Introduction to Behavioral Neuroscience · The Chemical Senses
The Olfactory System
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In 30 seconds
Smell (olfaction) is the sense that detects volatile chemical molecules in the air. Unlike taste, which recognizes just a handful of basic qualities, the olfactory system can discriminate an enormous range of odorants, and it does so with a logic that is unlike any other sensory system. The receptor neurons sit directly in the nasal cavity, exposed to the outside world, and their axons travel a short, ancient route into the brain — bypassing the thalamic relay that every other major sense requires.
Three features make olfaction special and worth studying carefully. First, the sensory neurons themselves are replaced throughout life, a rare example of ongoing neurogenesis in an adult nervous system. Second, odor identity is encoded combinatorially: no single receptor type is dedicated to a single odor. Third, the olfactory pathways connect directly to emotion and memory structures such as the amygdala and hippocampus, which is why a single smell can instantly transport you back to a childhood kitchen. This topic walks through the anatomy and signaling of the olfactory system, from airborne molecule to cortical perception, and finishes with the clinical conditions that arise when smell goes wrong.
Why this matters
- Flavor depends on smell: most of what we call "taste" is actually retronasal olfaction (odorants traveling up from the mouth). Loss of smell makes food bland — a huge quality-of-life issue.
- Safety and survival: smell warns us of smoke, natural gas leaks, spoiled food, and spoiled people — Anosmia Loss of the sense of smell. Full entry → (loss of smell) removes an early-warning system.
- Clinical diagnosis: smell loss is often one of the earliest signs of Parkinson's disease and Alzheimer's disease, and it is a hallmark symptom of COVID-19 (parosmia and anosmia).
- Unique neuroanatomy: olfaction's direct route to cortex (no obligatory thalamic relay) is a classic exam contrast with vision, hearing, and touch.
- A model for brain repair: because olfactory receptor neurons regenerate, they are a model system for studying adult neurogenesis and axonal regeneration.
The college version
Core Concepts
The olfactory epithelium: neurons in the open air
The Olfactory epithelium The sheet of tissue in the nasal roof containing ORNs, supporting cells, and basal cells. Full entry → lines the roof of the nasal cavity, just below the brain. It contains three main cell types:
- Olfactory receptor neurons (ORNs): bipolar neurons whose dendrite ends in several cilia that extend into a layer of mucus. Odorant molecules dissolve in this mucus (with help from odorant-binding proteins) and bind to receptor proteins on the cilia.
- Supporting (sustentacular) cells: provide structural and metabolic support, and help clear odorant molecules.
- Basal cells Stem cells in the olfactory epithelium that generate new ORNs. Full entry →: stem cells that divide throughout life to produce new ORNs. An ORN lives roughly a month before being replaced (exact timing varies and is an active research area); this makes the olfactory system one of the few places in the adult mammalian nervous system where new neurons are continuously born.
Receptors and the combinatorial code
Odorant receptors are G-protein-coupled receptors (GPCRs). Humans have roughly 400 different functional Odorant receptor G-protein-coupled protein on ORN cilia that binds odorant molecules. Full entry → genes (out of a much larger family of related genes, many of which are pseudogenes — the commonly taught estimate is ~350–400 working receptors), while mice have around 1,000.
The key rule: each ORN expresses only one type of odorant receptor, and all the ORNs expressing the same receptor send their axons to the same target in the brain. But one odorant molecule can activate several different receptor types, and one receptor type can be activated by several different odorants. The brain therefore reads smell as a pattern of receptor activation rather than a single lock-and-key match — this is called Combinatorial coding The rule that odors are encoded by patterns of activation across many receptor types. Full entry →. A coffee aroma, for example, activates a specific subset of receptors; change one receptor, and the perceived odor changes.
Signal transduction: from molecule to action potential
When an odorant binds its receptor, a G protein (called Golf) activates adenylyl cyclase, which raises cyclic AMP (cAMP) inside the cilium. cAMP opens cyclic-nucleotide-gated (CNG) ion channels, letting sodium and calcium enter, depolarizing the neuron. If depolarization reaches threshold, the ORN fires action potentials that travel down its axon toward the brain. This GPCR → cAMP → ion-channel cascade is a classic pathway to know.
