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Chemical Senses II - Olfaction

Introduction

  • Smell combines with taste to help identify foods & increases enjoyment of food

  • Can also warn against harmful substances or places

  • We can smell thousands of substances but only some of them smell good

  • Smell can also be used for communication

    • Pheromones are a signal for reproductive behaviors & identification of individuals

Organs of Smell

  • Do not smell w/nose, smell w/small thin sheet of cells in nasal cavity called olfactory epithelium

  • Olfactory epithelium has three cell types:

    • Olfactory receptor cells

      • Genuine neurons

      • Have axons that go into CNS

    • Supporting cells

      • Similar to glia

      • Produce mucus

    • Basal cells

      • Source of new receptor cells

        • Olfactory receptors regenerate every 4-8 weeks, one of very few neurons that do this

  • When you sniff only small percentage of air passes over olfactory epithelium

  • Epithelium contains thin coating of mucus

    • Flows constantly, replaced every 10 mins

    • Odorants dissolve in mucus layer before reaching receptors

    • Mucus contains antibodies that protects brain (and body) from viruses & bacteria

    • Odorant binding proteins: small & soluble molecules that help concentrate odorants in the brain

  • The bigger the olfactory epithelium & the more receptors there are, the more a species can smell

    • Humans are actually weak smellers

Olfactory Receptor Neurons

  • Have single thin dendrite that ends w/small knob @ surface of epithelium

    • Knob has small cilia

  • Odorants dissolve → bind to cilia → transduction pathway activates

  • On opposite side of olfactory receptor cell is thin unmyelinated axon

  • Collectively axons constitute olfactory nerve

    • Axons penetrate cribriform plate & course into olfactory bulb

  • Anosmia: inability to smell

Olfactory Transduction

  1. Odorants bind to membrane odorant receptor proteins

  2. G-protein stimulates

  3. Adenylyl cyclase activates

  4. cAMP forms

  5. cAMP binds to cyclic nucleotide gated cation channel

  6. Cation channel opens & allows influx of Na+ & Ca2+

  7. Open Ca2+ activated Cl- channels

  8. Cl- current flow & membrane depolarization (receptor potential)

  9. If reaches threshold will propagate action potential

  • Reasons olfactory response may terminate

    • Odorants diffuse away

    • Scavenger enzymes in mucus break odorants down

    • cAMP in receptor cell may activate other signaling pathways that end transduction process

  • In continuing presence of an odorant the strength of smell fades bc response of receptor cell adapts to odorant in a minute 

    • Adaptation ^ 

Olfactory Receptor Proteins

  • Receptor proteins have odorant binding sites on extracellular surface

  • Many diff types of odorant receptors

    • More than 1000 diff receptor genes in rodents (largest receptor genes in rodents yet discovered)

    • Humans have about 350

  • Each receptor gene has a unique structure allowing receptor proteins encoded by these genes to bind to different odorants

  • Olfactory receptor cells express few types of genes, often just one 

  • Olfactory epithelium is organized into large zones that contain receptor cells that express a different subset of receptor genes

  • Olfactory receptor proteins belong to the G-protein coupled receptors family (have transmembrane alpha helices)

    • Large evidence that the only second messenger in olfactory transduction is cAMP

  • How does one discriminate among tens of thousands of odors 

    • Involves population coding scheme

    • Each receptor protein binds different odorants more or less readily

      • Each receptor cell is less or more sensitive to odorants 

    • Some cells are more sensitive to the chem structure of odorants they respond to than other cells are

    • Each odorant activates many of the receptors

    • More concentration of odorant = stronger response until strength saturates

    • Central olfactory pathways classify odors further after receiving info

Central Olfactory Pathways

  • Olfactory receptor neurons send axons in two olfactory bulbs

  • Bulbs contain glomeruli

    • Each glomerulus has endings of 25k primary olfactory axons converging & terminating on dendrites of 2nd order olfactory neurons

    • Mapping of receptor cells onto glomeruli is precise

    • Each glomerulus receives input from only receptor cells of one particular type

      • Array of glomeruli within bulb is map of receptor genes expressed in the olfactory epithelium

  • Structures that receive olfactory connections

    • Output of axons from olfactory bulbs → olfactory tracts → olfactory cortex & temporal lobes

    • Anatomy of olfaction is unique, other sensory systems first pass info through the thalamus before it goes to the cerebral cortex

      • This produces direct & widespread effect on parts of forebrain that have roles in:

        • Odor discrimination

        • Emotion

        • Motivation

        • Certain kinds of memory

    • Conscious perceptions of smell may follow the path: olfactory tubercle → medial dorsal nucleus → orbitofrontal cortex

Spatial and Temporal Representations of Olfactory Information

Olfactory Population Coding

  • Uses responses of large population of receptors to encode specific stimulus

  • e.g. when presented w/citrus smell non of three different receptor cells can individually distinguish it clearly from other odors

    • Looking @ combination of responses from all three cells brain could distinguish the smell from others

  • Due to population coding humans can discriminate one trillion different combinations of odor stimuli

Temporal Coding

  • Evidence temporal patterns of spiking in olfactory neurons are feature of olfactory coding

  • Odors are slow stimuli, rapid timing of APs not needed for encoding timing of odors; rather, it is the sustained activation of olfactory receptors that contributes to our perception and differentiation of smells.

  • Temporal coding depends on timing of spikes to encode quality of odors