Chapter 8 Chemical Senses: Taste Flashcards
What are the common steps of all 5 senses?
- Physical stimulus
- Set of events by which stimulus is transduced into a set of nerve impulses
- A response to the message
Why do animals need chemical senses?
To identify nourishment, poison, potential mate
Which are chemical sense?
Gustation and Olfaction
What are appetitive tastes?
Sweet- sugars/carbohydrates provide metabolic energy
Umami- meaty/savory taste
Salty- senses ions critical for cellular function
What are aversive tastes?
Bitter- Senses a broad-spectrum of potentially poisonous
Sour- common sense to rotten food
Too salty- excessive salt causes dehydration
Where are the taste receptors?
Tongue, pharynx, palate and epiglottis
What locations on tongue are there taste buds?
Circumvallate, foliate, fungiform
What are the distinct anatomical types?
Type I, Type II, Type III cells
What are taste cells?
Taste cells are not neurons but some cells have voltage-gated Na+ channels and can generate APs
What is the simple process of a taste bud?
Taste cells in taste buds transduce sensory stimuli into signals that are sent to the brain via the gustatory nerves
Where are microvilli?
Microvilli at the apical end poke into taste pore-receptors on microvilli
What is a Type I cell?
Glial-like support cells
dark
long apical microvilli
What is a Type II cell?
They are receptor cells.
Express metabotropic receptors for sweet, bitter, and umami
Cells tend to be narrowly tuned to a single taste quality
“Light”
Short apical microvilli
What is a Type III cell?
They are presynaptic cells.
Express receptors for sour and salty taste
This cell on average responds to about 3 taste qualities
single think apical process
What receptors do Type II cells use?
They use G-protein-coupled receptors
What is the process of transduction for Type II cells? Steps 1-5
- Molecules activate taste receptor.
- The receptor activates G-protein (gustducin)
- G-protein activates PLC
- PLC sends second messengers DAG and IP3
- Second messengers activate Ca2+ to be released.
What is the process of transduction for Type II cells? Steps 6-10
- Ca2+ opens calcium dependent cation channel
- Na+ flows through channel causing depolarization.
- Depolarization opens voltage-gates Na+ channels .
- Na+ flows through, depolarizing causing an action potential.
- Releases ATP (transmitter) through hemichannel. (non-vesicular release)
What are the channels for Type III salty cell?
Epithelial Na+ channel, ENaC and Amiloride-Insensitive Na+ channel (AI)
What is an Epithelial Na+ channel?
It is inhibited by amiloride.
Low activation threshold
What is an Amiloride-Insensitive Na+ channel?
Higher threshold of activation
Receptor unknown
What is the transduction mechanism for salty Type III cell?
- Amiloride-sensitive sodium channel allows Na+ through.
- Na+ flows through causing membrane depolarization.
- Depolarization activates voltage-gated sodium channel and calcium channel.
- Causing vesicle fusion so serotonin-filled synaptic vesicles are released.
- They form conventional synapses with gustatory nerves where serotonin is released.
What is the transduction mechanism for sour Type III cell?
- Depolarization of apical membranes by a proton current
- Intracellular acidification results in membrane depolarization by inhibiting a K+ channel
- Voltage-gates Na+ and Ca2+ opening causing vesicle fusion.
- So serotonin filled vesicles released into gustatory afferent axon.
What is the labelled-line model?
The model that says that receptor cells for sweet, bitter, and umami express only one type of receptor and the taste bud is a structure of parallel independent processing units.
How does Type II cells communicate with gustatory nerve?
Type II cells express hemichannels for non-vesicular release of the neurotransmitter ATP for cell-cell communication and communication with gustatory nerve. Since ATP receptor are found on gustatory afferents innervating taste buds.
