NEURO: The Chemical Senses

Question 1

List some examples of chemical senses.

Answer 1

• TASTE
• SMELL
• CO2/O2 LEVELS
  (Chemoreceptors - in arteries of the neck measure CO₂/O₂ levels in our blood
  E.g. during altitude (hypoxia), chemoreceptors activated to drive oxygenated blood into the body)
• CHEMICAL IRRITANTS
  (nerve endings in skin/mucous membranes warn us of chemical irritants)
• ACIDITY
  (sensory nerve endings in muscle respond to acidity - burning feeling that comes with exercise (e.g. lactic acid) and O2 debt).

Question 2

How do we develop taste preferences?

Answer 2

Some of our taste preferences are inborn / innate
Humans innately enjoy sweet flavours and avoid bitter flavours - this is evolutionary (avoids toxins, distinguishes food sources)

However our experiences can strongly modify our innate preferences = we can learn to tolerate or enjoy the bitterness of some substances (e.g. coffee)

Question 3

What are the 5 basic tastes?

Answer 3

The 5 basic tastes:

• Sweet
• Sour
• Bitter
• Salt
• Umami (or savoury) (monosodium glutamate)

Question 4

How do we perceive flavour?

Answer 4

Flavour is perceived due to the mixture of the tastes, the texture and smell.

Question 5

Describe the organs of taste.

• see pics in slide.

Answer 5

Taste is primarily a function of the tongue - however, other areas of the mouth, throat and nasal passages also have important roles in taste.

These include:

• Palate: the roof of mouth separating oral and nasal cavities - taste buds present in palate.
• Epiglottis: leaf shaped cartilage laryngeal inlet upon swallowing - taste buds present in epiglottis.
• Pharynx and Nasal cavity: odours pass via pharynx to the nasal cavity to be detected by olfactory receptors.

Question 6

What aids the tongue in perceiving taste (gustation)?

Answer 6

All areas of the tongue can detect all flavours, but certain flavours are recognised better in specific areas (tip - sweetness, back - bitterness, sides - saltiness and sourness).

On the tongue we have various papillae. The 3 types of papillae are:

• foliate (ridge-shaped)
• vallate (pimple-shaped)
• fungiform (mushroom shaped)

Within the papillae, there are ridges/valleys in which the taste buds reside:

• Taste buds contain taste receptor cells
• Taste buds are surrounded by basal cells (precursors of taste cells) and gustatory afferent axons.
• There are also taste pores which are the chemically sensitive end of a taste receptor cell where chemicals dissolved in saliva can interact directly with the taste cells.

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 7

Describe the taste receptor cells.

Answer 7

Taste receptor cells express different types of taste receptors - it has been shown that most receptor cells respond primarily (or even exclusively) to one of the 5 basic tastes.

Taste receptor cells display different sensitivities. Taste receptor cells form synapses with gustatory afferent axons to transmit this gustatory information. - this was discovered by an experiment by recording their membrane potential to different tastes.

Question 8

The five different tastes are transduced via different mechanisms. What type of receptor governs each type of taste receptor?

Answer 8

Ion Channel mechanisms:

• saltiness
• sourness

GPCR mechanisms (via T1 and T2 taste receptors):

• bitterness
• sweetness
• umami

Question 9

Describe the taste transduction mechanism with saltiness.

Answer 9

Prototypical salty chemical is table salt (NaCl) - taste of salt is mostly the taste of the cation Na+.

Transduction mechanism for detecting saltiness:

• Na+ passes through Na+-selective channels, down its concentration gradient (high to low from outside to inside cell).
• This depolarises the taste cell, activating the voltage-gated Ca2+ channels (VGCCs) and voltage-gated Na+ channels (VGSCs).
• This triggers the vesicular release of neurotransmitter (serotonin) is elicited, and activates gustatory afferents.

Special selective channel (amiloride sensitive) used to detect low concentrations of salt - insensitive to voltage and generally stays open.

Question 10

Describe the taste transduction mechanism with sourness.

Answer 10

Protons (H+) are the determinants of acidity and sourness.

