Taste and Olfaction

Taste

• Concept of Flavour (not just taste)

• FLAVOUR INCLUDES:

  • Taste

  • Smell

  • Other nerve endings in the mouth

What are the tastes we perceive?

• Sweet

• Salt

• Sour

• Bitter

• Umami (monosodium glutamate -MSG-savory)

How tastes are detected on the tongue?

• Circumvallate papillae/cranial nerve IX

• Foliate papilla

• Fungiform papillae/cranial nerve VII

• Epiglottis/cranial nerve X

Taste Bud

• Taste molecules must dissolve in saliva

• Individual receptor cells are chemoreceptors (Life span 10 days, renewable)

• Different tastants are transduced by different kinds of channels (K+, Na+, second messengers):

  • Salt: Na+ influx;

  • Sour: H+ blocks K+ channel

  • Sweet: glucose or similar chemicals activates G-protein –cAMP – phosphorylation and blockage of K+

  • Bitter: more diverse tastants (e.g. alkaloids) – G-protein gustducin – inhibit phosphodiesterase – increase cAMP

  • Umami: triggered by amino acid, especially glutamate – G-protein – IP3 – Ca2+ release from internal store – activation of a monovalent-selective cation channel.

Type II Cell

• Bitter, sweet, umami

• T1R2/T1R3 or T2R (GPCR)

• Increase cAMP

• K+ channel closes → depolarization

• CALHM1/3 releases ATP

• P2X2/P2X3 (purinergic receptors)

• Na+/Ca2+ influx → afferent excitation

Type III Cell

• Sour + some salt

• Sour: PKD2L1 (acid sensitive) → K+ channel closes → depolarization

• Salt: ENaC (amiloride sensitive) → Na+ influx → depolarization

• Serotonin (5-HT) ± GABA, ATP release via vesicular exocytosis

• 5-HT3 receptors

• Na+/Ca2+ influx → afferent excitation

Type I Cell

• Supportive (glial-like)

• No direct taste transduction

• Regulate extracellular milieu (e.g., K+ via Kir channels)

Cranial Nerves Conveying Taste

• VII: ant. 2/3 of tongue, there may also be taste buds on the palate (roof of mouth)

• IX: posterior 1/3 of tongue

• X: a minor contribution, especially posteriorly around epiglottis

Labelled-line model of taste:

• Implies that the activity in one neuron type is both necessary and sufficient to represent a given sensory modality

• Taste receptor cells are generally tuned to a single taste modality, but their outputs converge onto shared gustatory afferent fibers, so individual taste sensory fibers carry mixed taste information

Across-fibre model 2 of taste:

Proposes that the pattern of responses to a particular stimulus across all fibers is the central feature of coding

Response of Individual fibers to Different Tastants:

Patterns of responses of sensory neurons in a population help discriminate tastes:

• Not just connectivity (topography) but activity patterns within the population that help discriminate tastes

Coding of Taste – Population Coding

• Not just connectivity (topography) but activity patterns within the population that help discriminate tastes

• Taste receptor cells are generally tuned to a single taste modality, but their outputs converge onto shared gustatory afferent fibers, so individual taste sensory fibers carry mixed taste information

• Patterns of neuronal activity appear to be more selective to tastes at ‘higher’ levels

Ageusia

no ability to taste

Dysgeusia

things taste odd (often bad)

Hypogeusia

diminished ability to taste

Clinical correlates of taste:

• Trauma (CN VII and IX injury)

• Chemotherapy (hypogeusia or dysgeusia)

• Radiation therapy (damage to renewing taste receptor cells)

• Decreased salivation (tastants must dissolve in saliva)

Olfaction

• Sense of smell

• Number of human olfactory receptor cells = 5 million

• Number of dog olfactory receptor cells = 4 billion

• Surface area of olfactory epithelium in humans = 10 cm²

• Area of olfactory epithelium in some dogs = 170 cm²

Medical Doctors can utilize the sense of smell for diagnoses:

• Infection after surgery from bacteria may smell like a wine cellar (musty)

• Breath of a diabetic in coma smells like apples

• Garlic smell can indicate arsenic poisoning

Location of the olfactory epithelium:

Below the olfactory bulb and cribriform plate

• Olfactory receptor cells are primary sensory neurons that generate action potentials

Olfactory receptor neuron (ORN)

• Olfactory receptor cells (neurons) have a life span of 30-60 days!

