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Olfaction is the ability to sense and perceive volatile chemicals that are suspended in the air. The typical human can distinguish up to 10,000 distinct odors, ranging from the sweet aroma of esters produced by apples and oranges, to the putrid smells of sulfurous compounds produced by skunks and rotten eggs. Because odors can drift along in the air, some chemicals can be detected long before the source is within eyesight: think about smelling a burning bonfire from miles away.
Smells affect our conscious behavior. They can motivate us to approach freshly-baked bread or avoid a rotting animal carcass. These chemicals serve as survival cues: bread gives us energy-rich carbohydrates while a decaying carcass can expose us to disease. Odorants can even affect our behaviors unconsciously: sleeping amidst the scent of a romantic partner increases the efficiency of our sleep, and subliminal exposure to citrus smells can lead people to clean their space more completely after eating a crumbly biscuit (an effect described in a study called “Smells Like Clean Spirit,” a nod to 90s punk band Nirvana).
Olfaction is one of the oldest functions we possess as animals. While our sense of smell has seemed to take a backseat to other senses relative to other animals (about 2% of the total mouse brain mediates smell, while only a scant 0.01% of the volume of the human brain is dedicated to this function), scientists now appreciate that our olfactory system is simply more specialized. For example, humans use the smells of sweat to clue us into the emotional state of others, and we can even subconsciously detect sickness through body odor.
9.1.1. Anatomy of the Olfactory System
Your sensation of smell begins when an odorant traverses your nostrils and passes through the nasal cavity, an empty, air-filled space just behind the front of the skull. The dorsal-most portion of the nasal cavity is a mucus-covered patch of tissue called the olfactory epithelium.
Embedded within the olfactory epithelium are the olfactory receptor neurons (ORNs, also called olfactory sensory neurons, or OSNs), neurons which begin processing smell. ORNs are bipolar neurons, with long dendritic projections protruding into the epithelium. Here, they peek out and contact the air, making them the only neurons that are directly exposed to the outside world. This, unfortunately, causes them to encounter all sorts of dangers such as toxins, particulates, and microbes. They are one of the few known populations of neurons where adult neurogenesis occurs, each having a lifespan in the range of 30 days to a year. The average human olfactory system has somewhere between 6-20 million ORNs.
Within the olfactory epithelium are a population of supporting cells, which function much like glial cells. They help dispose of dead and dying cells, metabolize pollutants, and may also help to physically maintain the epithelium.
On the dendrites of ORNs are olfactory receptors. It is estimated there are about 1,000 different genes (about 3% of the total human genome) that code for roughly 400 different olfactory receptors. Each ORN expresses only one type of olfactory receptor, and each receptor is believed to respond to activation by a different chemical. For example, the receptors expressed on ORN293s are highly responsive to cadaverine, the smell characteristic of death and decay. Even though scientists have identified the odorants that activate some receptors, most of the olfactory receptors are still “orphaned,” and have not yet been matched with their corresponding odorant. The initial research into the genetics underlying these neurons earned Drs. Linda Buck and Richard Axel a Nobel prize in 2004, but there is still much work to be done.
Olfactory receptors are transmembrane G protein-coupled receptors that signal downstream effectors using the intracellular transduction molecule \(\mathbf{G_{\alpha{\text {olf}}}}\). This protein complex is 90% similar to the stimulatory G-protein Gs, and likewise triggers activation of adenylate cyclase, elevating the intracellular concentration of cyclic AMP (see section 5.3 for a refresher on this signaling pathway). Activation of \(\text{G}_{\alpha{\text {olf}}}\) causes depolarization, causing the ORN to fire action potentials.
ORNs encode the intensity of smells through the frequency of action potential firing, which changes in accordance with the concentration of odorant molecules. Imagine standing over a fresh-baked pizza and inhaling deeply. Due to the high concentration of odorant molecules in the air, several receptors will be activated, leading to frequent neuronal firing. Now, imagine you are down the block from a pizza restaurant, getting only a slight whiff of those same scents. Here, the concentration of odorants is low, meaning that the ORNs fire less frequently.
Following activation of the receptors, the axons of the ORNs pass through the skull through a tiny series of holes at the cribriform plate, a sieve-like section of the ethmoid bone. These primary neurons form synaptic connections onto neurons in the olfactory bulb, the beginning of the olfactory nerve (CN I). Like the optic nerve, the olfactory nerve runs along the ventral surface of the brain.
The site of synaptic connectivity between the ORNs and the secondary neurons in the olfactory bulb is a highly specialized clump of tissue called a glomerulus. The typical human has a little under 2,000 glomeruli, and each glomerulus only receives inputs from ORNs that express the same type of olfactory receptors.
Within each glomerulus are the dendrites of the secondary neurons, which exist in two types: The mitral cells and the tufted cells. These two cell populations project axons directly into the olfactory cortex. This makes the olfactory system the only sensory system that does not pass signals through the thalamus before cortical processing.
There are also two types of inhibitory neurons that regulate this pathway: granule cells found within glomeruli, and periglomerular cells, which send axonal projections into the glomeruli, both help refine the synaptic processing of scent information using lateral inhibition, a system similar to that found in the retina of the visual system (Chapter 7).
