Olfaction/Taste

List the components of the olfactory system

Components:

• Olfactory epithelium

• Olfactory bulb

• Olfactory cortex

Describe the olfactory epithelium

• Location

• Mucus composition

• Function of basal cells

Olfactory epithelium:

• Location:

  • 10 cm2 area @ top of the nasal passages

• Mucus (covering neuroepithelium) composition:

  • olfactory receptor neurons (ORNs)

  • basal cells (ORN progenitor cells)

  • supporting cells

  • Bowman glands (secrete mucus)

• Function of Basal cells:

  • ORNs = continually replaced from basal cells and last about 1-2 months

Describe the structure of ORNs

Olfactory Receptor Neurons Structure:

• bipolar neurons

• contain knoblike olfactory vesicle

  • nonmotile cilia projecting into mucus

    • contains olfactory receptors for detection of odorant molecules

  • axon projecting to olfactory bulb

Describe olfactory receptors

• Type

• Odor Discrimination prop.

Olfactory Receptors:

• Type:

  • Largest subfamily of G-protein-coupled receptors (900 genes in human but only 357 functional)

• Odor Discrimination properties:

  • each ORN = only one odorant receptor

    • thus, different ORNs → respond to different odorants

Describe the Combinatory strategy for odorant discrimination

Combinatory strategy:

• Each odorant is recognized by specific combination of receptors:

  • single odorant = recognized by multiple receptors with different affinities

  • Single receptor = recognize multiple odorants with different affinities

• Mech: number of possible receptor combinations is way larger than the number of known chemicals

1. Describe the signal transduction pathway for odor detection

2. Describe the mechanism for Olfactory Detection & Adaptation

Signal Transduction Pathway

• Volatile hydrophobic odorant → binding proteins w/in mucus layer → receptors on cilia of olfactory neurons → Gs/Gq activation

• Outcome: Increase [Ca++} → AP generation in activated ORNs → glutamate (excitatory) NT release → OB neurons

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Olfactory Detection & Adaptation Mech:

• Ca+2 inhibition of activity of cation channel & adenylate cyclase

• Desensitization of olfactory receptor by PKA via cAMP

Describe the pathway of ORNs → OB:

• Pathway

• Odor Discrimination

ORNs → OB:

• Pathway:

  • ORNs axons → organized as bundles called fila olfactoria → form CNI → pass through cribiform plate → OB (in ant. cranifal fossa)

    • ORNS synapses w/ OB in complexes of synapses called olfactory glomeruli

• Odor Discrimination:

  • All ORNs expressing a given receptor converge on only one or two glomeruli

Describe the Olfactory Bulb

• Type of neurons

• Odor Discrimination Mech

Describe the Olfactory Tract:

• What is it?

• Contains?

• Medial vs Lateral olfactory Stria

• Importance?

OB:

• Types of Neurons:

  • Excitatory projection neurons: mitral & tufted (M/T) cells

    • receive synapse from ORNs

    • synapse in olfactory cortex

  • Inhibitory interneurons: periglomerular & granule cells

    • modulate activity of projection neurons

• Odor Discrimination Mech:

  • via excitatory & inhibitory synapses btw ORNs and OB:

    • amplify & filter signal generated by high affinity odorant in strongly activated glomeruli

    • Reduce noise (non-specific signal) by inhibiting activity in weakly activated glomeruli

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Olfactory Tract:

• Connects OB to Brain

• Contains:

  • projections: OB → primary cortex

  • brain projections → olfactory bulb

• medial vs lateral olfactory stria

  • Medial:

    • OB axons sends collateral → synapse @ anterior olfactory nucleus → contact Contralateral OB

  • Lateral:

    • OB Axons → Continues in Lat. → synapse @ ipsilateral Primary Olfactory Cortex

• IMPORTANCE: NO THALAMUS RELAY

1. List the components of the primary olfactory cortex

2. Connections of Primary

3. Describe the Secondary olfactory complex:

   1. Includes

   2. Function

   3. importance of medial orbitofrontal cortex

Components:

• anterior olfactory nucleus

• olfactory tubercle

• amygdala (anterior cortical amygdaloid nucleus)

• periamygdaloid cortex

• piriform cortex (located in uncus)

• lateral entorhinal cortex

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Connections:

• extensive associational connections w/ each other

• extrinsic connections w/

  • Lateral hypothalamus (feeding behavior),

  • hippocampus (learning & memory),

  • olfactory bulb

  • Secondary olfactory cortex

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Secondary Olfactory Complex:

• Includes:

  • orbitofrontal cortex

  • ventral insular cortex

• Function:

  • discrimination and identification of odors.

