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