Taste and Smell

Question 1

What is the function of taste?

Answer 1

Taste has an appetitive role identifying major nutrients contained in the food being ingested. A second major function of taste is to recognise food that may potentially be harmful so that ingestion can be avoided/limited.

Question 2

What is signalled by a sweet taste?

Answer 2

Sugar content, simple sugars such as sucrose, glucose and fructose taste sweet. This signals that it is high in carbohydrates and therefore has a high energy content.

Question 3

What is signalled by umami taste?

Answer 3

Amino acids, especially glutamate, which is commonly produced during the hydrolysis of proteins. Strong in meat, aged cheese, as well as some fruits such as tomatoes. Umami is an appetitive signal of the protein content of food, which helps to maintain the level of protein intake.

Question 4

What is signalled by a bitter taste?

Answer 4

Plants often produce toxins which deter animals from eating them by stimulating a bitter taste. This does not mean they are always rejected due to butter taste however, as animals can learn to like and ingest bitter tasting substances in limited quantities or if they are not that harmful.

Question 5

What is signalled by a salty taste?

Answer 5

Sodium chloride, which is vital for maintaining body fluid homeostasis. Small concentrations of salt are appetitive. Animals will actively lick at salt licks that have been provided, or seek out salty minerals to lick in the environment. Too much salt is bad for body fluid homeostasis, so not surprisingly high concentrations of salt are aversive and will be rejected.

Question 6

What is signalled by a sour taste?

Answer 6

Acidity. Generally averse but animals can develop preference for it, sourness can provide information about how ripe fruits are and therefore potential nutritive value for frugivores. But sour can also be a sign of bacterial metabolism and decay and something to be avoided.

Question 7

What is siganlled by a chalky taste?

Answer 7

Calcium receptor mechanisms and sensed via taste pathways, which could play a role in calcium homeostasis in certain species.

Question 8

What is signalled by a creamy/fatty taste?

Answer 8

Triglyceride receptors could be a possibility, but must be distinguished from the smooth, silky mouthfeel, also produced by fats via somatosensory system.

Question 9

What is signalled by a rancid taste?

Answer 9

Free fatty acids have also been found to stimulate taste sensation, but more often are associated with unpleasant tastes of rancidity.

Question 10

What are 2 tastes not mediated by the gustatory system?

Answer 10

Capsaicin is the molecule in chilli peppers that makes them hot.

Menthol in mint and toothpaste gives a colling sensation

Question 11

Explain capsaicin sensation.

Answer 11

It chemically stimulates the TrpV1 ion channel in the trigeminal somatosensory pathway that are normally opened by heat, which is why chillies generates the sensation of being hot.

Question 12

Explain menthol sensation.

Answer 12

Not due to its taste but due to menthol chemically stimulating the TrpM8 ion channels on cool sensing trigeminal somatosensory nerve endings.

Question 13

Explain drying, mouth puckering sensation.

Answer 13

Caused by tannins, a group of polyphenolic molecules produced by plants, such as those found in tea or red wine. May be due to stimulation of trigeminal somatosensory endings.

Question 14

Explain metallic sensation.

Answer 14

Could be galvanic reaction causing current flow, which could activate trigeminal somatosensory or gustatory sensory systems.

Question 15

Explain water taste.

Answer 15

Water taste first evidence is thought to be mediated by gustatory receptors, maybe associated with sour taste sensation. But requires more research. May be important to signal water consumption as a short term satiety signal for body fluid homeostasis.

Question 16

Where are taste receptor cells found?

Answer 16

• Within taste buds on the tongue
• In isolation in the soft palate, epiglottis, pharynx and throughout the GI tract epithelium

They signal digestion of foodstuffs, the presence of toxins and bacterial metabolism.

Question 17

Where are taste buds?

Answer 17

Taste buds are found within 3 of the 4 types of papillae on the tongue.

Question 18

Name and describe the 3 papillae on the tongue.

Answer 18

Fungiform papillae - small bumps in the middle and anterior regions of the tongue.

Foliate papillae - series of epithelial folds on the lateral edges at the back of the tongue.

Circumvallate papillae – back of the tongue. Circular cleft in which the taste buds are embedded in the walls.

