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Olfaction and Taste: The Chemical Senses

Introduction

  • Chemical senses include both taste and olfaction, essential for evaluating substances ingested in food and air.

Transduction by Sensory Systems

  • Mechanical Senses: Transduce mechanical energy into nerve impulses.

    • Includes somatosensation and hearing.

  • Chemical Senses: Transduce energy from chemical reactions into nerve impulses.

    • Includes nociception (pain), taste (gustation), and olfaction (smell).

  • Photosenses: Transduce electromagnetic energy into nerve impulses, associated with vision.

Overview of Chemical Senses

  • Taste and olfactory senses evaluate qualities of substances:

    • Assess the food and drink consumed.

    • Evaluate the air breathed into lungs.

  • Taste: Arises from taste buds on the tongue that contact substances directly.

  • Olfaction: Arises from sensory receptors in the nasal cavity that detect aerosolized molecules from the air.

Olfactory Processes

  • Inhalation and Olfaction:

    • As we breathe in, aerosolized molecules are drawn into the nasal cavity.

    • Air passes over the olfactory epithelium, the sensory organ for smell.

  • Binding of Odor Molecules:

    • Odor molecules bind to receptors on cilia extending from olfactory epithelium, which are the dendrites of bipolar olfactory sensory neurons.

  • Types of Olfaction:

    • Orthonasal Olfaction: Sensing odor molecules from external air during inhalation.

    • Retronasal Olfaction: Sensing odor molecules from the mouth during exhalation.

Olfactory Chemotransduction

  • Mediated by G-protein-coupled receptors in the cilia of olfactory sensory neurons.

  • Terminology: Olfactory sensory neurons can be referred to as 'olfactory receptors,' but differentiate from G-proteins, though both are sometimes called 'olfactory receptors.'

Olfactory Receptor Gene Family

  • Approximately 1000 unique olfactory receptor genes account for about 3% of all human genes.

    • Only about 400 of these genes are functional in humans.

  • Each gene codes for a unique receptor binding various odor molecules; most odor molecules bind to multiple receptors.

  • All olfactory receptors are G-protein coupled receptors.

  • Nobel Laureates: Richard Axel and Linda Buck were awarded the Nobel Prize in Physiology & Medicine in 2004 for their work on olfactory receptors.

Olfactory Sensory Neurons

  • Coated with olfactory sensory neurons in the olfactory epithelium.

  • Humans possess about 40 million olfactory sensory neurons; some dog breeds have as many as 220 million.

  • Each sensory neuron expresses one type of olfactory receptor.

  • Neurons expressing the same receptor are clustered in the olfactory epithelium.

Signal Processing in the Olfactory Bulb

  • Olfactory neurons send axons to glomeruli of the olfactory bulb, synapsing with mitral cells.

  • Diagram detailing axon convergence of mouse olfactory sensory neurons expressing the same receptor.

Regeneration of Olfactory Receptors

  • Both taste and olfactory receptors can regenerate throughout life, contrasting with most neurons.

  • Replacement cells originate from stem cells in taste buds and olfactory epithelium, dividing to form new sensory neurons.

  • Axon Guidance: New olfactory receptor cells send their axons to a specific glomerulus, guided by chemical signals from olfactory ensheathing cells (OECs).

    • OECs promote axon growth and can be transplanted to help spinal cord axon regrowth.

Odor Coding

  • Identifiable odors consist of unique combinations of atoms/molecules.

  • Each unique chemical composition activates a specific pattern of olfactory sensory neurons and glomeruli.

  • Example: A specific odor may activate two glomeruli, creating a unique brain activity pattern.

  • Research estimates that humans can discriminate over one trillion odors despite limited olfactory capabilities (Bushdid et al., 2014).

Sparse vs Distributed Coding

  • Neural representation can use sparse or distributed coding schemes:

    • Sparse Coding:

    • Information represented by a small percentage of activated neurons.

    • Advantages: easier to decode.

    • Disadvantages: limited capacity.

    • Distributed Coding:

    • Information represented by a larger percentage of activated neurons.

    • Advantages: increased capacity due to numerous activation patterns.

    • Disadvantages: harder to decode.

Population Coding of Odor Identity

  • Specific odors activate unique patterns of glomeruli in the olfactory bulb.