From nose to bulb: the first relay
ORN axons are thin, unmyelinated fibers that pass through tiny holes in the Cribriform plate Perforated bone plate through which ORN axons pass into the olfactory bulb. Full entry → of the ethmoid bone to reach the Olfactory bulb First relay structure; contains glomeruli, mitral/tufted cells, and interneurons. Full entry →, the first processing center, which sits just above the nasal cavity. Inside the bulb, the axons end in spherical structures called glomeruli. All ORNs expressing the same receptor type converge on the same Glomerulus Spherical neuropil in the bulb receiving axons from ORNs of one receptor type. Full entry →, so each glomerulus represents one receptor type.
The bulb's output neurons are mitral cells and tufted cells, which receive input from glomeruli and send axons out of the bulb along the lateral olfactory tract. Local inhibitory interneurons — periglomerular cells and granule cells — shape the signal, and lateral inhibition between glomeruli sharpens discrimination between similar odors. Different odorants produce different spatial patterns of glomerular activation, so the bulb acts as an early "odor map."
Cortical targets: the direct, ancient route
The lateral olfactory tract carries mitral and tufted cell axons directly to the primary olfactory (piriform) cortex, plus the amygdala and entorhinal cortex (which feeds the hippocampus). Notably, olfaction reaches the cortex without an obligatory relay through the thalamus — a distinction from vision, audition, and somatosensation. (There is a pathway from the Piriform cortex Primary olfactory cortex in the temporal lobe. Full entry → to the orbitofrontal cortex via the mediodorsal thalamus, but it is not required for initial cortical access.) This direct wiring to limbic structures explains why smells trigger such strong emotional and memory responses.
The vomeronasal organ (VNO) deserves a mention: in many mammals it detects pheromones and feeds the accessory olfactory bulb. In adult humans the VNO is vestigial (and its functional significance is debated), so human chemical communication relies mainly on the main olfactory system, including trace-amine-associated receptors (TAARs) that detect amines.
Adaptation, individual variation, and disorders
Olfaction adapts quickly: receptor-level desensitization, enzymatic breakdown of odorants in the mucus, and central habituation all contribute, which is why you stop noticing your own perfume or your own house. People also vary enormously in smell ability due to genetic differences in receptor genes (specific anosmias, such as an inability to smell androstenone), age (smell declines with aging), and experience.
Disorders of smell include anosmia (no smell), hyposmia (reduced smell), parosmia (distorted smell), and phantosmia (smelling something that is not there). Common causes: head trauma (the cribriform plate can shear ORN axons), viral upper respiratory infections and COVID-19, nasal polyps and chronic sinusitis, normal aging, and neurodegenerative disease. In Parkinson's and Alzheimer's disease, smell loss often precedes other symptoms — a clinically important early marker.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Smell (olfaction) | Taste (gustation) | Smell detects airborne molecules via the nasal epithelium; taste detects five basic qualities via tongue receptors. Flavor is mostly smell. |
| Olfactory epithelium | Olfactory bulb | Epithelium = sensory surface in the nose (receptors live here); bulb = first brain relay above the cribriform plate. |
| One receptor = one odor | One receptor = one odorant type per neuron | Each ORN expresses ONE receptor type, but that receptor can be activated by MANY odorants; odor identity is a population pattern. |
| "Pheromones work in humans like mice" | Human chemical communication | The human VNO is vestigial and its function is debated; do not assert a human pheromone system as established fact. |
| Anosmia | Ageusia | Anosmia = loss of smell; ageusia = loss of taste. Flavor loss from a cold is mostly anosmia, not ageusia. |
| Olfaction needs a thalamic relay like other senses | All senses relay through thalamus | Olfaction is the exception: it reaches cortex directly (piriform), although a later thalamic route to orbitofrontal cortex exists. |
| Smell loss is just "getting older" | Always benign | Age-related decline is normal, but sudden or progressive anosmia can signal trauma, infection, or neurodegenerative disease — worth clinical evaluation. |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Your nose is lined with tiny "smell antennas" that stick into a layer of wet mucus. Each smell molecule is like a key, and each antenna has a lock. One smell molecule can fit several different locks, so your brain figures out the smell by seeing which combination of antennas lights up. The antennas send their message straight to the parts of the brain that handle memories and feelings — that's why one whiff of grandma's cookies can make you feel happy before you even think about it.
Worked example
Imagine you have a cold. Your nose is blocked, and your favorite soup suddenly tastes like salted warm water — sweet, salty, sour, bitter, and umami are all still detected by your tongue, but the soup seems flat. Why? Because most of what you experience as "flavor" is actually odor: as you chew and swallow, volatile molecules from the soup travel up the back of your throat (retronasally) to your olfactory epithelium. With the nasal passages blocked, those molecules never reach the ORNs, so the olfactory component of the flavor vanishes. This everyday experience is a perfect demonstration of the olfactory system's anatomy: the receptors are in the nose, not the mouth, and their signals dominate flavor perception.
