The Chemical Senses (neuro) Flashcards
Intro to chemical senses
- in addition to taste and smell, our bodies can detect a whole range of chemical signals
- CO2/O2 levels: chemoreceptors in arteries of 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
Is taste innate or learned?
Innate?:
- 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)
Learned?:
- However, experience can strongly modify our innate preferences
- Humans can learn to tolerate or enjoy the bitterness of some substances (e.g. coffee)
Organs of taste
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
Tongue
Papillae:
- The surface of the tongue contains papillae:
I. Ridge-shaped (foliate)
II. Pimple-shaped (vallate)
III. Mushroom-shaped (fungiform)
Taste buds:
- 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
- Taste receptors cells express different types of taste receptors – it has been shown that most taste receptor cells respond primarily (or even exclusively) to one of the five basic tastes.
- 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
Mechanisms of taste transduction
- the 5 different tastes are transduced via different mechanisms
- saltiness and sourness via ion channel mechanisms
- bitterness, sweetness and umami via GPCR mechanisms via T1 and T2 taste receptors
Saltiness
- The prototypical salty chemical is table salt (NaCl) - taste of salt is mostly the taste of the cation sodium (Na+).
- 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
- Protons (H+) are the determinants of acidity and sourness
- Transduction mechanism:
• H+ may affective sensitive taste receptors in several ways – although these processes are not well understood
• Vesicular release of neurotransmitter is elicited, and gustatory afferents activated
• 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
Bitterness
- Transduction mechanism:
• 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
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
• Taste cells express either bitter or sweet receptors – not both
• In turn, bitter and sweet taste cells connect to different gustatory axons
Umami
- Transduction mechanism:
• Umami tastants bind to dimer receptor formed from T1R1 and
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
• Taste cells express either bitter, sweet or umami receptors
• In turn, bitter, sweet and umami 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:
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 epithelium
- Olfactory receptor cells
• Site of transduction – genuine neurons unlike taste receptor cells - Supporting 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