  • Receives Taste Input

• medial orbitofrontal cortex

  • important in integration of olfactory, taste and cues → experience of flavor.

secondary olfactory complex

List the components of gustatory system

Components

• Taste buds (receptor cells)

• First order taste neurons (CN VII, IX & X)

• Solitary nucleus

• Thalamus

• Gustatory cortex

  • Primary cortex

  • Secondary cortex

• Associated nuclei

  • Hypothalamus

  • Amygdala

  • Brainstem motor nuclei

  • Etc.

Draw out a taste bud and its neuron

Describe the gustatory system

• Detects?

• Mech?

gustatory system

• Detects 5 basic tastes:

  • sweet, salty, sour, bitter and umami (savoriness > amino acids).

• MECH: Tastants directly stimulate receptor cells in taste buds

  • located throughout the oropharyngeal cavity.

1. Describe Taste Buds

   • What is it?

   • Taste Transduction Location

   • Innervation

   • Turnover Rate

2. Describe the Location/Function of Taste buds

Taste Buds:

• What is it?

  • complex of supporting cells, basal cells and taste receptor cells (NOT NEURONS!)

• Taste Transduction Location:

  • apical end of receptor cells = covered w/ microvilli where transduction occur.

• Innervation:

  • innervated by >1 nerve afferent fiber

  • an individual nerve fiber innervate multiple taste buds.

• Turnover Rate:

  • continuous turnover with cycle of about 10-14 days.

  • New cells are thought to arise from basal cells.

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Location:

• variable # on tongue, soft palate, oral/laryngeal pharynx, larynx and upper esophagus.

• On Tongue:

  • fungiform, foliate and circumvallate papillae

• NOTE: function of extralingual taste buds is not clear, and their number is decreased in adults. They may stimulate brainstem-mediated reflexes associated with feeding

Describe Taste Sensitivity

• Location

• Unique prop of sweet/unami/bitter

• Thresholds

Taste Sensitivity:

• Dif. Taste sensation = varies in location:

  • Sweet: most sensitive @ tip

  • Sour: edges

  • Bitter: Back

  • Salty: Ant. edges of tongue

• Sweet, unami, bitter = detected by distinct subsets of taste cells that express different receptors

  • Some of these cells can also detect sour and salty.

• Thresholds:

  • citric acid: 2mM

  • salt (NaCl): 10mM

  • sucrose: 20 mM

  • strychnine 0.0001mM (bitter>poisonous or noxious are the lowest)

Describe taste transduction

Taste Transduction Pathway:

• Some sweet, sour, bitter and unami: tasting compounds used G protein coupled receptors.

  • Gs/Gq → cAMP,IP3 → Increases [Ca++] → opens/closes ion channels → Ca++ increases → Release NTs towards afferent nerve

• Some bitter, salty and sour-tasting compounds

  • Ion Channels → change move. of ions (Na+, K+, H+) → depolarize membrane/ receptor cell → open voltage-sensitive calcium channel → Release NTs towards afferent nerve

[REVIEW] Afferent Nerve Endings

1. Describe the peripheral taste pathway

2. Describe Taste-Central Projections

   • Main Pathway

   • Other Projections

• CN VII → geniculate gnaglia

• CN IX → petrosal ganglia

• CN X → Inferior Vagal Ganglia

• central process enter brainstem and terminate in the solitary nucleus.

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Taste Central Projections:

• Main:

  • Rostral solitary nucleus → Ascends Ipsilaterally w/ CTT → VPM (thalamus) → posterior limb of the internal capsule → frontal operculum and anterior insular cortex

    • This pathway is involved in taste discrimination.

• Other Projections:

  • lateral posterior orbitofrontal cortex (Secondary Gustatory Cortex)

    • receives inputs from primary taste cortex

    • Function:

      • integrates taste, olfactory and visual cues → appreciation of flavor, food reward and control of feeding.

  • Solitary nucleus → brainstem motor nuclei → control oromotor responses associated with feeding

    • EX: (tongue movement, swallowing, gaging, coughing etc.).

  • Some projections to amygdala (emotions, food aversion) and hypothalamus (feeding behavior).

Describe Trigeminal Chemoreception

• Function

• Mech

• Threshold

• Pathway

Trigeminal system:

• Function:

  • chemoreception associated w/ ingestion of food and olfaction → localization of flavors.

  • Responsible for

    • “other taste experiences”

      • astringent (cranberries and tea),

      • pungent (hot peppers and ginger),

      • fatty, starchy, and various metallic tastes.

