NEURO: The Chemical Senses Flashcards
List some examples of chemical senses.
- taste
- smell
- CO2/O2 levels
- chemical irritants
- acidity
What are the five basic tastes?
- sweet
- salty
- sour
- bitter
- umami
What three things contribute to our perception of flavour?
- smell
- touch (texture, temperature)
- taste
Describe the organs of taste.
It is not only the tongue that helps us with tasting, there are other structures that contribute towards it. Examples would include out hard palate, soft palate and pharynx. It gives us information such as temperature and texture of food.
Describe the structure of the tongue.
On the tongue we have various papillae. The three types of papillae are:
- foliate
- fungiform
- vallate
Within the papillae, there are ridges/valleys in which the taste buds reside. 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.
What type of receptor governs each type of taste receptor?
Ion Channel mechanisms:
- saltiness
- sourness
GPCR mechanisms (via T1 and T2 taste receptors):
- bitterness
- sweetness
- umami
Describe the taste transduction with saltiness.
Na+ is a major component of salt (NaCl).
Na+ passes through Na+-selective channels, down its concentration gradient.
This depolarises the taste cell, activating the voltage-gated Ca2+ channels (VGCCs).
The vesicular release of neurotransmitter (serotonin) is elicited, and gustatory afferents are activated.
Describe the taste transduction with sourness.
H+ is the determinant of acidity and sourness.
H+ can pass through the same Na+-selective channels that mediate saltiness, down its concentration gradient. H+ also blocks H+-selective channels.
Both of these actions depolarise the taste cell activating the voltage-gated Ca2+ channels (VGCCs).
The vesicular release of the neurotransmitter (serotonin) is elicited, and gustatory afferents are activated.
Describe the taste transduction with bitterness.
Bitterness is detected by the T2 taste receptors, of which there are over 3- types. T1/2 receptors are αGq coupled, so when activated PLC converts PIP2 to IP3 and DAG.
The IP3 intracellularly activates a type of Na+ ion channel, and releases Ca2+ from the endoplasmic reticulum. Both of these action depolarise the taste cell, activating voltage-gated Ca2+ channels (VGCCs).
The vesicular release of the neurotransmitter (ATP) is elicited, and gustatory afferents are activated.
Describe the taste transduction with sweetness.
Sweetness is detected by a dimer receptor formed from T1R2 + T1R3.
It follows the same signal transduction mechanism as bitterness.
Describe the taste transduction with umami.
Umami is detected by a dimer receptor formed from T1R1 + T1R3.
It follows the same signal transduction mechanism as bitterness and sweetness.
List the cranial nerves connected to the tongue, and where they transmit information from.
CN VII (7) transmits from the anterior tongue. CN IX (9) transmits from the posterior tongue. CN X (10) transmits from the epiglottis.
Describe the central taste pathways.
The 3 cranial nerves all transmit information to the gustatory nucleus (in the medulla), which then transmits it to the ventral posterior medial nucleus (in the thalamus), which again transmits the information to the gustatory cortex.
Describe the organs of smell.
We don’t smell with our noses. We actually smell using the olfactory epithelium, which are dendrites of the olfactory cells that protrude from your olfactory bulb through holes in our skull called the cribriform plate and into the top of our nasal passage.
The olfactory receptor cells have protrusions called cilia, which are covered by a layer of mucus. The mucus is a water base containing sugars, enzymes, antibodies, and odorant binding proteins.
Describe the mechanism of the olfactory receptor neurons.
The odorant molecules bind to the odorant receptor proteins on the cilia, and the olfactory-specific G(olf) G-protein is activated.
Adenylyl cyclase activation increases cAMP formation. This causes the cAMP-activated channels to open, allows the Na+ and Ca2+ influx.
Ca2+-activated chloride channels open, enabling Cl- efflux.
This leads to a depolarisation in the cell.