Question 19

Name and describe the 4th papillae.

Answer 19

Filiform papillae - finger-like epithelial projections on eth surface of eth tongue, which do not possess taste buds, but do provide the rough texture of the tongue surface for manipulation of the food bolus.

Question 20

What is the cause of variation of the number of taste buds on the tonge?

Answer 20

Varies from a few hundred in carnivores that have a restricted, ,high quality and predictable diet, to tens of thousands of herbivores, such as ungulates that eat highly variable, low quality and potentially toxic foods.

Question 21

Describe the physiological structure of taste buds.

Answer 21

Bundles of a few to at most 150 or so taste receptor cells. They are specialised epithelial cells that communicate either directly or indirectly with gustatory afferent nerve fibres.

Question 22

How are taste buds joined?

Answer 22

Joined by tight junctions, which separate the apical fluid in the taste pore from the basolateral fluid in which synaptic transmission and action potential firing occur.

Question 23

Why is continual turnover necessary in taste buds?

Answer 23

Continual turnover of receptor cells in the taste bud, from basal cell mitosis, with about a 10 day cell cycle. This is necessary due to the exposure of eth taste receptor cells to damaging environmental stimuli and infection.

Question 24

What are type I and IV taste receptor cells?

Answer 24

4 types of taste receptor cell, but type I is thought to have a supporting role mainly. Do not need to know much about type IV receptor cells, as they are likely to be immature stages of development of the other receptor cell types.

Question 25

What does diffusion of tastants allow?

Answer 25

Diffusion of tastants dissolved in saliva into the taste pore allows them to activate transduction mechanism located in the cell membrane of microvilli in apical region of the type II and III taste receptor cells.

Question 26

What are type II taste receptor cells?

Answer 26

A family of 7-transmembrane domain, G-protein linked receptors, which signal via the alpha G-protein subunit gustducin activating phospholipase C, inositol trisphosphate signalling pathway leading to intracellular calcium ion release and extracellular signalling via ATP release.

Question 27

What activates type II taste receptor cells?

Answer 27

• Activated by sweet, umami or bitter tastants, so molecules and not ions.
• It is the receptor protein that is expressed by the cell that determines the stimuli that will activate it.
• Involve receptor based transduction mechanism

Question 28

How is bitter taste mediated in type II taste receptor cells?

Answer 28

Mediated by T2Rs ad as plants toxins have a wide variety of different molecular structures, this has led to a much larger family of bitter receptors than receptors that detect sweet and umami tastants.

Question 29

How are sweet and umami tastes mediated by type II taste receptors?

Answer 29

These require a receptor dimer to form the ligand binding site, if the cell expresses T1R1 and T1R3 then the dimer they form will respond to amino acids such as glutamate and result in umami sensation.

A combination of T1R2 and T1R3 expression results in a dimer that responds to sucrose, other sugars and saccharin at the extracellular binding site.

Question 30

Explain how genetics of taste sensitivity cause dogs to be more attracted to sweet tastes?

Answer 30

• Dogs have similar taste sensation to humans and rodents with T1R1, T1R2 and T1R3 receptor expression giving sweet and umami sensations.
• All cats are able to taste umami and not sweet.
• So cats and dogs eating chocolate will cause problems in both but dogs are attracted to the sweet taste so is more of a problem.

Question 31

Describe how type III taste receptor cells work.

Answer 31

• Detect ionic stimuli via gating of ion channels, which depolarises the receptor cells.
• For action potentials, which in turn lead to local calcium ion influx and release of eth neurotransmitter serotonin onto gustatory nerve afferents.

Question 32

How is salt taste mediated by type III taste receptor cells?

Answer 32

Salt taste transduction occurs simply by the influx of sodium ions down the electrochemical gradient via a non-gated sodium channel, which directly depolarises the cell.

Question 33

How is sour taste mediated by type III taste cell receptors?

Answer 33

Possess apical H+ channels and H+ flow down electrochemical gradient. Has a depolarising effect and causes intracellular acidification, closing ion channels. Inhibits potassium ion efflux, which depolarises the cell. Weakly dissociating acids can freely diffuse and then dissociate intracellularly.

