Olfaction and Gustation

What is olfaction?

The sense of smell.

Where are the olfactory organs located?

In the nasal cavity.

What are the two layers of the olfactory organs?

The olfactory epithelium and the lamina propria.

What are odorants?

Small airborne organic water-soluble or lipid-soluble substances that stimulate olfactory receptors.

What areas does the olfactory epithelium cover?

The inferior surface of the cribriform plate, the superior portion of the perpendicular plate, and the superior nasal conchae of the ethmoid.

What lies beneath the olfactory epithelium?

The lamina propria.

What does the lamina propria contain?

Areolar tissue, blood vessels, nerves, and olfactory glands.

What do the olfactory glands secrete?

Mucus

What are olfactory sensory neurons?

Highly modified nerve cells involved in the sense of smell.

What is found in the dendritic bulb of olfactory sensory neurons?

Dendrites that extend into the surrounding mucus.

Where are olfactory receptors (odorant-binding proteins) located?

On the surface of the dendrites of olfactory sensory neurons.

What do the axons of olfactory sensory neurons form?

The olfactory nerves.

What is the function of supporting cells in the olfactory epithelium?

They provide structural and metabolic support to olfactory sensory neurons.

What are basal epithelial cells?

Stem cells that can differentiate into new olfactory sensory neurons.

What is the first step of olfaction?

An odorant binds to a G protein–coupled receptor in the plasma membrane of an olfactory dendrite.

What happens after the odorant binds to the receptor?

The second messenger cAMP opens sodium ion channels, and the olfactory neuron depolarizes, creating a generator potential.

What occurs if there is sufficient depolarization in the olfactory neuron?

It triggers action potentials in the axon of the olfactory neuron.

Where do the axons of olfactory sensory neurons synapse after passing through the cribriform plate?

With neurons in the olfactory bulbs.

What forms the olfactory tracts?

Axons of the olfactory bulb neurons.

Where do the olfactory tracts send sensory information?

To the olfactory cortex in the cerebrum, the hypothalamus, and the limbic system.

What is unique about how olfactory information reaches the cerebral cortex?

It is the only sensory information that does not pass through the thalamus first.

What causes olfactory adaptation?

Central adaptation due to innervation of the olfactory bulbs by other brain nuclei.

How many odorants can humans typically distinguish between?

Between 2,000–4,000 odorants.

Why can we detect so many different smells?

Because of populations of olfactory neurons with distinct sensitivities.

How does the nervous system interpret more than 50 primary smells?

Based on the overall pattern of receptor activity.

Does sensitivity to different smells vary between individuals?

Yes, sensitivity to different smells varies widely.

How are olfactory neurons replaced?

They are frequently replaced by basal cells in the olfactory epithelium.

What happens to the number of olfactory neurons with age?

The total number declines with age.

define gustation

the sense of taste

• Provides information about foods and liquids consumed

define taste receptor cells (gustatory epithelial cells)

Found in taste buds distributed on superior surface of the tongue and portions of

the pharynx and larynx

define Lingual papillae

epithelial projections on the surface of the tongue

define Filiform papillae

Found in the anterior two-thirds of the tongue

• Provide friction to move food around mouth and do not contain taste buds

define Fungiform papillae

Scattered around the tongue with higher concentrations along the tip and sides

• Contain about five taste buds each

define Vallate papillae

Found in a V-shaper pattern along the posterior margin of the tongue

• Contain about 100 taste buds each

define Foliate papillae

Found in folds along the lateral margin of the tongue

• Contain taste buds

What are taste buds?

Sensory organs involved in the sense of taste.

What are the three main cell types found in taste buds?

Basal epithelial cells, transitional cells, and taste receptor cells (gustatory epithelial cells).

What is the function of basal epithelial cells in taste buds?

They are stem cells that replace other cells in the taste bud.

What is the role of transitional cells in taste buds?

They act as supporting cells.

What are taste receptor cells also called?

Gustatory epithelial cells.

What do taste receptor cells extend into the surrounding fluid?

Microvilli, also known as taste hairs.

Where do the taste hairs extend through in the taste bud?

Through a taste pore (a narrow opening at the exposed end of the taste bud).

With what do taste receptor cells synapse?

The dendrites of sensory neurons.

How are taste receptor cells activated?

By depolarizing stimuli that cause them to release neurotransmitters.

What do taste receptor cells stimulate after releasing neurotransmitters?

Sensory neurons.

Which cranial nerve innervates the taste buds of the anterior two-thirds of the tongue?

The facial nerve (CN VII).

Which cranial nerve innervates the vallate papillae?

The glossopharyngeal nerve (CN IX).

Which cranial nerve innervates extralingual taste buds?

The vagus nerve (CN X).

Where do the sensory afferent fibers from taste neurons synapse?

In the solitary nucleus of the medulla oblongata.

Where do the axons of postsynaptic neurons from the medulla oblongata travel?

They enter the medial lemniscus and synapse in the thalamus.

Where do thalamic neurons send gustatory information?

To the gustatory cortex in the insula.

What factors influence taste perception besides taste receptor activity?

Texture of food and olfactory information.

Why might food taste dull when you have a cold?

Because inflammation of the nasal mucosa affects olfaction, which contributes to taste perception.

What are the primary taste sensations

Sweet, salty, sour, and bitter.

How is sweetness detected?

By G protein–coupled receptors.

How is saltiness detected?

When sodium ions diffuse into the receptor cell via sodium leak channels.

How is sourness detected?

When hydrogen ions diffuse into the receptor cell via sodium leak channels.

How is Sourness detected

When hydrogen ions diffuse into the receptor cell via sodium leak channels.

What is the taste sensation of umami?

A pleasant, savory taste imparted by glutamate.

How is umami detected?

By G protein–coupled receptors.

How is the taste sensation of water detected?

By water receptors in the pharynx.

Where is sensory output from taste processed?

In the hypothalamus.

Are we more sensitive to pleasant or unpleasant tastes?

We are more sensitive to unpleasant tastes such as bitter and sour.

How does taste sensitivity vary among individuals?

It differs significantly among individuals.

Is taste sensitivity inherited?

Yes, taste sensitivity is often inherited.

What is an example of a taste sensitivity variation?

Phenylthiocarbamide (PTC) can taste bitter to some people and have no taste for others.

Do children or adults have more taste receptors?

Children have more taste receptors than adults.

When does the number of taste receptors begin to decline?

The number of taste receptors begins declining rapidly at age 50.