  • Distributed coding allows for discrimination between different odors (Rubin & Katz, 1999).

Olfactory Pathways

  • First Order Neurons: Olfactory sensory neurons project to the second-order mitral cells in the olfactory bulb.

  • Second Order Neurons: Mitral cells relay information to third-order targets.

    • Targets Include:

    • Prepyriform cortex (primary olfactory cortex): responsible for odor discrimination (does not pass through thalamus).

    • Amygdala: assigns emotional valence to odors.

    • Hypothalamus: regulates homeostasis and hormonal responses to odors.

    • Medial dorsal thalamus: relays signals to other cortical areas including:

      • Insular cortex: integrates olfaction, taste, and somatosensation to encode flavors.

      • Orbitofrontal cortex: influences decisions and behaviors related to odors.

      • Entorhinal cortex and hippocampus: link odors to memories.

Vomeronasal Organ

  • Many vertebrates possess a vomeronasal organ for pheromone detection.

  • Humans are believed to lack a functional vomeronasal organ.

Taste Mechanism

  • Tastant: Any substance capable of being tasted.

  • When a tastant contacts the tongue, it dissolves in saliva and enters grooves surrounding papillae (tiny bumps on the tongue).

  • Each papilla contains taste buds made of taste receptor cells, with taste receptors located in the taste bud sac.

  • The taste pore allows tastants in saliva to flow into taste buds.

The Five Basic Tastes

  • Salty: Specialized for sodium detection.

  • Sour: Specialized for detecting acids.

  • Sweet: Specialized for sugar detection.

  • Bitter: Sensitive to noxious and toxic compounds.

  • Umami: Detects savory flavors, particularly proteins.

  • Common misconception: Different parts of the tongue do not taste different; multiple taste cells exist within a single taste bud for all taste types.

Taste Cell Types and Mechanisms

  • Salty Taste Cells: Express sodium channel ENaC which allows sodium influx to depolarize the cell.

  • Sour Taste Cells: Express PKD2L1 which functions as a cation channel gated by protons in acids (ionotropic).

  • Umami Taste Cells: Utilize metabotropic glutamate receptors and a G-protein receptor made of T1R1 and T1R3 subunits.

  • Sweet Taste Cells: Have a G-protein receptor composed of T1R2 and T1R3 subunits.

  • Bitter Taste Cells: Utilize G-protein receptors associated with approximately 30 members of the T2R family.

Taste Pathways

  • Taste cells release neurotransmitters (ATP and potentially glutamate) onto afferent cranial nerves.

  • Second-order taste neurons travel to the brain via the:

    • Facial nerve (VII)

    • Glossopharyngeal nerve (IX)

    • Vagus nerve (X)

  • Second-order neurons synapse onto third-order neurons in cranial nuclei like the nucleus of the solitary tract in the medulla.

  • Third-order neurons project to the ventral posterior medial (VPM) nucleus of the thalamus.

  • VPM projects to gustatory cortex, which consists of two main regions:

    • Anterior insula.

    • Frontal operculum.

Flavor Experience

  • The gustatory cortex detects the palatability of foods integrating multiple sensory experiences: taste, odor, texture, temperature, and visual appearance.

  • Internal states (like hunger and nausea) also influence flavor perceptions.

  • The insular cortex coordinates external and internal states and forwards this information to decision-making and memory regions (amygdala, hippocampus, orbitofrontal cortex, striatum, and midbrain dopamine system).

Motivational Valence of Tastes

  • Sweet tastes are innately perceived as pleasant; bitter tastes are perceived as unpleasant.

  • Uncertainty regarding whether sweet and bitter receptors project to pleasure centers differentially.

Genetically Engineered Taste Cells

  • Synthetic opiate peptide spiradoline is tasteless to normal mice.

  • Taste cells can be genetically modified to respond to spiradoline via transfection with the RASSL receptor.

    • Bitter cells made responsive via T2R19 promoter.

    • Sweet cells made responsive via T1R2 promoter.

Hardwired Valences in Mice

  • Mice with genetically modified taste cells underwent a taste preference test.

  • Normal mice showed no preference for spiradoline water; however, modified sweet and bitter cells displayed respective preferences.

  • Suggests taste cells are inherently wired to pleasure or discomfort centers.