• They fire APs → odor is first detected at cilia of olfactory receptor neuron

• Use vesicular glutamate release at synapses,

• Target well-defined second-order neurons in the CNS

Signal Transduction

Odorant → Golf –→ cAMP ↑ → open cyclic nucleotide-gated channels (CNG channels)

Odor Adaptation

The Ca2+ signal controls both excitation and adaptation:

• Ca2+ -CAM binds to the CNG channel, reducing its affinity for CAMP

• The extrusion of Ca2+ through the activation of Na+ /Ca2+ exchange proteins

• Longer-lasting adaptation mechanisms (CO/cGMP, NO etc)

• At receptor level → current detection in individual receptor cell is transient despite the continued presence of the odor

Coding at the ORN - Glomeruli

2004 Nobel Prize in Physiology or Medicine to Linda Buck and Richard Axel for:

• Their discoveries of odorant receptors and the organization of the olfactory system

  • 1 receptor – 1 molecular feature

  • 1 receptor – multiple odors sharing the same molecular feature

  • All receptors encoding the same molecular features – 1 glomerulus

  • 1 odor activates several glomeruli

  • Different odors activate distinct patterns of glomeruli

  • Distinct patterns of glomeruli carry distinct odor information to the cortex (chemotopic odor map – basis to distinguish ~10,000 different odors)

• Discovered a large gene family → 1000 genes (3% of our genes!) that give rise to an equivalent number of odorant receptors

• Individual odor receptors expressing the same gene are dispersed throughout the olfactory epithelium but the axons of the receptors converge on 1 or 2 glomeruli in the olfactory bulb.

Olfactory Bulb Organization

• In the olfactory bulb - glomeruli - these contain the 1st order neuron axons which carry information about one particular component of scent(s)

• Separate components of an odour are separated into different glomeruli

• These neurons then synapse on mitral cells --> refines the smell sense and relays it to the brain for further processing

• Info goes from OB →

1. Cortical/subcortical route

• Olfactory receptor neurons → olfactory bulb → lateral olfactory tract → primary olfactory areas: perception, discrimination, emotion, memory

2. Thalamo-cortical

• Olfactory bulb → piriform cortex → mediodorsal thalamus → orbitofrontal cortex (OFC): conscious odor evaluation, multisensory integration, flavour

Orbitofrontal Cortex

• integration of odour, taste, and reward

• flavour perception

Primary Olfactory Cortex (Piriform Cortex)

• conscious perception of odour

• odour identification and discrimination

Amygdala

emotional and motivational aspects of odour

Hypothalamus

Autonomic and behavioural responses (e.g., feeding)

Entorhinal Cortex/Hippocampus

• odour memory

• contextual associations

How does the brain discriminate odors?

• Olfaction also follows “population coding” as in taste

• Activity patterns within the population that help discriminate tastes

Anosmia

near total loss of sense of smell

Hyposmia

diminished ability to smell many or some odors

Dysosmia

distorted sense of smell to some odors

Clinical correlates of olfaction:

• Car accidents (head trauma) often lead to anosmia

• Nasal polyps. Anosmia may be cured by surgery.

• Flu/cold. Very often leads to short-lived anosmia

• Unknown (idiopathic)

• Brain tumor in temporal lobe. Rare, but can cause dysosmias.

• Progressive diseases such as Alzheimer’s and Parkinson’s often decreases ability to detect and discriminate odors.

• Covid-19 associated anosmia (often acute)

Vomeronasal organ (VNO)–accessory olfactory bulb pathways (pheromone detection):

• Much more restricted (amygdala & hypothalamus) than the main olfactory system.

• Vomeronasal/accessory bulb pathways in mammals are sexually dimorphic and likely involved in mating preferences.

• Evidence for existence of VNO in humans is not so clear....but sexually dimorphic nuclei are present in olfactory related structures.

How does the olfactory system interact with the immune system?

• There is evidence in animals that the immune system, via major histocompatibility (MHC) molecules, can leave a distinct odor in urine that can be detected by mice.

• The mice will more likely mate with distantly related mice rather than closely related mice if confronted with multiple potential mates. This is a means to prevent the defects associated with in-breeding.

• Could a similar unconscious mating scheme be working in humans? There is evidence that this is the case.

  • 1995 Claus Wedekind “sweaty T-shirts” study