Collectively, the olfactory cortex is made up of several different regions.
1. Piriform cortex
The piriform cortex is the main cortical input site for axonal projections from the olfactory bulb. In some species of mammals that rely very heavily on their sense of smell for survival, up to 10% of total cortical volume is piriform cortex. The outputs of the piriform cortex project into other structures of the olfactory cortex, as well as the mediodorsal thalamus, a relay center that contributes to learning and decision making.
2. Cortical amygdala
The amygdala is a part of the brain that helps mediate complex emotional states (chapter 15). Generally, it is subdivided into a few different subnuclei. One of them in particular, the cortical amygdala, receives strong inputs from the olfactory nerve. The cortical amygdala sends projections into the hippocampus, a brain structure critically important for the formation of new memories.
The strong neural connections between the olfactory nerve and amygdala are believed to be the reason why smell and memory are strongly linked. Think about a time when you caught a slightest whiff of a scent, and that scent mentally transported you to a distant place from a long time ago (this experience was also described by author Marcel Proust in his highlyregarded novel, In Search of Lost Time.) We form associations between specific smells and emotionally salient memories. Some odorants, like the volatile Maillard reaction products that are made as a product of cooking, remind us of the “good ole days” watching a parent bake or grill. Many bakeries take advantage of these positive memories and point their exhaust out into the street to entice passersby.
Unfortunately, the memories associated with smells can also have negative valence. For example, the smell of petroleum and oil fumes can trigger a sickness-like conditioned response in Gulf War veterans.
3. Entorhinal cortex (EC)
The entorhinal cortex is a small section of the medial temporal lobe. As with the cortical amygdala, the EC sends strong connections into the hippocampus, indicating that olfactory signals contribute to the strong associations formed between smell and memory through EC, as well.
Other than receiving and processing olfactory signals, the entorhinal cortex is also involved in spatial navigational tasks.
4. Orbitofrontal cortex (OFC)
As the name implies, the orbitofrontal cortex (OFC) is found just behind the orbit, the bony socket of the skull where the eyes sit. OFC is found at the ventral surface of the frontal lobe. Circuits in this brain area function as an integration site for sensory inputs, since it also receives projections from visual, taste, and somatosensory cortices.
The full extent of the OFC is still under examination, but it is also implicated in decision making and social behaviors.
Some species secrete chemicals which influence the behavior of other members of the same species. These pheromones trigger complex responses in others, such as motivating social insects like ants to follow meandering trails towards faraway food sources or causing a group of bees to swarm and attack a predator.
There is still debate over whether or not humans produce and release pheromones, and whether or not other humans are sensitive to these signals. Many nonhumans, notably snakes and dogs, have a specialized structure in their nasal cavity called the vomeronasal organ, which detects organic compounds produced by predators and reproductionrelated hormones produced by the opposite sex. These signals are sent into the brain via cranial nerve 0, which can then trigger behavioral changes. Humans have these anatomical structures, but they are not believed to carry any functional information. However, other evidence suggests that smelling certain compounds influences mood, alters neuroendocrine signaling, and even affects mate selection.
9.1.2 Disorders of the Olfactory System
Like other sensory systems, the structures involved in olfaction can be injured. An injury to the olfactory system can result in hyposmia, a reduced ability to smell, or anosmia, a complete loss of smell.
The most common insult to the olfactory system is simple nasal congestion, a temporary, physical blockage of the entry to the nasal cavity that decreases airflow and, therefore, the number of particles that reach the olfactory epithelium. Congestion can be caused by allergies, the common cold, upper respiratory bacterial or viral infections, or sinus infections. Hyposmia is also one of the main neurological symptoms of COVID-19.
Hyposmia is common among healthy, older adults, affecting about half of the population between 65 and 80 years old. As a person ages, spontaneous calcification causes the holes in the cribriform plate to shrink, which can impinge on and damage ORN axons.
Hyposmia can also be caused by abrupt head injuries. The ORN axons that project through the holes of the cribriform plate are particularly sensitive to blows to the head.
Neurodegenerative disorders, such as Parkinson’s disease and Alzheimer’s disease, contribute to smell deficiency. Usually, hyposmia precedes the major clinically observed symptoms of these disorders, hinting that smell deficiency may serve as an early diagnostic biomarker.
Another olfactory deficit, phantosmia, is when a person perceives “phantom” scents, or in other words, experiences an olfactory hallucination. Phantosmia may be triggered by a temporal lobe seizure or a stroke. It can also be caused by a brain tumor affecting the olfactory nerve (CN I), or the subsequent surgical removal of the tumor, leading to injury. Schizophrenia, a psychiatric condition characterized by auditory hallucinations, may also cause phantosmia.
This page titled 9.2: Olfactory System was last modified on Wed, 08 Jul 2026 03:46:39 GMT and is shared under a CC BY-NC 4.0 license and was authored, remixed, and/or curated by Austin Lim via source content that was edited to the style and standards of the LibreTexts platform.
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