Lecture 4: Gustation

Question 1

What are papillae and where are they located?

Answer 1

Papillae contain sensory receptors that transduce taste - they cover the surface of the tongue, as well as around epiglottis and oesophagus

Question 2

What is the function of gustation

Answer 2

To draw you away from things that are toxic and to draw you to foods that are nutritive, beneficial or pleasant.

Question 3

Where are tastes distributed?

Answer 3

Taste is distributed throughout the whole oral cavity, there are not particular areas sensitive to a particular taste

Question 4

Describe the structure of a taste bud and how they work

Answer 4

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 -

Question 5

Which sensory cells are able to regenerate and from which cells?

Answer 5

Olfactory receptors and taste receptors are able to regenerate from basal cells.

Question 6

Why does our sense of taste reduce when we age if we are able to regenerate new taste cells

Answer 6

Due to the reduction in the ability of basal cells to produce new taste cells

Question 7

What is the function of saliva?

Answer 7

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.

Question 8

Describe the taste spectrum

Answer 8

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

Question 9

Describe how taste cells respond to saltiness and draw a diagram of this process

Answer 9

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.

Question 10

Describe how taste cells respond to sourness and draw a diagram of this process

Answer 10

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.

Question 11

Describe how taste cells respond to bitter, sweet or umami and draw a diagram of this process

Answer 11

1. 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

Question 12

What is the main difference between taste cells that respond to saltness or sourness and taste cells that respond to bitter, sweet or umami?

Answer 12

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

Question 13

How many receptor subtypes are there for bitterness and why? How many subunits within a receptor?

Answer 13

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

Question 14

Describe the structure of the sweet receptor

Answer 14

Has only one type of receptor containing 2 subunits which are require to bind to sugars.

Question 15

How many subunits do umami receptors have

Answer 15

2 subunits

Question 16

Describe the gustatory pathway from taste receptors

Answer 16

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

1. Taste perception

NTS –> parabrachial nucleus –> caudal VPM of thalamus –> primary gustatory cortex (insula-operculum)

2. Taste reflexes

NTS –> reticular formation (salivatory nuclei, orofacial motor neurons, trigeminal complex)

NTS –> parabrachial nucleus –> hypothalamus

3. Taste affect (how taste makes us feel) NTS –> parabrachial nucleus –> amygdala

Question 17

What are the cranial nerves that carry taste, which region(s) do they carry and where are their cell bodies located?

Answer 17

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).

Question 18

How does the taste of chilli get transmitted

Answer 18

Interestingly, via somatosensory fibres in trigeminal nerve (CN5)

Question 19

What is the function of the VPM of the thalmaus

Answer 19

Relay for somatosensation, pain, temperature of the head. Taste is also relayed in the caudal end of the VPM

Question 20

What 3 things contribute to taste experience

Answer 20

1. Somatosensation (feeling of taste. Eg. oysters are slimy 2. chemosensation (actual taste) 3. Palatibility (whole sense of taste)

Question 21

Describe 2 taste reflexes and their pathways

Answer 21

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.

Question 22

What areas of the brain do smell and taste converge?

Answer 22

Orbitofrontal cortex, insula/operculum and cingulate gyrus.