
the cigarette smoke from grandma’s house, smells play a vital role in cognition
and memory of key experiences in our lives. The mammalian nose contains a patch
of sensory receptors embedded in a mucous layer called the epithelium. Chemical
odorants wafting about in the ambient air enter the nose, dissolve into the
mucous layer of the epithelium, and bind to these sensory receptors. Like a
lock and key mechanism, the shape of each individual receptor determines which
odor molecule it will accept. For example, the chemical brew of an apple pie
fits into certain group of receptors, while the recipe for grandma’s cigarette
smoke fits into another. The human genome contains roughly 1,000 separate genes
for encoding different odor receptors, although only about 40% of these genes
are expressed. In other words, the human nose is capable of discerning about
400 different chemical odorants. Once bound to the appropriate receptor, combinations
of odor molecules get encoded through a complicated chemical pathway, sent to
the brain, and are finally interpreted by the brain as a specific smell. Yesterday,
during a complete circumnavigation of the island by boat, we came upon this
raft of crested auklets, and for the first time in the history of my nose,
odorants from this group of birds made their way into my nasal passage, bound
to specific receptors in my epithelium, got encoded and sent to my brain, and I
experienced for the first time the tangerine smell of a crested auklet. It was
awesome.
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