Olfactory System

Components of the Olfactory System

• Olfactory epithelium

• Olfactory bulb

• Olfactory cortex

Olfactory Epithelium

Location

• ~10 cm² area at the top of nasal passages

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Cellular Components (in mucus-covered neuroepithelium)

• Olfactory receptor neurons (ORNs)

• Basal cells (progenitors)

• Supporting cells

• Bowman glands (secrete mucus)

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Basal Cells Function

• Continuously replace ORNs

• ORN lifespan: ~1–2 months

Olfactory Receptor Neurons (ORNs)

Structure

• Bipolar neurons

• Knob-like olfactory vesicle

• Non-motile cilia in mucus

• Cilia contain odorant receptors

• Axon projects to olfactory bulb

Olfactory Receptors

Type

• Largest G-protein-coupled receptor (GPCR) family

• ~900 genes (≈357 functional)

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Key Property

• Each ORN expresses only ONE receptor type

Odor Discrimination — Combinatorial Coding

• One odorant → activates multiple receptors

• One receptor → responds to multiple odorants

📌 Result:

• Huge number of detectable odors due to combinations

Signal Transduction

Pathway

1. Odorant binds proteins in mucus

2. Binds receptor on cilia

3. Activates Gs/Gq

4. ↑ Ca²⁺

5. Action potential generated

6. Glutamate released → olfactory bulb

Adaptation Mechanisms

• Ca²⁺ inhibits:

  • Cation channels

  • Adenylate cyclase

• PKA (via cAMP) → receptor desensitization

ORNs → Olfactory Bulb

Pathway

• ORN axons → fila olfactoria

• → CN I

• → through cribriform plate

• → olfactory bulb

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Synapse Location

• Olfactory glomeruli

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Odor Mapping

• ORNs with same receptor → converge on 1–2 glomeruli

Olfactory Bulb

Neuron Types

Excitatory:

• Mitral cells

• Tufted cells

Inhibitory:

• Periglomerular cells

• Granule cells

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Function

• Amplifies strong signals

• Suppresses weak/non-specific signals
  → Improves discrimination

Olfactory Tract

What is it?

• Connection from bulb → brain

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Contains

• Output from bulb

• Input back to bulb

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Striae Medial Stria

• Projects to anterior olfactory nucleus

• Communicates with contralateral bulb

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Lateral Stria

• Projects to ipsilateral primary olfactory cortex

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⚠ Key Fact

• NO thalamic relay (unique sensory system)

Primary Olfactory Cortex

Components

• Anterior olfactory nucleus

• Olfactory tubercle

• Amygdala (anterior cortical nucleus)

• Periamygdaloid cortex

• Piriform cortex (uncus)

• Lateral entorhinal cortex

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Connections

• With each other

• With:

  • Hypothalamus → feeding

  • Hippocampus → memory

  • Olfactory bulb

Secondary Olfactory Cortex

Includes

• Orbitofrontal cortex

• Ventral insular cortex

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Function

• Odor identification & discrimination

• Integrates taste input

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Medial Orbitofrontal Cortex

• Combines:

  • Smell

  • Taste

  • Sensory cues
    → Produces flavor perception

Gustatory System — Organized Notes

• Components

• Taste buds

• CN VII, IX, X (1st-order neurons)

• Solitary nucleus

• Thalamus (VPM)

• Gustatory cortex

• Secondary cortex

• Hypothalamus

• Amygdala

• Brainstem motor nuclei

Function

• Detects:

  • Sweet

  • Salty

  • Sour

  • Bitter

  • Umami

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Mechanism

• Tastants directly stimulate receptor cells in taste buds

Taste Buds

Structure

• Supporting cells

• Basal cells

• Taste receptor cells (NOT neurons)

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Transduction

• Occurs at microvilli (apical end)

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Innervation

• Multiple afferent fibers per bud

• One fiber → multiple buds

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Turnover

• Every 10–14 days

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Locations

• Tongue:

  • Fungiform

  • Foliate

  • Circumvallate papillae

• Also:

  • Soft palate

  • Pharynx

  • Larynx

  • Esophagus

Taste Sensitivity Regional Sensitivity

• Sweet → tip

• Sour → edges

• Salty → anterior edges

• Bitter → back

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Receptor Specificity

• Sweet, umami, bitter → distinct receptor cells

• Some overlap with sour/salty

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Thresholds

• Citric acid: 2 mM

• NaCl: 10 mM

• Sucrose: 20 mM

• Strychnine: 0.0001 mM (very sensitive)

Taste Transduction

GPCR Pathway (sweet, bitter, umami)

• Gs/Gq → cAMP/IP3

• ↑ Ca²⁺ → neurotransmitter release

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Ion Channel Pathway (salty, sour, some bitter)

• Na⁺, K⁺, H⁺ flux

• Depolarization

• Ca²⁺ influx

• Neurotransmitter release

Taste Pathway

Peripheral

• CN VII → geniculate ganglion

• CN IX → petrosal ganglion

• CN X → inferior vagal ganglion

→ All → solitary nucleus

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Central Pathway

• Solitary nucleus → ipsilateral central tegmental tract

• → VPM thalamus

• → internal capsule

• → frontal operculum + anterior insula

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Other Projections

• Orbitofrontal cortex → flavor

• Brainstem → reflexes (swallowing, gagging)

• Amygdala → emotion

• Hypothalamus → feeding

Trigeminal Chemoreception

Function

• Detects:

  • Pungent

  • Astringent

  • Irritating chemicals

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Mechanism

• TRP channels (temperature + irritants)

Examples:

• Capsaicin → hot

• Menthol → cool

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Threshold

• ~100× higher than smell/taste

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Pathway

• CN V (± IX, X)

• → spinal trigeminal nucleus

• → contralateral VPM

• → somatosensory cortex

Anosmia / Hyposmia

Transient

• Nasal obstruction

• Infection/allergy

• Chemical damage

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Chronic

• Aging

• Head trauma (cribriform plate damage)

• Tumors

• Neurodegenerative disease:

  • Alzheimer’s

  • Parkinson’s

  • Huntington’s

Phantosmia / Gustatory Hallucinations

• Seizures (uncinate fits)

• Psychiatric disorders

• Tumors, stroke, trauma

• Migraine

Parosmia

• Distorted smell (often unpleasant)

• Causes:

  • Infection (e.g., COVID)

  • Head trauma

  • Parkinson’s

Ageusia / Hypogeusia

Ageusia

• Bilateral CN VII/IX damage (rare)

• Taste bud damage

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Hypogeusia

• Neurological disease

• Diabetes

• Aging

Dysgeusia

• Distorted taste (metallic)

• Causes:

  • Medications

  • Chemotherapy/radiation

  • Nerve damage

Drug-Induced Disturbances

• Affect taste + smell

• Most common:

  • Dysgeusia

  • Parosmia

Specific Anosmia / Ageusia

Mechanism

• Genetic receptor deficiency

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Examples

• 1/1000: can’t smell skunk odor

• 1/10: can’t detect gas additives

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PTC Tasting

• 30–40% cannot taste

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Supertasters

• More taste buds

• Highly sensitive to bitter