Question 34

What are sub modalities of taste cells?

Answer 34

Each taste receptor cell is specific for 1 taste sub-modality with about 20% of cells in taste buds being type II cells and 5-15% type II cells. But stimulus specificity in the receptor cells is lost in the transmission of the information to the CNS. There are only a few that respond to a single sub-modality.

Question 35

How is gustatory information coded for?

Answer 35

Population coding can extract information about taste sub-modality from the non-selective responses of taste afferents. Although the firing of any single afferent is not selective, the pattern of response over a population of afferents provides specific information about the tastant. The CNS compares non-selective responses of many neurones rather than relying on small number of precisely coded neurones.

Question 36

Describe the innervation of the gustatory system.

Answer 36

• Anterior 2 thirds of the tongue in innervated by gustatory afferent from eth corda tympani branch of the facial nerve.
• Posterior third of the tongue is innervated by the gustatory afferents of the glossopharyngeal nerve.
• Oropharynx is innervated by gustatory afferent from the vagus nerve.

Question 37

Where do gustatory afferents project?

Answer 37

Nucleus of the solitary tract (NTS)

• Local brainstem circuits control autonomic digestive reflexes via adjacent salivary centres in the NTS
• Neurones in the NTS also project via the thalamus to somatosensory cortex and to the insular cortex for taste perception.
• The proportion of selective responses increases going from primary gustatory afferent to thalamus and finally cortex, which suggests a convergence of information at higher levels of the gustatory system.

Question 38

What are the innate responses to tastes?

Answer 38

• Taste with smell stimulates salivation via the salivary centres in the NTS
• Cephalic phase of digestion/digestive tract anticipates the arrival of food via taste receptors in the GI tract monitor passage of digesta and local control of digestive reflexes and gut motility.
• Sense of taste in acceptance of food with the swallowing reflex and the rejection of food via gag reflex.
• Behavioural attraction or avoidance, which involves hypothalamus and amygdala.
• Conscious perception of pleasantness or disgust mediated via the insular corte and cingulate cortex.

Question 39

What are learned responses to taste?

Answer 39

• Adaptive in a natural setting: herbivores in a poorly productive environment with bitter tasting toxin plants, animals have learnt to ingest small quantities of a variety of different toxic plants rather than avoiding them due to their taste.
• Rabbit kittens cam learn about food preferences to foods eaten by their mother during gestation.
• Flavour is a complex integration between olfactory and gustatory systems and involve information about temperature, spice and texture of food via the trigeminal somatosensory afferents as well as the motor systems controlling mastication.
• Food consumed days prior to illness can result in aversion to the food

Question 40

What are the functions of the olfactory system?

Answer 40

• Sensitivity of the olfactory system to food odours.
• Local navigating and migration.
• Warn of danger from environmental threats, such as fire, the presence of predators or even the levels of carbon dioxide and oxygen in burrow living rodents.
• Social communication, such as coordinating male and female sexual behaviour, mother-infant bonding, scent marking and territorial behaviour.

Question 41

Distinguish hyposmia and anosmia.

Answer 41

A reduction in olfactory sensitivity is known as hyposmia and the complete lack of smell known anosmia.

Question 42

How is hyposmia and anosmia caused?

Answer 42

These can arise due to infection or blockage. Hard to detect clinically so test by obscuring hidden treats, but be careful with which odour you choose – many pungent odours can elicit a refex withdrawal response via stimulation of pain afferents of the trigeminal system rather than a pure odour response via the olfactory system.

Question 43

Describe sniffing.

Answer 43

• Little airflow reaches the main olfactory epithelium.
• Exploratory behaviour
• Rapid, small movements of air causing turbulence in airflow, which dramatically increases the proportion of air reaching the main olfactory epithelium.
• Rapid sniffing may help to localise odour sources as the nose scans across odour plumes/trails and by adaption to reduce sensitivity to background odours.

Question 44

Define odour.

Answer 44

The perception of a smell and is caused by 1 or a complex mixture of many molecules that have an odour, called odorants.

Question 45

Define odorants.

Answer 45

Sensed by specialised olfactory sensory neurones in the main olfactory epithelium.

