What the Nose Knows
The human sense of smell often goes unconsidered, but it is the sense with the most direct path to the emotional centers of the brain.
The nose is often assumed to be the organ of smell reception, when in fact, it is not. The primary function of the nose is to regulate the temperature and the humidity of the air we breathe, in order to protect the delicate linings of each nostril.
The olfactory receptors themselves are located at the top of the nose. There are 50 million smell receptors in the human olfactory epithelia, the total size of which, in humans, is about that of a small postage stamp, with half being at the top left and half at the top right nostril. Olfactory cells are not very long-lived. In fact there is a constant turnover of these cells. Their average active life has been estimated to be about 28 days.
A major neurological feature of the sense of smell is the large number of receptor cells that converge upon a relatively small number of secondary cells, located in the olfactory bulbs, which lie on the under surface of the brain. This convergence indicates that the sense has evolved in terms of sensitivity. It has been calculated that as few as seven or eight molecules striking an olfactory cell will produce a nerve impulse with about forty nerve cells needing to be stimulated before a smell sensation is reported. There have been numerous theories of olfaction, but all have failed to fully explain how we detect smells.
Studies have shown that a pleasant aroma, even a very light one, enhances the impression a room makes on those that enter it.
In addition to small receptors, the nose of humans contains another system, a touch system, which is often mistakenly assumed to be part of the sense of smell. This second system is the trigeminal system and it is part of the extensive 5th cranial nerve. Somatosensory, or touch nerves, detect pungent substances, such as ammonia, carbon dioxide and acetic acid. The chemosensory system is best illustrated by the protective head averting reflex when a pungent substance, such as household ammonia, is inhaled.