NEURO: The Chemical Senses

Question 1

List some examples of chemical senses.

Answer 1

• taste
• smell
• CO2/O2 levels
• chemical irritants
• acidity

Question 2

What are the five basic tastes?

Answer 2

• sweet
• salty
• sour
• bitter
• umami

Question 3

What three things contribute to our perception of flavour?

Answer 3

• smell
• touch (texture, temperature)
• taste

Question 4

Describe the organs of taste.

Answer 4

It is not only the tongue that helps us with tasting, there are other structures that contribute towards it. Examples would include out hard palate, soft palate and pharynx. It gives us information such as temperature and texture of food.

Question 5

Describe the structure of the tongue.

Answer 5

On the tongue we have various papillae. The three types of papillae are:

• foliate
• fungiform
• vallate

Within the papillae, there are ridges/valleys in which the taste buds reside. Once the taste buds die, they are replaced by the basal cells situated behind them.

The taste cells have microvilli that project towards the taste pore in the wall of the ridge. It’s the microvilli that have those various receptors on them to detect the different tastes.

Question 6

What type of receptor governs each type of taste receptor?

Answer 6

Ion Channel mechanisms:

• saltiness
• sourness

GPCR mechanisms (via T1 and T2 taste receptors):

• bitterness
• sweetness
• umami

Question 7

Describe the taste transduction with saltiness.

Answer 7

Na+ is a major component of salt (NaCl).

Na+ passes through Na+-selective channels, down its concentration gradient.
This depolarises the taste cell, activating the voltage-gated Ca2+ channels (VGCCs).

The vesicular release of neurotransmitter (serotonin) is elicited, and gustatory afferents are activated.

Question 8

Describe the taste transduction with sourness.

Answer 8

H+ is the determinant of acidity and sourness.

H+ can pass through the same Na+-selective channels that mediate saltiness, down its concentration gradient. H+ also blocks H+-selective channels.

Both of these actions depolarise the taste cell activating the voltage-gated Ca2+ channels (VGCCs).

The vesicular release of the neurotransmitter (serotonin) is elicited, and gustatory afferents are activated.

Question 9

Describe the taste transduction with bitterness.

Answer 9

Bitterness is detected by the T2 taste receptors, of which there are over 3- types. T1/2 receptors are αGq coupled, so when activated PLC converts PIP2 to IP3 and DAG.

The IP3 intracellularly activates a type of Na+ ion channel, and releases Ca2+ from the endoplasmic reticulum. Both of these action depolarise the taste cell, activating voltage-gated Ca2+ channels (VGCCs).

The vesicular release of the neurotransmitter (ATP) is elicited, and gustatory afferents are activated.

Question 10

Describe the taste transduction with sweetness.

Answer 10

Sweetness is detected by a dimer receptor formed from T1R2 + T1R3.

It follows the same signal transduction mechanism as bitterness.

Question 11

Describe the taste transduction with umami.

Answer 11

Umami is detected by a dimer receptor formed from T1R1 + T1R3.

It follows the same signal transduction mechanism as bitterness and sweetness.

Question 12

List the cranial nerves connected to the tongue, and where they transmit information from.

Answer 12

CN VII (7) transmits from the anterior tongue.
CN IX (9) transmits from the posterior tongue.
CN X (10) transmits from the epiglottis.

Question 13

Describe the central taste pathways.

Answer 13

The 3 cranial nerves all transmit information to the gustatory nucleus (in the medulla), which then transmits it to the ventral posterior medial nucleus (in the thalamus), which again transmits the information to the gustatory cortex.

Question 14

Describe the organs of smell.

Answer 14

We don’t smell with our noses. We actually smell using the olfactory epithelium, which are dendrites of the olfactory cells that protrude from your olfactory bulb through holes in our skull called the cribriform plate and into the top of our nasal passage.

The olfactory receptor cells have protrusions called cilia, which are covered by a layer of mucus. The mucus is a water base containing sugars, enzymes, antibodies, and odorant binding proteins.

Question 15

Describe the mechanism of the olfactory receptor neurons.

Answer 15

The odorant molecules bind to the odorant receptor proteins on the cilia, and the olfactory-specific G(olf) G-protein is activated.

Adenylyl cyclase activation increases cAMP formation. This causes the cAMP-activated channels to open, allows the Na+ and Ca2+ influx.
Ca2+-activated chloride channels open, enabling Cl- efflux.

This leads to a depolarisation in the cell.

Question 16

How can we smell so many different things if it’s based on the same mechanism?

Answer 16

We have about 350 olfactory receptor proteins that are found in the human genome [over 1000 are found in the mouse genome].

The olfactory receptor cells may express only one receptor protein. Each receptor protein responds to odours to a greater or lesser extent. The smells are differentiated due to population coding.

Temporal and spatial cues seem to be important for decoding olfaction.

Question 17

Describe the central olfactory pathways.

Answer 17

Olfactory cells expressing the same receptor proteins project to the same glomeruli in the olfactory bulb via CN I (1).

The olfactory bulb then transmits the information to three places:

• frontal cortex (for the conscious perception of smell)
• hypothalamus and amygdala (for motivational and emotional aspects of smell)
• hippocampus (for odour memory)

Question 18

What are CO2/O2 levels measured by?

Answer 18

Chemoreceptors in arteries of the neck measure CO2/O2 levels in our blood.

Question 19

What are chemical irritants detected by?

Answer 19

Nerve endings in skin/mucous membranes warn us of chemical irritants.

Question 20

What is acidity detected by?

Answer 20

Sensory nerve endings in muscle respond to acidity –burning feeling that comes with exercise and O2 debt.

Question 21

Why do we not confuse bitter, sweet and umami tastes?

Answer 21

Taste cells express either bitter, sweet or umami receptors.

In turn, bitter, sweet and umami taste cells connect to different gustatory axons.

Question 22

What are present in the olfactory epithelium?

Answer 22

Olfactory receptor cells: Site of transduction – genuine neurons unlike taste receptor cells.

Supporting cells: Supporting cells function like glia cells and help produce mucus.

Basal cells:
Basal cells are immature olfactory receptor cells – source of new olfactory receptor cells.

Question 23

What is population coding for gustation and olfaction?

Answer 23

In population coding, the responses of a large number of broadly tuned neurons are used to specify the properties of a particular stimulus (e.g. taste, smell).

Gustatory and olfactory receptor cells may express only one specific receptor protein.

However, gustatory and olfactory axons and the neurons they activate in the brain respond more broadly.

Only with a large population of neurons, with different response patterns, can the brain then distinguish between specific tastes and smells…