Question 46

Describe olfactory sensory neurones.

Answer 46

• Olfactory sensory neurones are exposed to damaging environmental stimuli so only live for about 30 days, continually being replaced by division of basal stem calls.
• They are unique in being the only neurones that have axons that can grow into and form synapses in the mature CNS.

Question 47

Describe olfactory epithelium.

Answer 47

Covered in a thin layer of mucus in which odorants dissolve and diffuse to non-motile cilia on olfactory sensory neurones, which project into the mucus layer and increase the surface area for odorant binding.

Question 48

Describe olfactory transduction.

Answer 48

• When an odorant activates receptor proteins on the cilia, it lead to olfactory transduction which generates a depolarising receptor potential.
• This graded depolarisation is propagated along the dendrite to depolarise the cell soma and increase action potential firing rate.
• The larger the receptor potential, the more rapid the membrane in the soma is depolarised and the higher the frequency of firing.

Question 49

Where are olfactory action potentials projected?

Answer 49

Olfactory bulb

1. Olfactory receptors that bind to the odorants are 7-trasnmembrane domain
2. Coupled by G[olf], which activated adenylyl cyclase.
3. cAMP generated, which directly opens a cyclic nucleotide gated, non-selective cation channel in the cell membrane.
4. Influx of sodium and calcium ions, depolarising the membrane and Cl- channels opening.
5. Different in olfaction, sensory neurones actively accumulate Cl- ions.
6. Electrochemical gradient for efflux of the negatively charged Cl- ions, further depolarising the membrane potential to enhance the signal.

Question 50

What are olfactory receptor repertoires?

Answer 50

Olfactory receptors evolve rapidly, by gene duplication and divergence and there are large species differences in their number of different olfactory receptor variants. The great the number of olfactory receptors, the greater the olfactory acuity.

Question 51

What are the species differences in olfactory receptor repertoires?

Answer 51

• Macrosmatic species have highly developed since of smell, with around 1000 or more different proteins that can take up as much as 1% of the coding region of their genome, such as rodents, dogs and opossums.
• Less in primates, due to increased importance of visual and auditory communication, around 400 in humans.
• Birds heavily rely on vision and so chickens have 40 olfactory receptors.

Question 52

What is the main olfactory system?

Answer 52

Main olfactory system is unusual in sensory system in that the OSNs project directly to a simple cortical area of the brain, the main olfactory bulb, without relaying in the thalamus.

Question 53

Describe the neurological pathway of olfactory sensory neurones.

Answer 53

1. OSNs expressing the same olfactory receptor variant converge their axons to input to mitral cell projection neurones in the main olfactory bulb, which represent information about the odorant features.
2. Conveyed to the piriform cortex, directly and via the anterior olfactory nucleus. Here it is able to learn to recognise the pattern of mitral cell activation as belonging to a single odour/object.
3. Piriform cortex is densely interconnected with the orbitofrontal cortex which forms the multimodal perception of flavour.
4. Projection from the piriform cortex, along with direct projections from the main olfactory bulb, convey olfactory information to the perirhinal/enterohinal cortex and on to the hippocampus, which puts the odour into the context of space and time.

Question 54

What are social odours?

Answer 54

Social odours and pheromonal information follows a different pathway via the amygdala to regions, including the hypothalamus to elicit integrated, behavioural endocrine and autonomic responses to social odours.

Question 55

What is an example of innate olfactory response in mice?

Answer 55

The trace amine associated receptor TAAR 4 mediates innate aversion to predator urine in mice.

Question 56

What is an example of innate olfactory response in burrowing rodents?

Answer 56

OSNs in the main olfactory epithelium of mice have been found to be stimulated by CO2 and oxygen, odourless to humans. So due to burrow behaviour, high CO2 levels are aversive for mice and low oxygen levels elicit an immediate escape response. So cannot humanely kill rodents by displacement of air with inert gasses.

Question 57

What is a pheromone?

Answer 57

Secreted into the environment and sensed by another individual of the same species.

Question 58

What was the first discovered pheromone?

