NEURO: The Chemical Senses Flashcards
How does our body detect CO2/O2 levels?
How does our body warn us of chemical irritants?
chemoreceptors in arteries of the neck measure CO2/O2 levels in blood
nerve endings in skin/mucous membranes warn us of chemical irritants
List some examples of chemical senses.
- taste
- smell
- CO2/O2 levels
- chemical irritants
- acidity
What are the five basic tastes?
What three things contribute to our perception of flavour?
- sweet
- salty
- sour
- bitter
- umami
- smell
- touch (texture, temperature)
- taste
How does our body detect acidity?
sensory nerve endings in muscle respond to acidity, giving us that burning feeling that comes with exercise and O2 debt
Describe the organs of taste.
The main organ of taste is the tongue
Others include:
Palate
-contains taste buds that detect the flavour
Epiglottis
-contains taste buds that detect the flavour
Pharynx and Nasal Cavity
-odours can pass via the pharynx to the nasal cavity to be detected by olfactory receptors
Concentrations of taste receptors
Whilst there are particular concentrations of taste receptors in different regions, all areas of the tongue can detect all tastes at slightly lower levels
Describe the structure of the tongue.
On the tongue, we have papillae (structures on the tongue containing taste buds). The three types 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.
the cellular structure of a papillae
- Papillae contain taste buds
- Taste buds contain taste pore (chemically sensitive end of taste bud)
- Taste pores contain taste cells (connect and synapse with gustatory afferent axons which transmit information to the brain)
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.
What do transduction processes underlying bitter, sweet & umami rely on?
What are bitter substances detected by?
G-protein coupled receptors (Gq)
- T1Rs & T2Rs
- evidence suggests they form dimers
~25 T2Rs
Describe the taste transduction with bitterness.
1) Bitter tastants bind to T2 receptor (~25 of these), which is Gq coupled
2) Stimulate enzyme phospholipase C (PLC), which converts PIP2 to IP3 (and DAG)
3) IP3 intracellularly activates a special type of Na+ ion channel and releases Ca2+ from the endoplasmic reticulum
4) Activation of Na+ ion channel and release of Ca2+ from ER depolarises the taste cell
5) ATP released through an ATP-permeable channel
6) ATP binds to the gustatory afferent and activated it, transmitting the signal to the brain
The vesicular release of the neurotransmitter (ATP) is elicited, and gustatory afferents are activated.
Describe the taste transduction with sweetness.
sweetness is detected by one receptor made of T1R2 & T1R3 proteins
It follows the same signal transduction mechanism as bitterness and sweetness.
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.
These cranial nerves contact the gustatory nucleus in the medulla. A message is then passed on to the ventral posterior medial nucleus in the thalamus. The message is then passed to a region in the cortex known as the gustatory cortex, which allows us to perceive our taste.
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.
Olfactory epithelium, comprised of:
· Olfactory Receptor Cells
>Site of signal transduction- genuine neurones unlike taste receptor cells
>have cilia which is covered with mucus
· Supporting Cells
>Function like glial cells and help produce mucus
>Mucus layer (water base containing sugars, enzymes, antibodies, and odorant-binding proteins) allows odourants to bind to the cilia of olfactory cells
· Basal Cells
>Immature olfactory receptor cells- the source of new olfactory receptor cells
Where is the olfactory epithelium located?
roof of nasal cavity
Describe the mechanism of the olfactory receptor neurons.
1) Odourant molecules bind to odourant receptor proteins (G-protein) on the cilia
2) Olfactory-specific G-protein (Golf) is activated
3) Adenylyl cyclase activation increases the cAMP formation
4) cAMP-activated channels open, allowing Na+ and Ca2+ influx
5) Ca2+ activated chloride channels open, enabling Cl- efflux
6) Causes membrane depolarisation of the cell
This leads to a depolarisation in the cell. The olfactory cells then project the signal back up to the olfactory bulb.
What happens after depolarisation of the olfactory receptor cell?
slow receptor potential in cilia generated which propagates along the dendrite and triggers a series of action potentials within the olfactory cell soma
action potentials then propagate continuously along the olfactory nerve axon and are transmitted to the brain
Population coding
neural representation of a stimulus by the pattern of firing of a large number of neurones:
In gustation and olfaction:
· Gustatory and olfactory receptor cells may express only one specific receptor protein
· However, those receptor cells then connect to gustatory or olfactory axons, and these neurones in the brain respond more broadly than the receptor cells themselves
· Only with a large population of neurones, with different response patterns to particular stimuli, can the brain then distinguish between specific tastes and smells
Describe the central olfactory pathways.
Olfactory receptor cells send axons (CN I) into glomeruli in the olfactory bulb
-glomerulus is a collection of olfactory axons in the bulb
once signals have reached the glomerulus, they are delayed in the glomeruli and transmitted to higher regions of the brain to detect smell:
- Frontal (Olfactory) Cortex: used for the conscious perception of smell
- Hippocampus: important in odour memory
- Hypothalamus and Amygdala: used for motivational and emotional aspects of smell
