The Chemosensory System
MEDS 5371 Spring 2012
Royce Mohan, PhD
Our wondrous chemosensory system
So how do we choose odorants for perfumes?
Penguins use odor-based perception for monogamous relationship for life
Ah, so many choices!
My mom and dad wear Solo per Te
At the nesting sites in March, there is intense competition between the
females for a mate followed by a necessarily brief courtship. After a single
egg is laid the females return to the sea for many months. This leaves the
males to incubate the eggs in the bitter cold until females return only when
the chicks begin to hatch.
The ChemoSensory System: the Nose
The olfactory epithelium
Odorant receptor proteins
Structure of odorant receptor
Molecular mechanisms of odorant transduction
Olfactory transduction starts with odorant binding to the GPCR and
involves second messengers and ionic currents
Very strong adaptation to smells – one of the senses that is
strongly or fast adapting
Responses of receptor neurons to selected odorants
The optimal
odorant is very
difficult to find.
Hence, the science
of the perfume
industry is still an
art.
$30 Billion in
revenues!
Organization of the mammalian olfactory bulb
Mitral, tufted cell dendrites
in glomerulus; axons to
pyriform cortex, amygdala,
entorhinal cortex
Reinnervate the glomerulus;
specific for 1 receptor type
Axons grow through
cribriform plate
Neurons replaced every
few weeks
Olfactory receptors here;
cilia up to 30 m long
Organization of the mammalian olfactory bulb
25,000 ORN inputs/
glomerulus
1,000 ORN inputs/
mitral/tufted cell
100 intrinsic
neurons
(periglomerular,
granule) =
interneurons /
mitral or tufted cell
MANY glomeruli /
mitral cell; MANY
Mitral cells/glomerulus
Inputs; much
“sharpening”
of signal here;
mitral cell tufts =
dendrites
Output to
cortex
interneurons
Transgenic mouse nasal cavity and brain showing olfactory epithelium (OE)
and olfactory bulb (OB) from a single olfactory receptor (OR) receptor labeled
with lacZ and stained with X-gal
Cribriform plate
OB
OE
ORs are widely distributed in the OE
Olfactory sensory neurons converge on a single glomerulus
Characteristics of OR genes
One gene expressed per OSN – pseudogenes [no AUG
start; premature STOPs] initially expressed, replaced by
functional one
Expression of receptor genes around olfactory epithelium
is stochastic – not topographic by place in epithelium
Humans have about 340 functional olfactory receptor
genes, on many chromosomes, in clusters (families)
55 receptor genes respond to octanol
Concentration dependent effects
Why is my air freshener smelling like celery?
oc
ta
n
ol
oc
ta
n
ol
ODORANT
Hexanol
Heptanol
Octanol
Nonanol
Hexanoic acid
Heptanoic acid
Octanoic acid
Nonanoic acid
ODOR
Sweet, herbal, woody
Sweet, violet, woody
Sweet, rose, orange
Fresh, rose, oily floral
Rancid, sour, goat-like
Rancid, sour, sweaty
Rancid, sour, repulsive
waxy, cheese, nut-like
The olfactory bulb has a stereotyped map of
OR inputs
Odor coding in the olfactory epithelium (OE)
and olfactory bulb (OB)
OE: Inputs from different ORs are indicated by
different colors; dispersed ensemble of neurons
A). A single OR gene probe hybridized to sensory axons in 1
or 2 glomeruli on either side of the olfactory bulb
OB: Specific combination of glomeruli whose spatial
arrangement is similar among individuals. Partially
B). Different OR probes hybridized to different glomeruli and
overlapping combinations of OR inputs generate
those glomeruli had similar locations in six different bulbs.
distinct odor perceptions
Schematic diagrams showing the organization of odorant receptor inputs in the
olfactory epithelium (OE), olfactory bulb (OB), and olfactory cortex (OC)
Inputs from different ORs are segregated in different neurons and glomeruli in the OE and
OB. In contrast, it appears that different receptor inputs overlap extensively in the OC (A)
and that single cortical neurons receive signals from a combination of receptors (B)
The lock and key theory of odorant-receptor interaction does not
address how diverse chemical structures have the same odor perception
Is the electronic swipe card model a better explanation than the lock and key?
The olfactory cortex has a stereotyped map of OR inputs
Organization of inputs from the M5 (yellow) and M50 (pink) odorant receptors in the olfactory
epithelium, bulb, and cortex.
Black lines and abbreviations indicate different areas of the olfactory cortex.
AON: anterior olfactory nucleus; PC: piriform cortex; OT: olfactory tubercle;
Amg: olfactory nuclei of amygdala; EC: lateral entorhinal cortex.
The human taste system
Cortical
processing
unclear
All
converge to
NTS, then
thalamus
Nerves VII,
IX, X from
different
regions of
the mouth
The human taste system
Human Olfactory System
Taste buds and taste papillae
Taste buds and taste papillae
50-100 receptor
cells in bud; renew
every 10 days or so
5-20 axons leave the
bud; integration must
be happening !!
Salty, acidic, sweet,
amino acid, bitter
ALL represented by
cells in the bud
Bitter cells do not directly contact
nerve – indirect through sweet and
amino acid cells
Salty, acidic, sweet,
amino acid, bitter
ALL represented by
cells in the bud
Peripheral innervation of the tongue
Sensory transduction in taste cells
Molecular mechanisms of taste transduction via ion channels
and G-protein-coupled receptors
Main points to remember
• Largest family of genes; ORs are GPCRs
• 1 OR expressed/neuron; can detect many odorants
• Even though many OR genes initially selected for expression, only one will
dominate and remain expressed
• Odorant sensory neurons converge on a few glomeruli in the OB
• Partially overlapping combinations of OR inputs generate distinct odor
perceptions in the olfactory cortex
• Taste system; salt and acids use ionic channels
• Other taste receptors use GPCRs
• The neural coding for taste system uses a labeled line code