    • “acrid (irritating) smell”

      • horseradish,

      • vinegar

      • ammonia

  • These experiences are combinations of somatosensory perceptions including pain, touch and temperature.

• MECH:

  1. Nerves in nasal/oral cavities, naso/oropharynx → polymodal nociceptive receptors → activated by irritants

     • (acetic acid in vinegar, capsaicin in chili peppers, etc.).

  2. Transduction via activation of cationic channels, the TRP (transient receptor potential)

     • TRP family also detects Hot/Cold

       • (this is why capsaicin and peppers taste “hot” while menthol is “cool”)

• Irritants Threshold:

  • many = recognized as odors or tastes

  • trigeminal threshold = 100x higher than normal odors/taste

    • (protective mechanism)

• Pathway:

  • Irritants sensory info → CNV ( to a lesser extent through CNIX and X) → spinal trigeminal nucleus → Contralateral VPM (thalamus) → somatosensory cortex where facial irritation is processed.

Describe Anosmia or hyposmia

• Transient impairment via?

• Chronic impairment

Transient impairment due to:

• Nasal obstruction

  • by polips or septal deviations

• Inflammation of olfactory epithelium

  • infections,

  • allergies,

  • chemicals like cocaine or smoking

    • recovery because of adult neurogenesis

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Chronic impairment due to:

• Aging: ORNs replacemen<degen

• Damage to CNI axons

  • head trauma → damage cribriform plate/ abulsion of OBs

  • can lead to CSF rhinorrhea & danger of meningitis

• Absence/reduced size of OB&tract:

  • Congenital anosmia

  • hyposmia / Kallman syndrome

• Intracranial lesions

• Subfrontal brain tumor

  • (most frequent reason to test CN1 function)

• Neurodegenerative diseases

  • Alzheimer,

  • Parkinson

  • Huntington diseases

• Psychiatric diseases:

  • Schizophrenia

• Diabetes

Describe the causes of Phantosmia & gustatory hallucinations

Causes:

• seizure activity:

  • those beginning in uncus (uncinate fits) → olfactory/gustatory auras → precedes full seizure activity.

• Psychiatric disorders:

  • phantosmia → olfactory delusions → believes existence of odor/source does not exist

  • gustatory hallucinations also exist

• Head trauma

• Stroke

• tumors

• migraine

• toxic or metabolic abnormalities

1. Describe parosmia

2. Causes

Parosmia

• Rare condition that has become relatively more widespread since 2020 as a side effect of Covid-19.

• Usually, odor is perceived as unpleasant (euosmia if pleasant).

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Causes:

• damage to ORNs by respiratory infection or exposure to harmful chemicals.

• COVID:

  • infection of sustentacular cells → down regulation of OR/Signal. Pathway genes → effects last even after virus cleared

• Head trauma & Parkinson’s disease

Describe causes of ageusia and hypogeusia

ageusia

• Bilat. Lesion of CN VII/IX:

  • rarely encountered

• local damage to taste buds:

  • radiation therapy,

  • inflammation

  • infection of tongue

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hypogeusia

• Neurological damage:

  • Bell palsy (facial nerve),

  • multiple sclerosis,

  • Alzheimer’s disease

• Peripheral neuropathy caused by diabetes

• Aging

Describe causes of dysgeusia

• What is it?

What is it:

• distortion in the perceptions of taste (bad taste → metallic)

Causes:

• complication of treatment or medication usage,

  • particularly in radiation therapy and chemotherapy.

• result of damage to taste bud receptor cells or afferent nerves

Describe Drug induced Disturbances in taste & smell

• Effect

• Most Common?

• Mech

Drug induced disturbace:

• Drugs in every major pharmacological category can impair both taste and smell function.

  • distortion, alteration, reduction, or presence of tastes or smells.

• Dysguesias and parosmias are the most common.

• Most of the disturbances due to sensation alteration @ molecular level,

  • but others may be generally neurotoxic.

1. Describe Specific Anosmia/ageusia:

   • MECH

   • STATs

2. What are supertasters?

• Specific anosmia:

  • MECH: lower sensitivity to specific odorant b/c lack of functional receptor/ genetic mutation

  • STATs:

    • 1/1000: insensitive to butyl mercaptan, skunk.

    • 1/10: cannot detect hydrogen cyanide or ethyl mercaptan-added to natural gas.

• Specific ageusia:

  • Mech: lack of functional receptor/mutation

  • STATs:

    • 30-40% of U.S. population: cannot taste phenylthiocarbamide (PTC) (the bitter compound )

• Supertasters:

  • People who are extremely sensitive to PTC have more taste buds than normal and tend to avoid bitter tasting foods.