Gustation & Olfaction - Stark home page

Gustation & Olfaction
Traditional and modern

Tongue
• Hanig (1901) - preferential localization:
• sweet - tip of tongue
• salt - front sides of tongue
• sour - back sides of tongue
• bitter - back middle of tongue
• Correlation is not exclusive is not really
true.

Papillae:
Circumvallate back of tongue
• foliate sides of tongue
• fungiform front of tongue
• also receptors in epiglottis

• Papillae > taste buds
• support cells,
• sensory cells,
• and basal cells
Histology
• As with olfaction, a unique feature is the
turnover of receptor cells

Recent papers
• genetic taste "blindnesses"
• Ptc= phenylthiocarbamide, taster is dominant.
• Use taste vs. non-taste to screen for G-protein
coupled receptors
• “ Family of bitter taste receptors found,” 2000
• Drayna, “Founder mutations.” 2005, 78-85.
• "...seven different forms of the PTC gene exist in
sub-Saharan Africa. But only the major taster and
nontaster forms have been found...outside of
African populations.

Papers (continued)
• "taster detects chemicals with C=N-S
• (1) taster and nontaster are ancient
• (2) tasters and nontasters populated the
world ("Out of Africa" hypothesis)
• (3) these people did not interbreed with
others (like Neanderthals).
• only taster in all other primates

Papers (continued)
• Kim ...Drayna, …cloning PTC, 2003
• human chromosome 7q, there are nine
TRA2R (bitter taste genes) and 7 olfactory
receptor genes in this area.
• PTC is 1002 bp and 1 exon
• 3 single nucleotide polymorphisms (SNPs)
explain PTC taste insensitivity, A49P,
V262A, I296V

Charles Zuker
• isolation of taste receptors T1R1, T1R2, T1R3,
T2R,
• also the involvement of the TRP channel. Earlier,
Drosophila phototransduction.
• HHMI = Howard Hughes Medical Institute which
has helped to fund innovative and productive
scientists like Charles.
• There are easy to read HHMI press releases here.

Tuning
• (how selective is receptor?)
• work by Carl Pfaffman, 1941, & since -
• receptors are not all that specific
• Contradicted by very modern data supporting
"labeled line hypothesis" (in book).
• This applies to G protein coupled receptors, T2R1
plus T1R3 for sweet, T1R1 plus T1R3 for umami,
and T2R for bitter

Projection
• (much simpler than for olfaction)
• epiglottis via nerve X (vagus),
• circumvallate (9 of them) IX glossopharyngial
• others via VII (facial)
• Gustatory (solitary) nucleus in medulla,
• there to thalamus and then to sensory cortex
• (overlap to touch area - postcentral gyrus)
• also from solitary to hypothalamus

Trigeminal chemoreception
• Capsaicin (covered in the chapter on pain,
Chapter 10)
• for polymodal nociceptive fibers
• Trigeminal (5)
• mediates irritants

Smell- Olfaction
• chemicals (air)
• Landmark paper:Karlson Pheromones
1959
• sex attractants like a 10 carbon acetate.
• It can attract male from a few miles who
flies upwind at first.
• Pheromones have been used to trap pests.

complications
• unusual primaries like “aromatic and putrid”
• many primaries, although mixtures give a single
perception confounding the ability to define
primaries
• Relative to other senses, receptors difficult to
stimulate
• Perhaps more than with the sense of touch,
olfaction is related to motivational "affect"
• The sense of smell is especially important in other
animals (dogs)

Properties
• olfactory epithelium.
• receptors are neurons
• Receptors turn over (this is unusual),
• (dividing stem cell and developing (immature)
receptor),
• since cells are very exposed (to dry air, pathogens,
etc.).
• New cells must establish connections.
• There are also sustaining cells

Properties
• Receptors are ciliary with "9 + 2"
arrangement of microtubules as seen
structurally.
• Cilia are in mucus
• slowly adapting (receptors) even though it
seems otherwise (processing)

Transduction
• G protein coupled receptor
• adenylate cyclase
• olfactory alpha subunit of the G protein (Golf)
• Na+- Ca2+ channel like that of photoreceptor:
• cAMP ligand to open the channel from inside
• Ca2+ opens Cl- channel
• PLC and IP3
• there is a Na+/Ca++ exchanger

Recent work
• Barnes,…Axel, relates to Axel & Buck 2004
Nobel
• Odorant Receptors on axon termini in the brain,
2004
• Each cell expresses only one type of receptor.
• Seemingly randomly arranged on olfactory
epithelium.
• axons with same receptors converge at glomeruli.
• same receptors are used in axon guidance.

Recent work
• Postnatal refinement of peripheral olfactory
projections, 2004.
• "A hallmark of mature glomeruli is that they are
innervated exclusively by axons from olfactory
sensory neurons expressing the same olfactory
receptor."
• (1) Glomeruli start out heterogeneous
• heterogeneous glomeruli decreases with
development.
• (2) Sensory stimulation contributes to the final
unique mapping.

From notes
• number and organizations of genes and
proteins in C. elegans, Drosophila, mouse,
human
• no introns in mammals
• distribution of genes in human, many on 11

Projection
• Glomeruli - > Mitral cells -> lateral
olfactory tract (stria)
• Also Periglomerular cells and Granule cells
for processing
• There is specificity of projection (space) of
specific odorants to olfactory bulb favoring
labeled line scheme of processing