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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
Histology
• Papillae > taste buds • support cells, • sensory cells, • and basal cells • 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