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A blotter dipped in lavender oil, one molecule of linalool, a distance of about seven centimetres from the nostrils to the roof of the nasal cavity. Smell begins there, and the scent people name is not inside the molecule but in the combination of receptors it touches.

A Molecule Leaves the Blotter

Formula C10H18O, weight about 154. Linalool is a monoterpene alcohol, small and light enough to leave a paper surface at room temperature. It occurs in more than 200 commercial essential oils. In lavender oil, linalool makes up about 20 to 38 percent, beside roughly 25 to 45 percent linalyl acetate.

When the blotter is lifted, the molecules on the paper gradually evaporate into the air. Only substances small and volatile enough can make this journey; a molecule that is too heavy stays on the paper and never reaches the nose. Every smell, then, is the smell of something leaving.

A breath draws air through the nasal cavity. Only part of that airflow reaches the roof of the cavity, where a tiny patch of tissue is devoted to smelling. Linalool has to reach that patch before it has any smell at all.

Linalool's smell is usually described as soft floral, lightly woody, faintly spicy. But that description is not a property the molecule carries like the colour of a stone. It is the outcome of a chain of events after the molecule touches tissue, and that chain is what this post follows.

The Olfactory Epithelium

In humans the olfactory epithelium measures about 5 square centimetres and lies on the roof of the nasal cavity, some 7 centimetres above and behind the nostrils. A person's entire sense of smell starts on this patch of tissue.

The surface of the epithelium is coated in mucus secreted by Bowman's glands. An odour molecule has to dissolve into that layer before it can touch a cell. The layer also flows constantly, carrying old molecules away, so the tissue is renewed for the next breath.

Beneath it lie the olfactory receptor neurons, about 10 to 20 million cells in humans. Each neuron sends a short branch up to the surface, ending in a small knob with about 20 to 35 cilia up to 100 micrometres long. The odour receptors sit on those cilia, spread through the mucus like strands of algae in water.

Unlike most neurons in the body, olfactory neurons are replaced throughout life. Stem cells at the base of the epithelium keep producing new neurons; according to review sources, the epithelium's cells turn over almost completely in about 6 to 8 weeks, although some mature neurons can persist for months.

About Four Hundred Receptor Types

In 1991 Linda Buck and Richard Axel reported in the journal Cell that they had cloned 18 members of an extremely large gene family encoding proteins with seven transmembrane domains, expressed only in the olfactory epithelium. They proposed that this was the family of odour receptors. In 2004 the two received the Nobel Prize in Physiology or Medicine for this work.

In humans, about 400 genes in this family remain functional; roughly 600 others are pseudogenes that no longer produce usable receptors. Odour receptors belong to the G protein coupled receptors, in the same class as rhodopsin, the light-sensing pigment in the eye.

The key rule lies at the level of the cell: each mature olfactory neuron generally expresses one allele of one receptor gene. Among some ten million neurons, each cell listens through a single receptor type. About 400 receptor types are shared across millions of cells, each type with a whole team of neurons of its own.

When an odour molecule binds a receptor, a G protein inside the cell is activated, switching on the enzyme adenylate cyclase, which turns ATP into cyclic AMP. Cyclic AMP opens ion channels, letting sodium and calcium into the cell. That flow of ions turns a chemical touch into an electrical signal running along the axon.

A Combinatorial Code: One Smell, Many Receptors

An odour receptor is not a lock that opens to only one key. In 1999 Malnic, Linda Buck and colleagues measured how single neurons responded to molecules with related structures but different smells. The result: one receptor recognises several molecules, one molecule is recognised by several receptors, and each molecule is recognised by a different combination of receptors.

That is the combinatorial code. Linalool does not activate a linalool receptor, because no such receptor exists. It activates a group of receptors, each to a different degree, and that pattern is what travels to the brain. With about 400 receptor types, the number of possible combinations far exceeds the number of smells anyone will ever meet.

The researchers also noted that a slight change to a molecule, or a change in its concentration, can change its code, which may explain why such changes alter how it smells. A material smelled dilute and smelled strong sometimes earns two different descriptions.

Carvone is the clearest example in an essential oil cabinet. Its (+) and (−) forms share the same formula and the same atoms, differing only as a left hand differs from a right. The (−) form smells of spearmint, the (+) form of caraway seed. The nose tells them apart, so the signal patterns they produce must differ somewhere. The post Two Mirror Molecules goes deeper into this.

From the Olfactory Bulb to the Cortex

The axons of olfactory neurons pass through a thin plate of bone pierced with small holes and reach the olfactory bulb, a structure lying just beneath the frontal lobe. There, all neurons carrying the same receptor type converge on the same nerve clusters, called glomeruli. Millions of scattered signals from the nose are gathered into an ordered map.

On this map each smell appears as a pattern of glomeruli lighting up. Projection neurons, the mitral and tufted cells, take the signal from the glomeruli and carry it onwards along the olfactory tract.

The route onwards differs from the other senses. Vision, hearing and touch all pass through a relay station in the thalamus before reaching the cortex. Smell does not: signals from the olfactory bulb go straight to the primary olfactory cortex, including the piriform cortex, and also project directly to the amygdala. From there information continues to the hippocampus and the orbitofrontal cortex.

This anatomy is often cited when discussing the link between smell and memory. The post A Trace of Vanilla and an Old Kitchen looks at that more closely. Here one point is enough: the route from a molecule to a name passes through very few stations.

The Perfumer's Nose

An identical set of receptors in two people does not guarantee they give a smell the same name. Receptor genes vary, and each person learns smell names from different things. A perfumer does not have a different kind of nose; they have a thicker dictionary, built by smelling the same materials again and again over years.

The tools of that learning are simple. A paper blotter, a dilution so the smell is not too harsh, a notebook recording descriptions over time. Smelling pure linalool, then lavender, then bergamot, a learner gradually picks out the linalool in each bottle. The nose training series begins with ten bottles and a notebook of this kind.

Fragrance wheels and classification tables, such as the scent wheel in the library, are ways of arranging language, not maps of receptors. They help name the patterns the brain has already recognised, so two people can talk about the same blotter.

What remains are the limits of the organ itself. The nose tires after a few minutes with the same smell, and there are molecules some people cannot smell at all. The next two posts in the olfaction series take up each of those limits.

Linalool does not carry its smell.

It carries a shape, and the nose translates that shape into a pattern.

The name comes last.