Answer 58

Bombykol, which is produced and released by the female silk moth. It is a sexual attractant pheromone that attract males across large distances. Individual sensory neurones in the male antennae show an electrical response to a single molecule of bombykol and it has been estimates that only 5 molecules need to be detected to elicit a full behavioural response. This response is for the male silk moth to switch it behaviour to fly upwind, up the pheromone plume, to locate female from as far as 2km away.

Question 59

Name the rabbit mammary pheromone.

Answer 59

2-mehylbut-2-enal

Question 60

Describe the effects of the rabbit mammary pheromone.

Answer 60

• Rabbit gives birth to their kittens in a separate nursery burrow.
• They bury their kittens immediately after birth before leaving the burrow and closing the entrance.
• They return 24 hours later and nurse the kittens for 3-5 minutes in which the kittens have to suckle the milk that will last them for the next 24 hours until their mother returns.
• Whole process depends on the kittens locating their mothers nipples quickly.
• But kittens are born at altricial stage of development, so are blind and deaf, with poor coordination. But they do possess a fully functional olfactory system at birth.
• The mother produces a mammary pheromone form the skin around her nipples, which has dual effect both to arise the kittens and to elicit a stereotyped nipple search behaviour.

Question 61

Describe the effect of the boar sexual attractant pheromone.

Answer 61

• In the presence of a sow, boar will salivate profusely and chomp at the mouth, which creates a foam that help to disperse their airborne sex attractant pheromones, androstenone and androstenol, which are secreted by the salivary glands.
• If the sow is in a receptive hormonal state, she will respond to the adnrosternone and androstenol by being attracted to the boar and showing lordosis behaviour – the female mating posture.
• Commercial application for AI in boar spray product at the snot of the sow to see if she is receptive – she will stand and not move away if you apply pressure to her hindquarters, signalling optimal timing for AI.

Question 62

Describe the ram effect pheromone.

Answer 62

• Caused by testosterone dependent odours from their fleece, which are sensed by main olfactory system.
• Exposure to vasectomised ram that infertile but still producing testosterone can be used to induce and synchronise oestrous in a flock of ewes, so lambs are born in a short and predictable period.
• In the ewes, induces immediate rise in LH release form the anterior pituitary, with the first LH pulse observed within 1- minutes of male odour exposure.

Question 63

Name and describe the effect of mouse alarm pheromone.

Answer 63

Thiazole.

Elicits behaviour and freezing behaviour typical of a fear response. Pheromonal effect is mediated by the Gruneberg ganglion, a cluster of chemosensory cells near the tip of the nasal cavity.

Question 64

Describe the structure of the vomeronasal organ.

Answer 64

The vomeronasal organ in a blin-ended tubular structure that is connected to both the oral and nasal cavities via the vomeronasal duct. Unlike the main olfactory epithelium, it is not exposed to airflow and relies on a pumping mechanism to move stimuli in and out of the organ in the liquid phase.

Question 65

Describe the vomeronasal organ in rodents.

Answer 65

Is a CVS pumping mechanism, activated in situations of novelty and exploration. Increased blood flow to the large longitudinal blood vessel decreases the size of the lumen, pumping fluid out of the organ. Reduction of blood flow to this vessel increases the size of the lumen moving fluid borne stimuli into the organ.

Question 66

Describe flehmen behaviour in other species.

Answer 66

• Baring the teeth and curling of the upper lip, changing the pressure of the organ, directly pumping stimuli in and out.
• Commonly seen in horses and bovids, also observed in some carnivores.
• Behaviour occurs after direct anogenital investigation of another [cat] which enables sensing of non-volatile stimuli, such as proteins and peptides by the vomeronasal organ.

Question 67

What are the 2 main classes of vomeronasal receptors?

Answer 67

Mice have around 187 functional membranes of the first class. These are located in the apical region of the vomeronasal epithelium and respond to small volatile molecules carried in solution.

Mice have 114 of the second class, located more deeply to the epithelium and respond to peptides and proteins. Around 5 members of the formyl peptide receptors family, scattered through the sensory epithelium and respond to stimuli associated with the immune respond to pathogens.

Question 68

What is the Lee-Boot effect?