Sourness transduction mechanism:

• H+ may affect sensitive taste receptors in several ways, although these processes are not well understood.
• It is likely that H+ pass through proton channels down it concentration gradient (from high outside to low inside taste cell) and bind to and block K+ selective channels.
• Both of these actions depolarise the taste cell activating the voltage-gated Ca2+ channels (VGCCs) and voltage-gated Na+ channels (VGSCs).
• This causes the vesicular release of the neurotransmitter (serotonin) is elicited, and gustatory afferents are activated.

Question 11

Describe the GPCR taste receptor proteins (T1Rs and T2Rs).

Answer 11

Transduction mechanisms underlying bitter, sweet and umami tastes rely on two families of related taste receptor proteins – T1Rs and T2Rs.

T1Rs and T2Rs are G-protein coupled receptors (GPCRs) and are Gq coupled – evidence suggests that they form dimers.

• Bitter substances are detected by approximately 25 different T2R subtypes.
• Sweet substances are detected by one receptor – comprised of T1R2 and T1R3 protein subunits.
• Umami substances are detected by one receptor – comprised of T1R1 and T1R3 protein subunits.

Question 12

Describe the taste transduction mechanism with bitterness.

Answer 12

Bitterness transduction mechanism:

• Bitter tastants bind to T2R, which is coupled to the G-protein Gq.
• This stimulates the enzyme phospholipase C leading to the production of inositol triphophate (IP3) from PIP2.
• IP3 intracellularly activates a special type of Na+ ion channel (unique to taste cells) allowing Na+ ions to enter the cell and release Ca2+ from intracellular storages sites.
• Both these actions depolarise the taste cell - eliciting the release of ATP and activating the gustatory afferent axons transmitting information to other regions of the brain.

Question 13

Describe the taste transduction mechanism with sweetness.

Answer 13

Sweetness transduction mechanism:

• Sweet tastants binds to dimer receptor formed from T1R2 and T1R3, which is coupled to the G-protein Gq.
• The same signal transduction mechanism as bitterness occurs.
• So enzyme phospholipase C is stimulated leading to the production of inositol triphophate (IP3) from PIP2.
• IP3 intracellularly activates a special type of Na+ ion channel (unique to taste cells) allowing Na+ ions to enter the cell and release Ca2+ from intracellular storages sites.
• Both these actions depolarise the taste cell - eliciting the release of ATP and activating the gustatory afferent axons transmitting information to other regions of the brain.

Question 14

Describe the taste transduction mechanism with umami.

Answer 14

Umami transduction mechanism:

• Umami tastans bind to a dimer receptor formed from T1R1 + T1R3, which is coupled to the G-protein Gq.
• The same signal transduction mechanism as bitterness and sweetness occurs.
• So enzyme phospholipase C is stimulated leading to the production of inositol triphophate (IP3) from PIP2.
• IP3 intracellularly activates a special type of Na+ ion channel (unique to taste cells) allowing Na+ ions to enter the cell and release Ca2+ from intracellular storages sites.
• Both these actions depolarise the taste cell - eliciting the release of ATP and activating the gustatory afferent axons transmitting information to other regions of the brain.

The Umami mechanism shares the T1R3 protein with sweetness - T1R1 subunit determines specificity to umami.

Question 15

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

Answer 15

Taste cells express either bitter, sweet or umami receptors - not all 3.

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

Question 16

List the cranial nerves connected to the tongue, and where they transmit information from. What is the flow of taste information to the CNS?

Answer 16

CN VII (7 - Facial Nerve) transmits from the anterior tongue.
CN IX (9 - Glossopharyngeal Nerve) transmits from the posterior tongue.
CN X (10 - Vagal Nerve) transmits from the epiglottis.

The main flow of taste information: the 3 cranial nerves carry gustatory axons and transmit information to the gustatory nucleus in the medulla, up to the ventral posterior medial nucleus (thalamus) and to the gustatory cortex.

The gustatory cortex mediates the conscious experience of taste.

Question 17

Is smell (olfaction) innate or learned?

Answer 17

Similarly to taste, some of our smell preferences are inborn (or “innate”).

Our experience can strongly modify our innate preferences (e.g. smoking, professional perfumers)

Question 18

What are pheromones?

Answer 18

Pheromones are olfactory stimuli used for chemical communication between individuals.

In some animals, pheromones are important signals for reproductive behaviours, marking territories and indicating aggression or submission.

