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The Chemosensory System
MEDS 5371 Spring 2012Royce Mohan, PhD
Our wondrous chemosensory system
So how do we choose odorants for perfumes?
Penguins use odor-based perception for monogamous relationship for life
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.
Ah, so many choices! My mom and dad wear Solo per Te
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 dendritesin glomerulus; axons topyriform cortex, amygdala,entorhinal cortex
Reinnervate the glomerulus;specific for 1 receptor type
Axons grow throughcribriform plate
Neurons replaced everyfew weeks
Olfactory receptors here;cilia up to 30 m long
Inputs; much“sharpening”of signal here;mitral cell tufts =dendrites
Output tocortex
interneurons
MANY glomeruli /mitral cell; MANYMitral cells/glomerulus
100 intrinsicneurons(periglomerular,granule) =interneurons /mitral or tufted cell
1,000 ORN inputs/mitral/tufted cell
25,000 ORN inputs/glomerulus
Organization of the mammalian olfactory bulb
ORs are widely distributed in the OE
Olfactory sensory neurons converge on a single glomerulus
OB
Transgenic mouse nasal cavity and brain showing olfactory epithelium (OE)and olfactory bulb (OB) from a single olfactory receptor (OR) receptor labeledwith lacZ and stained with X-gal
OE
Cribriform plate
55 receptor genes respond to octanol
One gene expressed per OSN – pseudogenes [no AUG start; premature STOPs] initially expressed, replaced by functional one
Humans have about 340 functional olfactory receptor genes, on many chromosomes, in clusters (families)
Expression of receptor genes around olfactory epithelium is stochastic – not topographic by place in epithelium
Characteristics of OR genes
Concentration dependent effects
Why is my air freshener smelling like celery?
octanol
octanol
octanoloc
tan
ol
Hexanol
Hexanoic acid
Heptanoic acid
Octanoic acid
Nonanoic acid
ODOR
Heptanol
Octanol
Nonanol
Rancid, sour, goat-likeRancid, sour, sweatyRancid, sour, repulsivewaxy, cheese, nut-like
Sweet, violet, woodySweet, rose, orange
Fresh, rose, oily floral
Sweet, herbal, woody
ODORANT
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
OB: Specific combination of glomeruli whose spatial arrangement is similar among individuals. Partially overlapping combinations of OR inputs generate distinct odor perceptions
The olfactory bulb has a stereotyped map of OR inputs
A). A single OR gene probe hybridized to sensory axons in 1 or 2 glomeruli on either side of the olfactory bulb
B). Different OR probes hybridized to different glomeruli and those glomeruli had similar locations in six different bulbs.
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
Corticalprocessingunclear
Allconverge toNTS, thenthalamus
Nerves VII,IX, X fromdifferentregions ofthe mouth
The human taste system
Human Olfactory System
Taste buds and taste papillae
Taste buds and taste papillae
50-100 receptorcells in bud; renewevery 10 days or so5-20 axons leave thebud; integration mustbe happening !!
Salty, acidic, sweet,amino acid, bitterALL represented bycells in the bud
Salty, acidic, sweet,amino acid, bitterALL represented bycells in the bud
Bitter cells do not directly contactnerve – indirect through sweet andamino acid cells
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