September 15, 2008
The Smells of Delhi
My brother in law (BIL) has ‘flu’: his dry throat and raspy cough bother him, but it his mucus laden stuffy nose that is the cause of his misery. He cannot smell and food has lost its flavor. If I were sentimental about food like BIL, I would sympathize; instead, I congratulate him, “Celebrate your anosmia BIL, you will loose a few pounds and come out slimmer.”
“Anosmia, what is that?”
“The loss of ability to smell.”
BIL, the laid off hedge fund manager, cannot unshackle himself from the clutches of his limbic system and hates any loss. He fears, he would not be able to enjoy the aroma of Indian cuisine during his first trip to India, where we plan to travel together in two weeks. I console him, “It is temporary, and you will get your smell back in a week.”
Sense of smell is perhaps the first sensory system where the molecular mechanism of the process of olfaction has been established. Richard Axel and Linda B. Buck jointly published the fundamental paper in the journal “Cell” on the functioning of olfactory system in 1991. They won the Nobel Prize in medicine in 1994 for their work on "Odorant receptors and the organization of the olfactory system." The researchers, working on mice, discovered a pool of more than 1,000 different genes (about 3% of all genes) that encode olfactory receptors in the nose, which can distinguish over 10,000 distinct odors.
BIL carries right number of genes but cannot smell; mucus has stuffed his nose, which prevents air from reaching the olfactory membrane at roof of his nose. Nerve cells or neurons, that line this area of about 10 square centimeters, dangle hair like projections (cilia) into the nasal cavity, where they work as receptors of smell. Odor molecules, mostly volatile organic oils and some inorganic compounds, fit snugly into of receptor sites – a pocket made by a chain of amino acids (protein molecules) - which triggers the coupling of G protein and the process of olfaction through a chain of chemical reactions generating electric signals, which the biologists call ‘transduction’. These olfactory receptor cells are the only neurons in the nervous system that regenerate regularly and replace the old ones every 4 to 8 weeks.
Most of the nasal cavity - about 95% - works as a conduit for air and does not participate in the act of smell. At normal air speed of 250 ml per second in the nose, only some inhaled air comes in contact with the olfactory membrane. Sniffing or deep breathing enhances turbulence in the nose, which gushes more air to contact olfactory receptors.
Nerve processes (axons) from about 10 million odor receptor cells travel into the base of skull to regroup into about 2000 micro-bunches (glomerulus) and form olfactory bulb. Each receptor cell carries only one type of odorant and signals from the same types of receptors end in same glomerulus.
The nerve extensions from the olfactory bulb emerge to form an olfactory tract, which relays information to two parts of the brain: primitive regions of limbic system and neo-cortex. Biologists believe that direct limbic connection of smell is due to its earlier appearance in evolution compared to sight and hearing.
Most common odoriferous substances emit complex mixtures of hundreds of different smells, which activate multiple receptors leading to a combined odor pattern. The cortex recognizes it as a pattern relying on about 10,000 patterns in its memory.
Olfactory sensitivity deceases with age; older people over 70 have over 10 times less sensitivity compared to young adults and older males are less sensitive than females. Alterations in the sense of smell carry various names: hyposmia for diminished sensation; dysosmia suggests distorted sensation; cacosmia is sensation of foul smell and parosmia describes smell without a stimulus. About 2 million people in the United States have no sense of smell, called anosmia.
How do our friends and foes – dogs and mosquitoes – compare with us?
The size of the olfactory lining and the number of receptors determine the prowess to smell. Dogs have 170 square centimeters of olfactory lining and have one hundred times more receptors per square centimeter than humans, hence their ability to recognize more odors.
Sensory organ of the mosquito is the maxillary palp on its head, which probably works as a long range smelling system. The palp contains specialized receptor cells that detect octenol and carbon dioxide, which leads it to its target: human prey. Knowing this, I had advised BIL to apply a mosquito repellant and wear long sleeves while in India.
When our plane landed, I was ready for the forthcoming assault: in a few moments my olfactory system would be overwhelmed by the first smell of Delhi. The volatile, water-soluble and partially lipid soluble molecules would fly into my nose and attach to the smell discriminating nerve receptors at the roof of my nose.
If a perfume maker were to imitate the aroma, he would have to mix early morning dew, tall grass, gasoline fumes, charcoal smoke, runway tar, summer dust, construction steel and human sweat. The product would be a mixture of nostalgia and hope; poverty and progress; a juncture of future and past.
Having landed often at the Palam airport, I should have got used to this expected welcome. But no: not to the nostril-piercing gust. India evokes strong emotions. Love or hate starts at the first whiff.
I wanted BIL to love his first trip and was grateful that BIL had anosmia.
I enquired to confirm. “Can you smell?”
BIL paused, stared at the steel scaffold holding the granite walls of the new construction. The greedy glint in his eyes betrayed his limbic system.
“Yes, I can smell opportunity.”
Posted by Shiban Ganju at 02:37 AM | Permalink