The chemical sense Flashcards
Chemoreceptors in the neck
Measure CO2/O2 levels in our blood
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 and O2 debt
Learned taste
Experience can strongly modify our innate preferences.
Humans can learn to tolerate or enjoy the bitterness of some substances (e.g. coffee)
Innate taste
- Some of our taste preferences are inborn (or “innate”)
- Humans innately enjoy sweet flavours and avoid bitter flavours – this is evolutionarily ancient (e.g. distinguish food sources, avoidance of toxins)
Palate
• Roof of mouth separating oral and nasal cavities – taste buds present in palate
Epiglottis
Leaf shaped cartilage covering laryngeal inlet – taste buds present in epiglottis
Pharynx and nasal cavity
Odours can pass, via the pharynx, to the nasal cavity to be detected by olfactory receptors
Papillae
I. Ridge-shaped (foliate)
II. Pimple-shaped (vallate)
III. Mushroom-shaped (fungiform)
Tatse bud
The papillae contain taste buds:
• Taste buds contains taste receptor cells
• Taste buds are surrounded by basal cells (precursors of taste cells) and
• gustatory afferent axons
Taste receptor cells
- Three taste receptor cells sequentially exposed to salt, bitter, sour and sweet stimuli – membrane potential recorded
- Taste receptor cells display different sensitivities
- Taste receptor cells form synapses with gustatory afferent axons to transmit this gustatory information
Saltiness and sourness
Ion channel mechanisms
Saltiness taste transduction mechanism
• Na+ passes through Na+ selective channels, down its concentration gradient
• This depolarises the taste cell, activating voltage-gated Ca2+ channels (VGCCs)
• Vesicular release of neurotransmitter is elicited, and gustatory afferents activated
Special Na+ selective channel (amiloride sensitive) used to detect low concentrations of salt – insensitive to voltage and generally stays open
Sourness - taste transduction mechanism
Protons (H+) are the determinants of acidity and sourness.
• H+ may affective sensitive taste receptors in several ways – although these processes are not well understood
• However, it is likely that H+ can pass through proton channels and bind to and block K+ selective channels
• This leads to depolarisation of the taste cell, activating VGSC and VGCCs
• Vesicular release of neurotransmitter is elicited, and gustatory afferents activated
Bitterness, Sweetness, umami
GPCR mechanisms via T1 and T2 taste receptors
T1 Rs and T2Rs are
G-protein coupled receptors (GPCRs) and are Gq coupled
Bitter
Detected by approximately 25 T2Rs
• Bitter tastants binds to T2R, which is coupled to the G-protein Gq
• This stimulates the enzyme phospholipase C (PLC), leading to the production of inositol triphosphate (IP3)
• IP3 intracellularly activates a special type of Na+ ion channel and releases Ca2+ from intracellular storage sites
• Both these actions depolarise the taste cell – release of ATP is elicited, and gustatory afferents are activated
Sweet
Detected by one receptor – T1R2 and T1R3 proteins
• Sweet tastants binds to dimer receptor formed from T1R2 and T1R3, which is coupled to the G-protein Gq
Umami
Detected by one receptor – T1R1 and T1R3 proteins
• Umami tastants bind to dimer receptor formed from T1R1and T1R3, which is coupled to the G-protein Gq
• The same signal transduction mechanism as bitterness and sweetness occurs
• Shares T1R3 protein with sweetness – T1R subunit determines specificity to umami
Why do we not confuse bitter and sweet tastes?
- Taste cells express either bitter or sweet receptors – not both
- In turn, bitter and sweet taste cells connect to different gustatory axons
Central gustatory pathways
• The main flow of taste information is from taste cells to gustatory axons, into the gustatory nucleus (medulla), up to the ventral posterior medial nucleus (thalamus) and to the gustatory cortex
• Three cranial nerves carry gustatory axons and bring taste information to the brain:
CN 7
CN 9
CN 10
Smell as a mode of communication
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 pheromones in humans is unclear
Olfactory receptor cells
• Site of transduction – genuine neurons unlike taste receptor cells
Suppoorting cells
• Function to produce mucus – odorants dissolve in mucus layer before contacting cilia of olfactory receptor cells
Basal cells
• Immature olfactory receptor cells able to differentiate into mature olfactory receptor cells – olfactory receptor cells continuously grow, degenerate and regenerate
Olfactory transduction mechanisms
- Odorant molecules bind to odorant 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
Central olfactory pathways
- 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
Population coding for gustation and olfaction
- 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
An example of population coding for olfaction
- Olfactory receptor cells express a single olfactory receptor protein – each can respond to different odours with differing preferences
- 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 cells
- It is estimated that humans can discriminate at least one trillion different combinations of odour stimuli
