W9- Neuro: The Chemical Senses Flashcards
What other chemical senses aside from taste and smell do we have?
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
Movement from lungs to blood stream in eg. high altitudes/states of hypoxia.
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. Eg. Lactic acid causing a stitch.
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)
How do we perceive flavour?
5 basic tastes:
- Saltiness
- Sourness
- Sweetness
- Bitterness
- Umami, defined by the savoury taste of the amino acid glutamates with monosodium glutamate (NSG).
-Each food activates a unique combination of these 5 tastes.
-Touch - texture, temperature also plays a role
-Smell
What are the organs of taste?
Taste is primarily a function of the tongue – however, other areas of the mouth, throat and nasal passages also have important roles in 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
What is on the 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 (transmits gustatory information to other regions of nervous system)
Each taste bud has approximately 50-150 taste receptor cells. The taste pore is the chemically sensitive end of taste receptor cells in which chemically dissolved in saliva can interact directly with the taste cells.
Tip of the tongue = sweetness
Back = bitterness
Sides = saltiness and sourness
Most of the tongue is receptive to all 5 basic tastes.
What are 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
What are the mechanisms of taste transduction?
The process by which an environmental stimulus causes an electrical response in a sensory receptor cell is called transduction.
The five different tastes are transduced via different mechanisms:
These can be subdivided into ion channel mechanisms and G protein coupled receptors or PCR mechanisms.
SALTINESS
SOURNESS
Both ion channel
What is saltiness - is it a transduction mechanism?
The prototypical salty chemical is table salt (NaCl) - taste of salt is mostly the taste of the cation sodium (Na+).
- 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. Amiloride is a diuretic - promotes urine production.
What is sourness - is it a 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
- 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
What are the mechanisms of taste transduction?
The five different tastes are transduced via different mechanisms:
BITTERNESS
SWEETNESS
UMAMI
GPCR mechanisms via T1 and T2 taste receptors
What are taste receptor proteins (T1Rs and T2Rs)?
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
I. Bitter substances are detected by approximately 25 T2Rs. Important to have more of these because they are poison detectors.
II. Sweet substances are detected by one receptor –
T1R2 and T1R3 proteins
III. Umami substances are detected by one receptor – T1R1 and T1R3 proteins
How is bitterness a taste 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
What are taste transduction mechanisms of sweetness?
- 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
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
How is umami a transduction mechanism?
- 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, SWEET AND UMAMI TASTES? - Taste cells express either bitter, sweet or umami receptors
- In turn, bitter, sweet and umami taste cells connect to different gustatory axons
What is a central gustatory pathway?
WHAT IS THE FLOW OF TASTE INFORMATION TO THE CNS?
* 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:
3 Gustatory axons:
- Anterior tongue = Central Nerve VII -> facial
- Posterior tongue = CN IX -> Glossopharyngeal
- Epiglotis = CN X -> vagus
Then then go to the Gustatory nucleus (medulla) -> Ventral posterior medial nucleus (thalamus)
-> Gustatory cortex, which mediates sensation of taste
