Neuro: The Chemical Senses

Question 1

What are some of the chemical senses that our bodies are able to detect?

Answer 1

• Taste (gustation)
• Smell (olfaction)
• CO₂/O₂ levels
• Chemical irritants
• Acidity

Question 2

How are we able to detect CO₂/O₂ levels?

Answer 2

Chemoreceptors in arteries of neck detect levels of CO₂/O₂

Question 3

How are we able to detect chemical irritants?

Answer 3

Nerve endings in skin/muscous membranes detect chemical irritants

Question 4

How are we able to detect Acidity?

Answer 4

Sensory nerve endings in muscle are able to detect/respond to acidity

Question 5

What flavours do humans have an innate preference/dislike for? Why is this?

Answer 5

• Humans innately enjoy sweet flavours and innately dislike bitter flavours
• Allowed humans to distinguish between food sources

Question 6

Can experience modify our innate taste preferences?

Answer 6

• Yes, this is why most people go on to enjoy bitter taste of coffee

Question 7

What are the 5 basic tatses?

Answer 7

• Sweet
• Sour
• Bitter
• Salt
• Umami

Question 8

How do we perceive the flavour of food?

Answer 8

Perception of flavour a combination of different factors:

• Particular combination of the 5 basic tastes within the food
• Smell of food
• Texture/temperature of food

Question 9

What is the primary organ of taste?

Answer 9

Tongue

Question 10

What are the other organs of taste and how do they contribute to it?

Answer 10

• Palate (roof of mouth) - taste buds present in palate
• Epiglottis - taste buds present in epiglottis
• Pharynx and nasal cavity - Odours can pass into nasal cavity, via pharynx, where it gets detected by olfactory receptors

Question 11

On the tongue what are different types of papillae?

Answer 11

• Fungiform papillae - mushroom shaped
• Foliate papillae - ridge shaped
• Vallate papillae - pimple shaped

Question 12

Where exactly are the taste buds located on the tongue?

Answer 12

Taste buds are located within the ridges formed between the papillae of the tongue

Question 13

Describe the structure of a taste bud

Answer 13

• Taste pore - chemically sensitive end of taste bud
• Microvilli project from taste pore which surrond gustatory afferent axons (send signals to brain)
• Between microvilli of taste pore there are taste cells which synapse with the gustatory afferent axons

Question 14

What structure within the taste cells mean they are able to respond to taste?

Answer 14

• Taste cell receptors
• Each taste cell expresses different taste receptors - most taste cells respond primarily or exclusively to one of the five basic tastes

Question 15

What taste transduction mechanisms are ion channel mechanisms?

Answer 15

• Saltiness
• Sourness

Question 16

Explain the saltiness taste transduction mechanism

Answer 16

• Na+ passes through Na⁺ selective channels down its concentration gradient
• This Na+ influx causes depolarisation which activates Voltage-gated Ca²⁺ channels leading to Ca2+ influx
• This leads to release of neurotransmitters from vesicles via exocytosis
• Neurotransmitter release leads to activation of gustatory afferent axons which send signals to brain

Question 17

What is the specific name of the Na⁺ selective channels within particular taste cells? What are some of the properties of this channel

Answer 17

• Amiloride-sensitive Na⁺ channel
• Can detect low salt concentrations
• Insensitive to voltage so always stay open

Question 18

Explain the sourness taste transduction mechanism

Answer 18

• H⁺ passes down (Amiloride-sensitive) Na⁺ selective channels down its concentration gradient
• H⁺ binds and blocks K⁺ selective channels so more K⁺ remains in the cell
• More H⁺ and K⁺ within the cell leads to depolarisation which activates the voltage-gated Ca²⁺ channels
• This leads to Ca²⁺ influx which results in neurotransmitter release from vesicles via exocytosis
• Neurotransmitters bind and activate gustatory afferent axons which send signals to brain

Question 19

What taste transduction mechanisms are controlled by G-protein couplled receptor mechanisms?

Answer 19

• Sweetness
• Bitterness
• Umami

Question 20

Sweet, bitter and Umami taste receptors all form dimers and are all G-protein coupled receptors. What combination of taste receptor protein make up each type of receptor?

