Olfaction

What does olfaction detect?

airborne chemical stimuli called odorants, which are then encoded into electrical signals.

How does olfaction guide survival-related behaviors?

It helps in the search for food and mates (through pheromones).

How does olfaction help avoid danger?

It warns about threats such as spoiled food or chemical dangers, helping to prevent harm, and helps avoid predators

What role does olfaction play in reproduction and hormones?

It can influence reproductive and endocrine functions, often through pheromone signaling.

How is olfaction involved in social bonding?

It influences mother-child interactions, strengthening bonds and recognition.

Besides survival, what other functions does olfaction serve?

It provides sensual pleasure, such as enjoying the odor of flowers or perfume.

Which sense is considered the “oldest” or most primitive?

Olfaction

How well is olfaction understood compared to other senses?

It is the least understood sense.

What kind of information does olfaction process?

It processes information about the identity, concentration, and quality of odorants.

Where are olfactory receptor neurons (ORNs) located?

in the olfactory epithelium, an epithelial sheet in the nasal cavity.

How do odorants activate the olfactory system?

Odorants interact with olfactory receptor neurons (ORNs) in the olfactory epithelium.

Where do the axons of olfactory receptor neurons project?

through the cribriform plate directly to neurons in the olfactory bulb.

What is the cribriform plate?

it is a thin, perforated region of the skull that separates the olfactory epithelium from the brain.

Where does the olfactory bulb send its projections?

to the pyriform cortex and other forebrain structures via the olfactory tract.

What makes the olfactory system unique compared to other sensory systems?

it does not include a thalamic relay from the primary receptors to the cortical region that processes sensory information.

What is the pyriform cortex, and what is its role?

it is a 3-layered archicortex dedicated to olfaction.

How is the olfactory pathway different from other sensory pathways at the start?

The initial pathway bypasses the thalamus, though the thalamus does play an important role in later stages of olfactory processing.

Where does olfactory information travel after the pyriform cortex?

It travels to association areas, where further processing occurs.

What functions are supported by higher-level olfactory processing?

It allows for the conscious appreciation of odors and the association of odors with other sensory characteristics of environmental stimuli.

How much is known about the neural representation of olfactory information in the CNS

Very little is known — it is still unclear whether olfactory maps exist like in other sensory systems.

How does human olfaction compare to other senses?

In humans, olfaction is the least acute of the senses.

How does human olfaction compare to that of many animals?

Many animals have far superior olfactory abilities compared to humans.

What factors contribute to stronger olfaction in animals?

• A higher number of olfactory receptor neurons,

• An expanded olfactory epithelium, and

• A larger proportion of the forebrain devoted to olfaction.

Are humans naturally skilled at following a scent trail?

Humans seem ill-suited for tasks like following a scent to a specific target.

Can humans track scents when challenged?

Yes — when challenged, humans can “sniff out” a scent trail and improve with practice.

What strategy do humans use when tracking scents?

They pursue a path that constantly bisects the linear scent trail.

How does sniffing behavior change during scent tracking?

Sniffing increases as scent tracking is learned.

Do humans rely heavily on olfaction for daily information?

No, humans do not rely on olfaction as a major source of information.

what capacity does the olfactory system have?

The olfactory system can still use chemosensory information to track targets and locate items of interest in space.

What is the detection threshold for odors?

The detection threshold occurs when the presence of an odor is correctly identified above chance level (50%).

Do all odorants have the same detection threshold?

No, different odorants have different detection thresholds.

How does concentration affect odor perception?

Perception can be concentration-dependent, meaning the same odorant can smell different at different concentrations.

Can molecules with the same chemical formula smell different?

Yes — enantiomers of the same molecule (mirror-image forms) can elicit different perceptions.

What is anosmia?

it is the inability to detect one or more odors.

What are some causes of anosmia?

• Genetic factors

• Exposure to toxins

• Acquired conditions such as chronic or temporary sinus infection

• Head injuries

• Aging

• Neurodegenerative diseases, like Alzheimer’s or Parkinson’s disease

Which odors are commonly tested to check for anosmia?

baby powder, chocolate, cinnamon, coffee, mothballs, peanut butter, and soap.

