BS530: 5.3 OLFACTION AND TASTE

What is olfaction?

Sense of smell

Where is the olfactory epithelium located?

Roof of nasal cavity covering superior nasal conchae

What is the olfactory epithelium?

Organ of smell containing olfactory sensory neurons

What type of neurons are olfactory sensory neurons?

Bipolar neurons

What structures radiate from olfactory receptor cells?

Olfactory cilia

Function of supporting cells in olfactory epithelium?

Surround and cushion olfactory receptor cells

Where are olfactory stem cells located?

Base of olfactory epithelium

What covers olfactory cilia?

Mucus

Function of mucus in olfaction?

Acts as solvent for odorants

What cranial nerve is responsible for smell?

CN I olfactory nerve

What forms the filaments of the olfactory nerve?

Bundles of nonmyelinated axons of olfactory receptor cells

How often are olfactory neurons replaced?

Every 30 to 60 days

Why are olfactory neurons unusual?

They can regenerate from stem cells

How many active smell genes do humans have?

About 400

What does each smell gene encode?

Unique receptor protein

Can one odor activate multiple receptors?

Yes

Can one receptor respond to multiple odors?

Yes

What other receptors are found in nasal cavity besides olfactory receptors?

Pain and temperature receptors

Why can menthol feel cold and chili peppers feel hot?

They stimulate temperature and pain receptors

What must a substance be like to be smelled?

Volatile and in gaseous state

What else must odorants do to be smelled?

Dissolve in olfactory epithelium fluid

What happens when odorants bind olfactory receptors?

Cation channels open causing receptor potential

What occurs if receptor potential reaches threshold?

Action potential travels to olfactory bulb

What G protein is involved in smell transduction?

Golf

What second messenger is produced in smell transduction?

cAMP

What ion channels does cAMP open in olfactory cells?

Na+ and Ca2+ channels

What causes depolarization in olfactory neurons?

Na+ influx

What causes olfactory adaptation?

Ca2+ influx decreasing response to sustained stimulus

What is olfactory adaptation?

Reduced ability to smell an odor after prolonged exposure

Where do olfactory nerve filaments synapse?

Mitral cells in olfactory bulb

What are mitral cells?

Second order neurons forming olfactory tract

Where does synapse between olfactory nerves and mitral cells occur?

Glomeruli

What happens to axons with same receptor type?

Converge on same type of glomerulus

Function of mitral cells?

Amplify refine and relay smell signals

What neurotransmitter do granule cells release?

GABA

Function of granule cells in olfactory bulb?

Inhibit mitral cells so only strong signals transmitted

Where do olfactory tract impulses travel?

Piriform lobe of olfactory cortex

What role does thalamus play in olfaction?

Some olfactory information passes through it to frontal lobe

Where is smell consciously interpreted?

Olfactory cortex and frontal lobe

Why can smells trigger emotions?

Signals travel to hypothalamus and limbic system

What is anosmia?

Loss or disorder of smell

Common causes of anosmia?

Head injury nasal inflammation Parkinson disease

What can tear olfactory nerves?

Head injuries

What are olfactory hallucinations?

Smelling odors that are not present

Common cause of olfactory hallucinations?

Temporal lobe epilepsy

What is an olfactory aura?

Smell sensation occurring before seizure

Why can aromatherapy affect mood?

Smell pathways connect to limbic system and hypothalamus

What is gustation?

Sense of taste

What are taste buds?

Sensory organs for taste

How many taste buds do humans have?

About 10000

Where are most taste buds located?

Tongue papillae

What are papillae?

Peglike projections of tongue mucosa

What are fungiform papillae?

Mushroom shaped papillae scattered across tongue housing taste buds

Where are foliate papillae located?

Side walls of tongue

What are vallate papillae?

Largest papillae arranged in V shape at back of tongue

Other locations of taste buds besides tongue?

Soft palate cheeks pharynx epiglottis

How many cells are in each taste bud?

50 to 100 epithelial cells

What are gustatory epithelial cells?

Taste receptor cells

What are gustatory hairs?

Microvilli projecting into taste pores

What bathes gustatory hairs?

Saliva

What surrounds gustatory epithelial cells?

Sensory dendrites

What neurotransmitters are released by gustatory cells?

Serotonin and ATP

What are basal epithelial cells?

Stem cells dividing every 7 to 10 days

What are the five basic taste sensations?

Sweet sour salty bitter umami

Examples of sweet tastants?

Sugars saccharin alcohol amino acids

What causes sour taste?

Hydrogen ions

What causes salty taste?

Metal ions especially sodium

What substances produce bitter taste?

Alkaloids like quinine nicotine caffeine aspirin

What produces umami taste?

Glutamate and aspartate

Possible sixth taste sensation?

Long chain fatty acids from lipids

Why are bitter and sour dislikes protective?

Warn against spoiled or poisonous substances

What must happen for a chemical to be tasted?

Dissolve in saliva diffuse into taste pore contact gustatory hairs

What happens when tastant binds gustatory cell?

Cell depolarizes and releases neurotransmitter

What happens after neurotransmitter release in taste?

Sensory neuron generates action potentials

Which taste receptors are most sensitive?

Bitter receptors

How quickly do taste receptors adapt?

3 to 5 seconds with complete adaptation in 1 to 5 minutes

How is salty taste transduced?

Na+ influx directly depolarizes cell

How is sour taste transduced?

H+ opens channels allowing cations to enter

What G protein is involved in sweet bitter and umami taste?

Gustducin

What happens when gustducin is activated?

Stored Ca2+ released opening cation channels

What neurotransmitter is released during sweet bitter and umami taste?

ATP

Which cranial nerve carries taste from anterior 2/3 of tongue?

Facial nerve CN VII

Which cranial nerve carries taste from posterior 1/3 of tongue?

Glossopharyngeal nerve CN IX

Which cranial nerve carries taste from epiglottis and lower pharynx?

Vagus nerve CN X

Where do taste fibers first synapse?

Solitary nucleus in medulla

Where do taste signals travel after medulla?

Thalamus then gustatory cortex in insula

What brain regions influence appreciation of taste?

Hypothalamus and limbic system

Functions of taste in digestion?

Increase saliva and gastric juice secretion

Protective reactions initiated by taste?

Gagging and vomiting

How much of taste depends on smell?

About 80 percent

Why does food taste bland with blocked nose?

Loss of smell contribution to flavor

What receptors besides taste receptors are in mouth?

Thermoreceptors mechanoreceptors nociceptors

How can spicy foods stimulate the mouth?

Activate pain receptors

Why are taste disorders less common than smell disorders?

Taste uses three cranial nerves

Causes of taste disorders?

Upper respiratory infections head injury medications radiation

What supplement may help radiation induced taste disorders?

Zinc

Could damage to cribriform plate affect taste?

Yes because loss of smell reduces flavor perception