Idiot Brain by Dean Burnett Page A

detecting the nutrient levels in our blood and body is another sort of sense. Most of these are concerned with our internalstate, and the five “proper” ones are responsible for monitoring and perceiving the world around us, our environment. These are, of course, vision, hearing, taste, smell and touch. Or, to be extra scientific, ophthalmoception, audioception, gustao-ception, olfacoception and tactioception, respectively (although most scientists don’t really use these terms, to save time). Each of these senses is based on sophisticated neurological mechanisms and the brain gets even more sophisticated when using the information they provide. All the senses essentially boil down to detecting things in our environment and translating them into the electrochemical signals used by neurons which are connected to the brain. Coordinating all this is a big job, and the brain spends a lot of time on it.
    Volumes could be and have been written about the individual senses, so let’s start here with perhaps the weirdest sense, smell. Smell is often overlooked. Literally, what with the nose being right below the eyes. This is unfortunate, as the brain’s olfactory system, the bit that smells (as in “processes odor perception”), is odd and fascinating. Smell is believed to be the first sense to have evolved. It develops very early; it is the first sense to develop in the womb, and it has been shown that a developing baby can actually smell what the mother is smelling. Particles inhaled by the mother end up in the amniotic fluid where the fetus can detect them. It was previously believed that humans could detect up to 10,000 separate odors. Sounds like a lot, but this total was based on a study from the 1920s, which obtained the figure largely from theoretical considerations and assumptions that were never really scrutinized.
    Flash forward to 2014, when Caroline Bushdid and her team actually tested this claim, getting subjects to discriminatebetween chemical cocktails of very similar odors, something that should be practically impossible if our olfactory system is limited to 10,000 smells. Surprisingly, the subjects could do it quite easily. In the end, it was estimated that humans can actually smell in the region of 1 trillion odors. This sort of number is usually applied to astronomical distances, not something as humdrum as a human sense. It’s like finding out the cupboard where you store the vacuum cleaner actually leads to a subterranean city with a civilization of mole people. *
    So how does smell work? We know smell is conveyed to the brain through the olfactory nerve. There are twelve facial nerves that link the functions of the head to the brain, and the olfactory nerve is number 1 (the optic nerve is number 2). The olfactory neurons that make up the olfactory nerve are unique in many ways, the most pronounced of which is they’re one of the few types of human neurons that can regenerate, meaning the olfactory nerve is the Wolverine (of X-Men fame) of the nervous system. The regenerative capabilities of these nose neurons means they are extensively studied, with the aim of exploiting their regenerating abilities to apply them to damaged neurons elsewhere—for instance, in the spine of paraplegics.
    Olfactory neurons regenerate because they are one of the few types of sensory neurons that are directly exposed to the “outside” environment, which tends to degrade fragile nerve cells. Olfactory neurons are in the lining of the upper parts of your nose, where the dedicated receptors embedded in them can detect particles. When they come into contact with aspecific molecule, they send a signal to the olfactory bulb, the region of the brain responsible for collating and organizing information about odor. There are a lot of different smell receptors; a Nobel Prize–winning study by Richard Axel and Linda Buck in 1991 discovered that 3 percent of the human genome codes for