How does aging affect olfaction?

here is a normal decline with age, which can reduce odor detection ability.

Which brain regions are involved in olfactory processing beyond the olfactory bulb?

The orbitofrontal cortex, pyriform cortex, and amygdala play important roles in higher-level olfactory processing. 

Where does the transduction of olfactory information begin?

in the olfactory epithelium, leading to the conscious sense of smell.

What is the olfactory epithelium composed of?

A sheet of neurons and supporting cells that lines about half of the nasal cavity’s surface.

What lines the remaining surface of the nasal cavity?

The respiratory epithelium, which helps condition inhaled air.

What are the functions of the respiratory epithelium?

• Maintains temperature and moisture of inhaled air

• Provides an immune barrier to protect against irritation and infection

What produces the mucus layer in the olfactory epithelium?

The mucus layer is secreted by Bowman’s glands.

What protective functions does the mucus layer serve?

• Contains enzymes and antibodies to protect exposed portions of cells to avoid passing infection to brain

• Maintains the ionic environment (ionic milieu) for olfactory cilia

How does mucus thickness affect olfaction?

If the mucus thickens, olfactory acuity significantly decreases.

What type of neurons are olfactory receptor neurons (ORNs)?

ORNs are bipolar neurons with unmyelinated axons.

What is found on the apical surface of ORNs?

A knob that extends olfactory cilia into the mucus layer.

What is the primary site of odorant transduction?

The olfactory cilia

What role do basal cells play in the olfactory epithelium?

Basal cells act as stem cells that regenerate the olfactory epithelium in adults.

What is the function of sustentacular (supporting) cells?

They detoxify potentially dangerous chemicals, helping to protect the olfactory system.

Why are olfactory receptor neurons (ORNs) particularly vulnerable?

Because they are directly exposed to the external environment.

How are ORNs protected from damage and infection?

They are protected by the mucus layer, which contains enzymes and antibodies.

How are ORNs maintained throughout life?

They are continuously regenerated by stem cells found in the layer of basal cells within the olfactory epithelium.

How often are olfactory neurons regenerated in rodents?

Approximately every 6–8 weeks.

Was anosmia a common symptom of COVID-19?

Yes, anosmia was frequently reported as an early or first symptom.

Which cells does (covid?) SARS-CoV-2 primarily infect in the olfactory system?

It seems to infect sustentacular (supporting) cells, which express ACE-2 receptors.

What role do ACE-2 receptors play?

ACE-2 receptors act as the entry point that allows SARS-CoV-2 to enter cells.

How does infection of sustentacular cells lead to anosmia?

Infection causes inflammation and a loss of support for ORNs, leading to impaired olfaction.

What is known about long COVID-related anosmia?

The exact mechanism of long-term anosmia in long COVID is still unclear.

Where does odor transduction in the olfactory epithelium begin?

when an odorant binds to specific odorant receptor proteins on the external surface of olfactory cilia.

What happens when odorants are presented to the cilia of an isolated ORN?

They elicit a robust electrical response.

What happens when odorants are presented to the cell body of an ORN instead of the cilia?

They do not elicit an electrical response.

What type of receptors are odorant receptors?

Odorant receptors are metabotropic, G protein–coupled receptors (GPCRs).

What happens when an odorant binds to its receptor?

The Golf alpha subunit dissociates and activates adenyl cyclase III (ACIII).

What is the role of adenyl cyclase III in olfactory transduction?

ACIII increases cAMP, which opens Na⁺ and Ca²⁺ channels, leading to depolarization.

How is the depolarization amplified?

By a Ca²⁺-activated Cl⁻ current, which further enhances depolarization.

How does the depolarization lead to action potentials?

Depolarization spreads to the axon hillock of the ORN, where voltage-gated Na⁺ channels generate action potentials.

Where are these action potentials sent?

They are conducted to the olfactory bulb.

