Taste and Smell

Tasting or smelling means detecting chemicals

  • Chemoreceptors are used for this purpose

    • Respond when specific chemicals bind to them

    • Chemoreception is the oldest and most widespread sense in the animal kingdom

      • Also used by unicellular organisms

        • Have contact chemoreception

          • Taste/gustation

        • Larger multicellular organisms also have distance chemoreceptors

          • Sense chemicals emitted from a distance

          • = olfaction/smell

            • Helps animals

              • Lure toward food, assessing food quality

              • Sense presence of other animals

In intraspecific communication, pheromones are released.

  • May trigger either

    • behavioural responses (releaser pheromones)

    • developmental changes (primer pheromones)

Difference between taste and smell

  • In insects and vertebrates

    • Taste and smell rely on receptors from separate gene families

    • Are expressed in different organs

    • Use separate brain centres for neuronal processing

Vertebrate taste

  • Taste receptors are found in taste buds in the mouth (except fishes, on body surface)

  • In mammals, the taste buds are found in the taste papillae

    • 3 types

      • Gustatory cells

        • Primary chemoreceptive cells

      • Sustentacular cells

        • Supporting and secreting cells

      • Basal cells

        • Involved in replacement of the other two types

Taste buds have a small pore, exposing the gustatory cells via tongue epithelium

  • Allows direct contact with the chemical compound

  • The gustatory cells are in direct synaptic contact with cranial nerves (VII, IX, X)

    • Take taste information to the brain for higher processing

Human tongue can distinguish five main taste sensations

  • Bitter

    • GPCRs

    • Signal transduction proceeds via second messengers (metabotropic)

  • Sour

    • Signal ionotropically (via ion channels)

  • Salty

    • Signal ionotropically

  • Sweet

    • GPCRs, metabotropic

    • More sensitive for sucrose than fructose. Glucose is least sweet

  • Umami (glutamate)

    • GPCRs, metabotropic

  • (possibly also fat)

Each gustatory cell only expresses receptors for one taste modality

  • Is therefore specifically devoted to detect e.g. sugar, salt, or bitter compounds

  • Every taste bud contains cells for most/all of the different tastants and their signalling to afferent neurons appears to be at least partly unspecific, suggesting that the lines are not “perfectly labeled”

Invertebrate taste

  • Receptors sensitive to either amino acids, sugar, nucleotides, salts, or bitter compounds

  • Insects have gustatory receptors (GRs) in mouthpart + other parts of the body

    • Receptive ends of taste receptor neurons are usually located on a raised peg/hair-like structure with a small pore at the tip

      • Come into contact with the chemical

Smell

  • Highly developed in both mammals and arthropods

Olfactory receptor neurons

  • ORNs

  • Consists of peripherally located cell body, a dendrite with fine, cilia-like projections reaching to the surface or opening, and an axon that projects to the CNS.

Insect ORNs are located in sensilla, usually on the antennae

  • Each sensillum has 2-4 ORNs in most species (bees 30 ORNs)

  • Have numerous very small pores which odorant molecules can enter

  • Some ORs are generalists and respond to a broad range of odorants

  • Some ORs only respond to a single chemical component, e.g. sex pheromones (specialists)

In vertebrates, the olfactory receptor neurons are embedded in the olfactory epithelium, and the long ciliary dendrites are embedded in a layer of mucus.

In terrestrial vertebrates, the olfactory epithelium lies exposed to the air-filled nasal cavity where odour molecules arrive via the nose. Odour molecules bind to specific odorant receptor (OR) proteins on the dendrites of the ORNs, where the chemical signal is transduced and cause a change in receptor potential. Transduction in vertebrate ORs is a complex process involving G-proteins and second messengers, whereas insect ORs signal ionotropically (perhaps in combination with metabotropic pathways). In both insects and verebrates, the ORN dendrites are located in an aqueous environment (mucus or sensillar lymph). Since most volatile odourants are hydrophobic, mechnisms to solubilize and transport odour molecules to the receptors have evolved. Solubilization and transport are achieved by odorant binding proteins, which are highly abundant in insect olfactory sensilla and in the mucus of the vertebrate olfactory epithelium.

Morphology in vertebrate noses varies

  • Fishes have highly sensitive noses

    • Olfactory receptor neurons are recessed in the nasal sacs

  • In tetrapod vertebrates, the small external naris (nostril) provides access to each enlarged nasal passage

    • The back of the nasal passage opens into the mouth via the internal naris.

      • In amphibians, the nasal sack enlarges and a short recess (vomeronasal/Jacob’s organ) is formed

  • Birds and reptiles generally not rely on the sense of smell so much

    • Nasal sacs becomes differentiated into the anterior vestibule

  • Most mammals rely mostly on olfaction

    • Even humans have 12 million ORNs in their noses

      • Each neuron expresses only one OR gene

        • In vertebrates, the OR is a GPCR

    • Have large nasal chamber that ensures the air is warmed and moistened before reaching the lungs