Special Senses

Basic structures of the organs of taste and smell

1. Organs and structures of Taste

   1. - Tongue → papillae → taste buds → taste cells → sensory neurons → brain

      1. Lingual Papilae: small bumps on ur tongue tht hav taste buds

      2. Tase buds: group of taste receptor cells tht detect tastants (chemicals in food)

      3. Taste pore: small opening at the top of a taste bud where dissolved chemicals enter

      4. Gustatory (taste cells): receptor cells inside taste buds that detect chemicals. They have tiny microvilli that stick into taste pore

      5. Supporting cells: help support the taste cells

      6. Basal cells: stem cells that make new taste receptor cells

Food chemicals dissolve in saliva → enter the taste pore → contact gustatory cells → signal goes to sensory neurons → brain recognizes the taste.

Basic structures of the organs of smell

Olfactory epithelium: Specialized tissue in the upper part of the nasal cavity that has smell receptors

Mucus: traps odor molecules and helps them dissolve so they can interact with receptors

Olfactory receptor cells: bipolar sensory neurons that detect odor molecules. They have cilia sticking into the mucus and cilia contain odorant receptors

Olfactory bulb: brain structure above nasal cavity where the axons of olfactory receptor cells meet and connect with second - order neurons

Supporting cells: support and protect the olfactory receptor cells

Basal cells: stem cells that make new olfactory receptor neurons

Odor molecules dissolve in mucus → bind to receptors on olfactory cilia → olfactory receptor neurons send signals → olfactory bulb → brain recognizes the smell.

Basic mechanisms of how different tastes or odorants are detected and perceived

1. Tastant dissolution

   1. The food chemicals dissolve in saliva and enter the taste pore (so tatse can detect them)

   2. Detection by receptors or ion channels

      1. The tastants interact with receptor or ion channels on the microvilli of gustatory cells which stars the signal

3. Depolarization and neurotransmitters released when gustatory cells activate it depolarizes and releases chemical messengers like ATP or serotonin onto primary gustatory neurons

4. Flavor perception: your brain doesn’t use just one tatse receptor

Basic mechanisms of how different odorants are detected and perceived

Odor molecule → mucus → odorant receptor → cAMP → channels open → depolarization → action potential → olfactory bulb → brain

1. Odorant dissolves in mucus

2. Binds to an odorant GPCR

3. Activates G-protein → cAMP increases

4. Cation channels open

5. Cell depolarizes and fires an action potential

6. Signal travels to the olfactory bulb

7. Brain recognizes the smel

Chemoreceptors for taste and smell

Chemoreceptors: sensory receptors that detect chemicals and chemicals inside the body like H+

Taste	What detects it?	Receptor type

Salty

Na⁺ enters ion channels

Ion channel

Sour

H⁺ affects ion channels

Ion channel

Sweet

Sugars

GPCR

Bitter

Potentially toxic compounds

GPCR

Umami

Glutamate/MSG

GPCR

Salty + sour= ion channels

Sweet, + bitter + Umami= GPCRs

Transduction and pathways for taste

Tastant → receptor/ion channel → receptor potential → neurotransmitter → sensory neuron → action potential

• The gustatory (tatse) cell depolarize and releases neurotransmitters

• Primary afferent neuron then fires an action potential

• Pathway to the brain

  Taste bud → cranial nerves → brainstem → thalamus → gustatory cortex. (Tatse signals goes thru thalamus before reaching brain)

Transduction and pathways for smell

Transduction

Odorant → GPCR → G-protein → cAMP → ion channels open → depolarization → action potential

• olfactory is a receptor so it can fire action potential directly

Pathway to the brain

Olfactory receptor neurons → CN I → olfactory bulb → olfactory tract → olfactory cortex ( Smell does NOT have to go through the thalamus first.)

Taste and smell disorders

1. Taste disorders

   1. Ageusia: complete loss of tatse

   2. Hypogeusia: reduced tatse ability

• common causes: aging, damage to the gustatory (tatse) pathways, and certain medication

2. Smell disorders

   1. Ansomia: complete loss of smell

   2. Hyposmia: reduced smell ability

• Common causes: sinus/upper respiratory infections, deviated septum, hormonal porblems