Answer 68

Effect of oestrogen dependent urinary pheromones to lengthen oestrous cycles and suppress cyclicity in grouped female mice.

Question 69

What is the Whitten effect?

Answer 69

Effect of male urinary pheromones to induce and synchronise oestrous cycles in female mice.

Question 70

What is the Vandenbergh effect?

Answer 70

Acceleration of puberty in response to male urinary pheromones (similar to basis in Whitten effect).

Question 71

How is aggressive behaviour in mice linked to the vomeronasal organ?

Answer 71

Both forms of aggression (male-male and female protecting litter) eliminated by removing the vomeronasal organ.

Aggression can be triggered by MUP, major urinary protein, or by testosterone dependent volatile ligands bound to MUP.

Question 72

What is MUP?

Answer 72

• From a family called lipocalins
• Hydrophobic cavity that can bind to testosterone dependent urinary volatile, which induce male aggression.
• Cows and dogs only produce a single MUP variant. Mice and rats have larger families of MUP variants due to expansion of vomeronasal function.

Question 73

Describe the effects of MUPs on behaviour.

Answer 73

• Territorial markers are vital for male territorial behaviour, which in turn determines female mate preference for the male who can maintain a resource rich territory and defend it from competing males.
• MUP20 elicits aggression and countermarking behaviour.
• MUPs provide a reservoir for their bound volatile ligands that are slowly released for the urine marks.
• Volatiles attract attention of other males and females and the intensity of the volatiles indicates its freshness.

Question 74

Distinguish vomeronasal sensory neurone and olfactory sensory neurone responses.

Answer 74

VSN has higher sensitivity and so pheromonal signals are normally present at very low concentrations in the environment.

VSNs appear to be higher selective for individual molecules with no overlap.

Question 75

Describe the receptor repertoire of vomeronasal neurones in fish.

Answer 75

Evolved in the ancestor of fish and both V1r and V2r classes are expressed in VSNs in the olfactory epithelium along with OSNs expressing main olfactory receptors. The large V2r repertoire of fish is not altogether surprising as peptides and proteins are soluble in water and easily gain access to the olfactory system.

Question 75

Describe the receptor repertoire of vomeronasal neurones in fish.

Answer 75

Evolved in the ancestor of fish and both V1r and V2r classes are expressed in VSNs in the olfactory epithelium along with OSNs expressing main olfactory receptors. The large V2r repertoire of fish is not altogether surprising as peptides and proteins are soluble in water and easily gain access to the olfactory system.

Question 76

Describe the receptor repertoire of vomeronasal neurones in terrestrial species.

Answer 76

Adaptation to the terrestrial environment to the common ancestor of terrestrial vertebrates segregating the VSNs and OSNs.

Question 77

Describe the receptor repertoire of vomeronasal neurones in amphibians.

Answer 77

Amphibians have a water nose that contains VSNs to detect involatile but soluble chemicals and an air nose with OSNs to detect volatile airborne chemicals.

Question 78

Describe the receptor repertoire of vomeronasal neurones in reptiles.

Answer 78

Reptiles, especially squamates, have evolved a specialised vomeronasal organ. Snakes is far better developed and important than their main olfactory system. Predominantly V2r class.

Question 79

Describe the receptor repertoire of vomeronasal neurones in mammals.

Answer 79

Mammalian VSNs located in a specialised vomeronasal organ but unlike other vertebrates there has been ab expansion of the V1r class. In some species, like cows, sheep and dogs, there are no functional V2rs, so some mammals have lost the ability to detect proteins and peptides directly, such as MUPS.

Question 80

Describe the receptor repertoire of vomeronasal neurones in rodents.

Answer 80

Rates, mice and opossums are exceptions to the evolutionary trend, perhaps due to high population densities and highly developed territorial behaviour.

Question 81

What are appeasement pheromones?

Answer 81

Treatments that are based on pheromonal effects are available but not necessarily effective. They are not classed drugs and do not need to pass the same rigorous trails of safety and efficacy.

Dog appeasement pheromone and Feliway are meant to provide a mixture of pheromones that mimic skin secretions from around the nipples of lactating females, which are thought to have a calming influence on their young. Evidence is unconvincing.