However, the importance in humans is unclear.
It has been shown that infants as young as 6 days old show a clear to their mother (as opposed to nursing mothers). Mothers can also distinguish the smell of their own baby amongst other smell choices.

Question 19

Describe the olfactory epithelium and its 3 cell types.

• see pics in slide

Answer 19

We don’t smell with our noses. We actually smell using a small thin sheet of cells high up in the nasal cavity, known as the olfactory epithelium.

The olfactory epithelium has 3 main cell types:

• Olfactory receptor cells: Site of transduction with axons penetrating into the CNS – genuine neurons unlike taste receptor cells.
• Supporting cells: Function to produce mucus – odorants dissolve in mucus layer before contacting cilia of olfactory receptor cells. The mucus comprises a variety of proteins for example antibodies and enzymes that have important protective functions.
• Basal cells: Immature olfactory receptor cells able to differentiate into mature olfactory receptor cells – olfactory receptor cells continuously grow, degenerate and regenerate.

Question 20

Describe the olfactory transduction mechanism.

Answer 20

• Odorant molecules dissolved in the mucus bind to odorant olfactory receptor proteins on the cilia.
• Olfactory-specific G-protein (Golf) is activated.
• Adenylyl cyclase activation increases cAMP formation.
• cAMP-activated channels open, allowing Na+ and Ca2+ influx.
• Ca2+ activated chloride channels open enabling Cl- efflux.
• Causes membrane depolarisation of the olfactory neuron.

Question 21

What structures form the olfactory receptor neuron?

Answer 21

• Cilia and Odorants
• Dendrite
• Olfactory cell body
• Olfactory nerve axon

Question 22

Describe how action potentials are generated at the olfactory receptor neurons.

Answer 22

• Odorants generate a slow receptor potential in the cilia.
• Receptor potential propagates along the dendrite and triggers a series of action potentials within the olfactory receptor cell soma.
• Action potentials propagate continuously along the olfactory nerve axon.

Question 23

What is the flow of smell information to the CNS?

Answer 23

We have many different odourant receptor proteins.

In order to transmit smell information into the CNS the olfactory receptor cells send axons into the olfactory bulb.

Olfactory receptor cells expressing the same receptor proteins project to the same glomeruli in the olfactory bulb.

Signals are relayed in the glomeruli and transmitted to higher regions of the brain.

Question 24

Describe the central olfactory pathways.

Answer 24

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 25

What is population coding?

Answer 25

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).

Question 26

Describe population coding in relation to gustation and olfaction.

Answer 26

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 distinguish between specific tastes and smells.

Question 27

Describe the concept of population coding using olfaction as an example.

• see example in slide.

Answer 27

Olfactory receptor cells express a single olfactory receptor protein – each can respond to different odours with differing preferences.

None of the receptor cells can individually distinguish the smell (e.g. citrus) from the other odours.

However, the brain can distinguish the citrus smell through the combination of responses from all three cells.

It is estimated that humans can discriminate at least one trillion different combinations of odour stimuli.

Question 28

How are we able to determine various smells and tastes?

Answer 28

Temporal and spatial population coding is used to discern sensory signals

Question 29

Glossary

Answer 29

Chemoreceptor – A protein responsive to chemical stimuli
Glomerulus – A cluster of nerve endings in the olfactory bulb
Gustation – The perception of taste
Gustatory nucleus – Region of the medulla that conveys gustatory signals
Olfaction – The perception of smell
Olfactory bulb – Region to which olfactory receptor cells project
Olfactory cortex – Region of the cortex that perceives smell
Olfactory epithelium – Mucus covered layer into which cilia of olfactory receptor cells protrude to facilitate odour detection
Olfactory receptor cell – Neuronal cell responsible for odour detection
Papilla – A small rounded protuberance on the tongue
Pheromone – A chemical substance produced and released into the environment by an animal that affects the behaviour or physiology of others of its species
Population coding – The temporal and spatial frequencies of sensory signals from multiple neurons
Primary gustatory cortex – Region of the cortex that perceives taste
Taste bud – A cluster of taste receptor cells
Taste receptor cell – Cells responsible for taste detection
Ventral posterior medial nucleus – Sensory thalamic nucleus through which gustatory signals are transmitted