Dr. N.L. Kachhara
Olfaction, the sense of odor (smell), is the detection of chemicals dissolved in air (or, by animals that breathe water, in water). In vertebrates smells are sensed by the olfactory epithelium located in the nose and processed by the olfactory system.
As discovered by Lind B. Buck and Richard Axel, mammals generally have about 1000 genes for odor receptors. Humans have 347 functional odor receptor genes. Odor receptor nerve cells may function like a key lock system: if the odor molecule can fit into the lock the nerve cell will respond. An alternative theory, the vibration theory proposed by Luca Turin (1996, 2002), a British biophysicist, posits that odor receptors detect the frequencies of vibrations of odor molecules in the infrared range by electron tunneling. Turin argued that two of our other senses vision and sound- are based on the brain's interpretation of vibrations and spectra so why not smell? The fact that smell is a spectral sense like sight and sound might also help explain a strange but very rare condition known as synaesthesia where the "sufferer's" senses are mixed up. Several musical composers claim to be able to hear in colours or smell sounds and very young babies are thought to have a mixed up sense of the world where various inputs sight, sound and smell are not processed separately by the brain. However, the major predictions of this theory have been found lacking (Keller and Vosshall, 2004), though other studies disagree.
According to Jain philosophy the entire body is potential Integrated Sensory Organ (Karan). The skin performs the function of all sense receptors. In practice light, waves and odour encounter the whole body and just not eyes, ears or nose. The signals may be generated in the whole body but have high concentration in the eyes ears and nose respectively. However, if high intensity signals from the specified organs are absent, the mind can amplify the weak signals produced elsewhere in the body. In a recent study a women was able to recognize colours simply by touching when her eyes were blind folded.
Olfaction and taste together contribute to flavor. The human tongue can only distinguish among five distinct tastes, while the nose can distinguish among hundreds of substances. This is the reason why food has little flavour when your nose is blocked, as from a cold.
The importance and sensitivity of smell varies among different organisms: most mammals have good sense of smell, whereas most birds do not, with the exceptions being the tubenoses and the kiwis. Among mammals it is well developed in the carnivores and ungulates, which must always be aware of each other, and in those, such as moles, who smell for their food. It is less well developed in the catarshine primates, and non-existent in cetaceans, which in compensation have a sensitive and well developed sense of taste. Insects primarily use their antennae for olfaction. Sensory neurons in the antenna generate odor-specific electrical signals called spikes in response to odor.
