Lecture 14 - Chemical Senses

Nasal vestibule

Nasal canal at front of nose with nostrils as openings

Nasal turbinate

Shell-like bones covered by epithelial tissue and blood vessels

Mucus

Concentrates odorants for binding to olfactory receptors (ORs)

Glomeruli

Synapses between olfactory receptor neuron (ORN) and mitral cells

Function of olfaction

Surroundings awareness

Associated with memories and emotions

Odor diversity

10,000 detectable odors

1,000 olfactory genes

1 gene = 1 OR

1 OR expressed per ORN

Each OR responds one set of structurally related odorant molecules

Strength of odorant depends on how well it fits to the OR

Each odorant can activate different types of ORNs

Where are ORs found?

The dendritic cilia of ORNs

Odor detection pathway

Odorant binds to OR

Transduction of odor detection into electrical signals

Same-type ORNs generate APs that converge at a glomerulus Activation of mitral cells in olfactory bulb

Mitral cells send signals to brain

Transduction of odor detection into electrical signals

GPCR activated by odorant

G protein dissociates into alpha and beta-omega subunits

Alpha subunit binds to AC

ATP is converted to cAMP

cAMP opens cation and Cl- channels

AP created

Function of gustation

Stay away from bitter poisons

Encourage eating sweet/salty foods

Lingual papillae

Small bumps located on top surface of tongue

Where are taste buds located?

Mostly on lingual papillae

Tonsils

Epiglottis

Taste cell cilia

Hold the taste receptors

Tastant

Molecule that excites taste receptor cells

Dissolves in saliva

Taste pore

Allows entry of food molecules

Taste (gustatory) cells

Convey taste signals to gustatory nerves

Gustatory cranial afferents

Transmit taste signals to the brain

Basal cells

Stem cells that can replenish worn out taste cells every 10 days

Taste diversity

1 taste receptor = 1 taste

1 taste cell = 1 type of taste receptor

25 taste cells = 1 taste bud

1 taste bud = multiple tastes

5 gustatory receptor cell families

Sweet

Salty

Sour

Bitter

Umami

Sweet

Sugar binds to GPCR

Salty

Na+ ions enter via cation channels

Sour

Alkaloids cause H+ ions to enter H+ channels

Bitter

Bitter compound binds to GPCR

Umami

Amino acid glutamate binds to GPCR

Transduction of taste stimuli to electrical signals (salt/sour)

Ions in salty/sour compounds excite ion channels (H+ for sour; Na+ for salty)

Graded potential turns to AP

ATP release stimulates gustatory cranial nerves

Transduction of taste stimuli to electrical signals (sweet/bitter/umami)

Sugar/alkaloid/amino acid binds to GPCR

Gusducin (Gq) protein activation

Graded potential turns to AP

ATP release stimulates gustatory cranial nerves

Olfactory signal transmission

ORNs

Mitral cells

Ipsilateral olfactory tract

Olfactory processing center

What is interesting about olfactory signal transmission?

It does not go through the thalamus and is processed in both hemispheres

Olfactory processing center

Anterior olfactory nucleus

Hippocampus

Amygdala

Anterior olfactory nucleus

Conscious perception of smell

Hippocampus

Olfactory memory

Amygdala

Associates smell with emotional memory

Olfactory adaptation/fatigue

Temporary and normal inability to smell an odorant after prolonged exposure

Gustatory signal transmission to the brain

Tastant passes through taste pores

Tastant binds to taste receptor

Generation of AP

Release of ATP

ATP binds to (1 of 3) gustatory cranial nerves

Generation of AP

Medulla

Thalamus

Primary gustatory cortex

Integration with other senses (especially olfactory)

Primary gustatory cortex

Manages conscious perception of taste

Limbic system

Creates emotional response and memory of taste sensation

Dysosmia

Dysfunction of sense of smell

Hyperosmia

Increased sense of smell

Causes of hyperosmia

Pregnancy

Autoimmune disorder

Exposure to lead and mercury

Hyposmia

Decreased sense of smell

Causes of hyposmia

Neurodegenerative diseases

Exposure to tobacco smoke and stimulation drugs (like cocaine)

Anosmia

No sense of smell

Causes of anosmia

Neurodegenerative diseases

Exposure to certain antibiotics and nasal sprays with zinc

Cribriform plate bone

Separates brain from nasal cavity

Contains holes that allow axons of ORNs from nose (which form cranial nerve I) to enter olfactory bulb under brain

Causes of covid-induced anosmia

Cytokines from inflammation and viral infected sustentacular cells inhibit olfactory receptor expression

Treating brain tumors with nasal spray

Introduce drugs into nasal cavity

Drugs enter brain via olfactory/trigeminal nerves

Drugs activate at specific brain region via electromagnetic waves