Are all olfactory receptor neurons (ORNs) equally sensitive to odorants?

No, individual ORNs are sensitive only to subsets of stimuli.

What kind of electrical activity do odorants trigger in ORNs?

Odorants elicit inward currents in ORNs.

How selective are most ORNs?

Most randomly selected ORNs show broadly tuned responses to a variety of odorants, rather than being extremely specific.

What type of receptor proteins are odor receptors?

Odor receptors are 7-transmembrane G protein–coupled receptors (GPCRs).

What happens when an odor receptor is activated?

Activation causes the release of a G-protein, which initiates a signaling cascade.

How many variations of odor receptor proteins do mammals have?

Mammals have roughly 1,000 different odor receptor proteins.

How does the large number of receptors contribute to smell?

The huge variety allows mammals to recognize many different odors.

What parts of odor receptor proteins are shared, and what parts differ?

All receptors have conserved domains in common, while the variable regions determine each receptor’s specific chemical affinity.

What do olfactory receptor neuron (ORN) axons form?

ORN axons form many bundles that together make up the olfactory nerve (cranial nerve I).

Where does each olfactory nerve project?

Each olfactory nerve projects ipsilaterally to the olfactory bulb.

What is the synaptic target of primary olfactory axons in the olfactory bulb?

They synapse in glomeruli, specialized structures in the olfactory bulb.

Which cells do ORN axons contact within the glomeruli?

ORN axons contact the apical dendrites of mitral cells.

What type of synapses do ORNs form with mitral cells?

They form excitatory glutamatergic synapses.

What is the role of mitral cells?

Mitral cells are the principal projection neurons of the olfactory bulb, relaying olfactory information deeper into the brain.

How many mitral cells typically contribute apical dendrites to a glomerulus in the mouse?

About 25 mitral cells.

How many ORN axons provide input to a single glomerulus?

Roughly 25,000 olfactory receptor neuron (ORN) axons.

Do these ORN axons come from neurons expressing different receptors?

No — most (if not all) axons come from ORNs that express the same single odorant receptor gene.

What is the functional benefit of this high degree of convergence?

It increases mitral cell sensitivity and ensures maximum fidelity of odor detection by averaging out uncorrelated background noise.

What other cell types contribute to each glomerulus besides mitral cells?

About 50 tufted cells and 25 periglomerular cells, though their precise functions remain unclear.

What role do granule cells play in the olfactory bulb?

Granule cells synapse on the basal dendrites of mitral cells, forming local inhibitory circuits and contributing to synaptic plasticity.

What are the five layers of the olfactory bulb (from outermost to innermost)?

• Glomerular layer

• External plexiform layer

• Mitral cell layer

• Internal plexiform layer

• Granule cell layer

What structures are found in the glomerular layer?

• Dendritic tufts of mitral cells

• ORN axon terminals

• Periglomerular cells, which define glomerulus margins

What is contained in the external plexiform layer?

• Lateral dendrites of mitral cells

• Cell bodies and lateral dendrites of tufted cells

• Dendrites of granule cells, which form dendrodendritic synapses

What defines the mitral cell layer?

It contains the cell bodies of mitral cells.

What structures are found in the internal plexiform layer?

It contains mitral cell axons.

What is located in the granule cell layer?

It contains granule cell bodies.

How do individual glomeruli respond to distinct odorants?

selectively

What happens when odorant concentration increases?

Both the activity of individual glomeruli and the number of glomeruli activated increase.

What coding mechanism does the olfactory system appear to use?

The olfactory system uses sparse coding, focusing on a small number of dominant chemicals within a mixture and representing the mixture with a relatively small subset of glomeruli.

What structure do mitral cell axons form?

They form the olfactory tract, which projects mostly ipsilaterally

Does olfactory information pass through the thalamus before reaching cortex?

No, there is no direct thalamic relay for primary olfactory information.

What is the main cortical target of the olfactory tract?

The pyriform cortex, where neurons show broad responses and may integrate different odors (exact function unclear).