Lecture 4: Gustation Flashcards
What are papillae and where are they located?
Papillae contain sensory receptors that transduce taste - they cover the surface of the tongue, as well as around epiglottis and oesophagus
What is the function of gustation
To draw you away from things that are toxic and to draw you to foods that are nutritive, beneficial or pleasant.
Where are tastes distributed?
Taste is distributed throughout the whole oral cavity, there are not particular areas sensitive to a particular taste
Describe the structure of a taste bud and how they work
It has a characteristic onion shape - its a cluster of sensory receptors that form a ball like structure. They are found across the entire surface of each papillae. At the top of the ball like structure there is a open pore with microvilli (where chemicals are able to access sensory cells) and these receptors transduce the chemicals into an electrical signal. Each taste cell makes a synapse with an axon -
Which sensory cells are able to regenerate and from which cells?
Olfactory receptors and taste receptors are able to regenerate from basal cells.
Why does our sense of taste reduce when we age if we are able to regenerate new taste cells
Due to the reduction in the ability of basal cells to produce new taste cells
What is the function of saliva?
Similar to the olfactory mucosa, where mucous assists the conversion from the air to the aqueous phase, saliva assists with the dilution of tastes and enabling them to present to the taste pore.
Describe the taste spectrum
Sensory taste cells within a taste bud have different patterns of activation in response to salty, bitter, sour, sweet and umami.. Some taste cells are more broadly tuned whilst others may only respond to one tastent (has specificity). For example, one cell in a taste bud may only respond to sucrose, whereas another may respond to salty and acidic tastes. Sensory fidelity is maintained
Describe how taste cells respond to saltiness and draw a diagram of this process
Taste cells that respond to saltiness have Na channels in cell membrane. 1. Saliva liberates Na ions from food and have access to sodium channels on taste cell pore surface. 2. Sodium enters the cell causing depolarisation in membrane, this causes voltage gated sodium and calcium channels to open. 3. Calcium enters cell and activates vesicles containing neurotransmitter 4. Vesicles migrate to surface and neurotransmitters is released from cell to activate gustatory afferent neuron.
Describe how taste cells respond to sourness and draw a diagram of this process
Sour things contain H ions 1. Saliva liberates H ions from food and have access to sodium channels on taste cell pore surface. 2. Hydrogen ions enter the cell, inhibiting the potassium channel, causing depolarisation in membrane, this causes voltage gated sodium and calcium channels to open. 3. Calcium enters cell and activates vesicles containing neurotransmitter 4. Vesicles migrate to surface and neurotransmitters is released from cell to activate gustatory afferent neuron.
Describe how taste cells respond to bitter, sweet or umami and draw a diagram of this process
- Saliva liberates tastents from food and are able to bind to G-protein coupled receptors on taste cell pore surface. 2. This activates phospholipase C which activates IP3 3. IP3 acts on intracellular Ca2+ stores and calcium is released. 4. Calcium acts on synaptic vesicles that release neurotransmitter on to the gustatory afferent neuron to cause depolarisation
What is the main difference between taste cells that respond to saltness or sourness and taste cells that respond to bitter, sweet or umami?
Unlike saltiness and sourness which use sodium channels, bitter/sweet/umami uses G-protein coupled receptors. The release of neurotransmitter from synaptic vesicles depend on extracellular calcium for saltiness and sourness whereas bitterness, sweetness and umami depend on intracellular stores of calcium
How many receptor subtypes are there for bitterness and why? How many subunits within a receptor?
Bitterness has 30-40 different subtypes. As bitterness is associated with high toxicity a large array of receptors may assist to identify toxic substances. There is only one subunit for each bitterness receptor
Describe the structure of the sweet receptor
Has only one type of receptor containing 2 subunits which are require to bind to sugars.
How many subunits do umami receptors have
2 subunits
Describe the gustatory pathway from taste receptors
Gustatory afferent neurons carrying information from a single taste receptor project to the rostral 1/3 Nucleus of the Solitary Tract via cranial nerves 7, 9 and 10 (depending on location of taste receptor).
There are then 3 pathways from the NTS
- Taste perception
NTS –> parabrachial nucleus –> caudal VPM of thalamus –> primary gustatory cortex (insula-operculum)
- Taste reflexes
NTS –> reticular formation (salivatory nuclei, orofacial motor neurons, trigeminal complex)
NTS –> parabrachial nucleus –> hypothalamus
- Taste affect (how taste makes us feel) NTS –> parabrachial nucleus –> amygdala
What are the cranial nerves that carry taste, which region(s) do they carry and where are their cell bodies located?
All cranial nerves carry taste from taste receptors to NTS. 1. Facial nerve (CN7) carries from taste buds in anterior 2/3 of tongue. Cell bodies are contained in geniculate ganglion 2. Glossopharyngeal nerve (CN9) carries taste sensation from posterior 1/3 tongue. Cell bodies are contained in petrosal ganglion (inferior glossopharyngeal ganglion) 3. Vagus nerve (CN10) carries taste fibres from epiglottis and upper oesophagus. Cell bodies are contained in the nodose ganglion (inferior vagal ganglion).
How does the taste of chilli get transmitted
Interestingly, via somatosensory fibres in trigeminal nerve (CN5)
What is the function of the VPM of the thalmaus
Relay for somatosensation, pain, temperature of the head. Taste is also relayed in the caudal end of the VPM
What 3 things contribute to taste experience
- Somatosensation (feeling of taste. Eg. oysters are slimy 2. chemosensation (actual taste) 3. Palatibility (whole sense of taste)
Describe 2 taste reflexes and their pathways
Gag/retching Reflex: NTS –> reticular formation (salivatory nuclei, orofacial motor neurons, trigeminal complex) - allow to expell something that is potentially toxic NTS –> parabrachial nucleus –> hypothalamus - increases salivation to ensure we have expelled remnants of potentially toxic food. These pathways can also result in facial expressions in babies and are conserved across species. For example with pleasant foods - it evokes a happy facial expression with tongue out.
What areas of the brain do smell and taste converge?
Orbitofrontal cortex, insula/operculum and cingulate gyrus.