Now trace the same soup through the full pathway: volatile molecules dissolve in mucus → bind GPCRs on ORN cilia → cAMP opens CNG channels → ORN fires → axon passes through the cribriform plate → glomerulus in the olfactory bulb → mitral cell → lateral olfactory tract → piriform cortex and amygdala. If a head injury shears those thin axons at the cribriform plate, the whole pathway stops — and the person may permanently lose the ability to smell, and with it, most of flavor.
Key takeaways
- Anatomy: olfactory epithelium (roof of nasal cavity) → cribriform plate → olfactory bulb → lateral olfactory tract → piriform cortex, amygdala, entorhinal cortex; orbitofrontal cortex via mediodorsal thalamus.
- Three cell types in the epithelium: ORNs, supporting cells, basal cells (stem cells → lifelong neurogenesis of ORNs).
- Combinatorial coding: ~400 functional odorant receptor types in humans (commonly taught estimate); each ORN expresses one receptor type; one odorant activates several receptor types.
- Transduction cascade: odorant → GPCR → Golf → adenylyl cyclase → cAMP → CNG channels → depolarization.
- Glomerular convergence: all ORNs of one receptor type synapse on the same glomerulus in the olfactory bulb; mitral/tufted cells are the output; periglomerular and granule cells are inhibitory interneurons.
- Unique route: olfaction reaches cortex without an obligatory thalamic relay.
- Clinical: head trauma, viral infections (incl. COVID-19), aging, and neurodegeneration (Parkinson's, Alzheimer's) can all cause smell loss; anosmia is an early sign of neurodegenerative disease.
- Adult neurogenesis: ORNs are continually replaced from basal cells — a rare example in mammals.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
List the three cell types of the olfactory epithelium and the job of each.
Show answer
Olfactory receptor neurons (detect odorants and fire action potentials), supporting/sustentacular cells (metabolic and structural support, odorant clearance), and basal cells (stem cells that generate new ORNs throughout life).
What does "combinatorial coding" mean, and why does it let ~400 receptor types encode so many odors?
Show answer
Each odorant activates several receptor types, and each receptor type responds to several odorants; the brain identifies an odor from the pattern of activity across the population, so a limited set of receptors covers a huge odor space.
Describe the transduction cascade from odorant binding to action potential.
Show answer
Odorant binds GPCR → G protein (Golf) activates adenylyl cyclase → cAMP rises → cyclic-nucleotide-gated channels open → Na⁺/Ca²⁺ influx depolarizes the ORN → action potentials fire if threshold is reached.
Where do ORN axons cross from nose to brain, and what happens to them in head trauma?
Show answer
They pass through the cribriform plate of the ethmoid bone. In head trauma, shearing forces can tear these delicate axons, producing anosmia.
How is the olfactory pathway different from vision or touch in its route to the cortex?
Show answer
Olfaction reaches the piriform cortex directly via the lateral olfactory tract without an obligatory relay through the thalamus; vision, hearing, and touch all require a thalamic relay.
Why does a blocked nose flatten the flavor of food even though the tongue still works?
Show answer
Because flavor depends heavily on retronasal olfaction: with the nasal passages blocked, food odorants cannot reach the olfactory epithelium, so the olfactory contribution to flavor is lost even though taste receptors still work.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Olfactory receptor neuron (ORN)
- Bipolar sensory neuron in the nasal epithelium that detects odorants and sends signals to the brain.
- Olfactory epithelium
- The sheet of tissue in the nasal roof containing ORNs, supporting cells, and basal cells.
- Odorant receptor
- G-protein-coupled protein on ORN cilia that binds odorant molecules.
- Combinatorial coding
- The rule that odors are encoded by patterns of activation across many receptor types.
- Cribriform plate
- Perforated bone plate through which ORN axons pass into the olfactory bulb.
- Olfactory bulb
- First relay structure; contains glomeruli, mitral/tufted cells, and interneurons.
- Glomerulus
- Spherical neuropil in the bulb receiving axons from ORNs of one receptor type.
- Mitral/tufted cells
- Output neurons of the olfactory bulb.
- Piriform cortex
- Primary olfactory cortex in the temporal lobe.
- Anosmia
- Loss of the sense of smell.
- Basal cells
- Stem cells in the olfactory epithelium that generate new ORNs.
Sources & references
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