Answer 20

• Bitter receptors - 2 T2Rs
• Sweet receptor - T1R2 and T1R3
• Umami receptor - T1R1 and T1R3

Question 21

Explain the bitterness taste tranduction mechanism

Answer 21

• Bitter tastant binds to T2R, couplled to Gq protein, causing it to activate
• Activated T2R cativates phospholipase C enzyme which converts PIP₂ into IP₃ and DAG
• IP₃ binds to a special Na⁺ ion channel causing influx of Na⁺
• IP₃ also binds to receptors on endoplasmic reticulum causing it to release Ca²⁺ into cytosol
• These 2 effects cause depolarisation which causes the release of ATP
• ATP activates gustatory afferent axons which send signals to brain

Question 22

Explain the sweetness taste transduction mechanism

Answer 22

• Sweet tastants bind to dimer receptor formed from T1R2 and T1R3
• Mechanism is the same as the bitterness taste transduction mechanism

Question 23

If the sweet and bitter taste transduction mechansims are the same why does the brain not get them confused?

Answer 23

• Taste cells express either bitter or sweet receptors NOT both
• Also, bitter and sweet receptors activate different gustatory afferent axons

Question 24

Explain the umami tatse transduction mechanism

Answer 24

• Umami tastants bind to dimer receptor formed from T1R1 and T1R3
• Transduction mechanism same as bitter and sweet mechanisms

Question 25

Why does the brain not confuse Umami, sweet and bitter tastes if all the transduction mechanisms are the same?

Answer 25

* Taste cells only express either Umami, sweet or bitter taste receptors * Each taste receptor also activates different gustatory afferent axons

Question 26

Explain how information from the taste buds gets to the CNS via the central gustatory pathways

Answer 26

* Taste buds on anterior tongue connect with cranial nerve 7 (CN VII) * Taste buds on posterior tongue connect with cranial nerve 9 (CN 9) * Taste buds on epiglottis connect with cranial nerve 10 (CN X) * Each of these cranial nerves project to the gustatory nucleus (in the medulla) * Gustatory nucleus sends message to the ventral posterior medial nucleus (in the thalamus) * Ventral posterior medial nucleus sends a signal to the Gustatory cortex

Question 27

Are some of our smell preferences innate? Can experinece modify these innate preferences

Answer 27

Yes some of our smell preferences are innate but experineces can modify our innate preferences

Question 28

What is the main organ of smell?

Answer 28

Olfactory epithelium - we DO NOT smell with our noses

Question 29

Describe the structure of the olfactory epithelium

Answer 29

Olfactory epitheleium contains: * Olfactory receptor cells - site of transduction (actual neurons) * Supporting cells - function like glia cells and produce mucus * Basal cells - immature olfactory receptor cells (source of new olfactory receptor cells)

Question 30

Explain the olfactory transduction mechanism

Answer 30

* Odorant molecules bind to olfactory receptor proteins on cilia * This causes Olfactory-specific G protein (G_olf) to become activated * Activated G_olf activates adenylate cylase which forms cAMP from ATP * cAMP causes cAMP-actiavted channels to open which causes Na⁺ and Ca²⁺ influx * Ca²⁺ opens Ca²⁺ activated Cl^- channels leading to Cl^- efflux * This leads to membrane depolarisation

Question 31

Describe the pathway an action potential would take through an olfactory receptor cell/neuron

Answer 31

* Odorants bind to receptors on cilia which generates a slow receptor potential * Receptor potential then propagates along dendrites and triggers series of action potentials within olfactory cell stroma * Action potential propagates continously along olfactory nerve axon

Question 32

Where do the olfactory receptor cell axons project to?

Answer 32

* Olfactory receptor cell axons project to the olfactory bulb * The olfactory bulb contains structures called Glomeruli * Olfactory receptor cells expressing the same receptor proteins project to the same Glomerulus within the olfactory bulb

Question 33

Explain how information from the olfactory receptor cells gets to the CNS via the central olfactory pathways

Answer 33

* Olfactory receptor cells connect to cranial nerve 1 * Cranial nerve 1 projects to the olfactory bulb * Olfactory bulb then sends signals to different brain areas to process different aspects of smell: * It sends signals to Frontal cortex - processes conscious perception of smell * It sends signals to hypothalamus and amygdala - processes motivational and emotional aspects of smell * It sends signals to the hippocampus - processes odour memory

Question 34

What is population coding?

Answer 34

Occurs when the responses of a large number of broadly tuned neurons are used to specifiy the properties of a particular stimulus e.g. taste or smell

Question 35

Give an example of population coding based on olfaction

Answer 35

* When presented with a citrus smell, none of the three receptor cells can individually distinguish it from the other odours. * However, the brain can distinguish the citrus smell through the combination of responses from all three olfactory receptor cells.