October 04, 2008

Smelling The Intentions Of Olfaction

a boy smelling a red roseOne simple thing puzzles me as to why the olfactory pathway has to bypass thalamus while most pathways from sensory receptors and sensory organs say hello to the thalamus, as they pass to the cortex. In fact, the cortex and thalamus are so intricately connected to each other, in a bidirectional way, they are sometimes referred together as thalamocortical system. Thalamic excitation of the cortex is necessary is necessary for almost all cortical activity. Why is there an exception in the case of olfaction (smell)?

How do we smell things? It is believed that volatile odorant molecules first mix ResearchBlogging.orgwith the mucus in the nose. They then interact with receptors in the olfactory cilia, called G-Protein coupled receptors. An enzyme called adenyl cyclase is produced as a result, which opens up a sodium channel. Sodium ions (Na+) pour into the olfactory cells, which then fire and smell is thus created. Marshall Stoneham and colleagues, at University College London, propose that olfaction may involve quantum mechanics. The odorant molecules, in addition to having a shape, have another property called vibration. They argue that this (vibration) property may allow electrons to tunnel through to the receptors within the nose to evoke the sense smell. They found phonon assisted electron tunneling, from a donor to an acceptor, mediated by the odorant activated a receptor, in perfect harmony with present physics provided the smell receptors fulfilled some criteria.

We all know that rats fear cats and avoid them so that they are not harmed. They can smell cat’s urine and don’t tread their paths. However, when cats are infected with the parasite Toxoplasma gondii, the rats no longer fear the cats’ urine. The parasite possibly alters the rats’ perception of smell in such a way, so that the intrinsic avoidance becomes passionate attraction. Manipulation of smell sensing thus ensures that the parasite is transmitted from their definitive host (cat) to their intermediate host (rat), thus completing its life cycle. In a similar way it has been conclusively found that dogs can smell some human cancers like lung cancer, breast cancer and skin cancer.

Scientists now can measure minute amount of acetaldehyde, a biomarker of lung cancer. Using tunable diode laser absorption spectroscopy, they could ‘smell’ the telltale signature of those odorant biomarkers in the exhaled breath of the patients. May be they would be able to devise practical explosive smelling devices as well. Sniffing with reliable accuracy and precision would herald a new era in diagnostic oncology.

Apart from its bypassing the thalamus, olfaction puzzles at another point: its stem cell reserve. Why out of the whole brain has it been selected to hold an adult neural stem cell reserve? May be it’s just a coincidence. These cells may come in very handy in the treatment of a variety of diseases.

While a newer pathway of olfaction has been discovered in the monkeys which pass through the thalamus, olfaction on the whole continues to elude me.

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Reference: Diagnostic Accuracy of Canine Scent Detection in Early- and Late-Stage Lung and Breast Cancers;
DOI: 10.1177/1534735405285096
Perception without a Thalamus How Does Olfaction Do It?
Jennifer C. Brookes, Filio Hartoutsiou, A. P. Horsfield, A. M. Stoneham (2006). Could humans recognize odor by phonon assisted tunneling? arXiv:physics/0611205v1
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