Olfaction, the sense of 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 Linda B. Buck and Richard Axel, mammals generally have about 1000 genes for odor receptors. Of these genes, only a portion code for functional odor receptors. Humans have 347 functional odor receptor genes; the other genes have nonsense mutations. This number was determined by analyzing the genome in the Human Genome Project; the number may vary among ethnic groups, and does vary among individuals. For example, some people can smell amyl acetate (which smells like bananas), whereas some others cannot.
Each olfactory receptor neuron in the nose expresses only one functional odor receptor. According to shape theory, each receptor detects a feature of the odor molecule. Odor receptor nerve cells function like a key lock sytem. If the odor molecules can fit into the lock the nerve cell will fire.
However, according to Vibration theory, recently proposed by Turin (1996, 2002), odor receptors detect the frequencies of vibrations of odor molecules in the infrared range by electron tunnelling. This would allow encoding of odors to be similar to the way wavelengths of light are encoded by cones of the eye. Currently, only vibration theory can explain why a molecule containing hydrogen atoms (such as decaborane, B10H14) and one containing deuterium atoms (such as deuterated decaborane, B10D14) smell differently. Both have the same shape and chemical characteristics but have different vibrations.
The axons from all the thousands of cells expressing the same odor receptor converge in the olfactory bulb. Mitral cells in the olfactory bulb send the information about the individual features to other parts of the olfactory system in the brain, which puts together the features into a representation of the odor. Since most odor molecules have many individual features, the combination of features gives the olfactory system a broad range of odors that it can detect.
Odor information is easily stored in long term memory and has strong connections to emotional memory. This is possibly due to the olfactory system's close anatomical ties to the limbic system and hippocampus, areas of the brain that have long been known to be involved in emotion and place memory, respectively.
To detect pheromones many vertebrates have an auxiliary olfactory sense organ called vomeronasal organ, located in the vomer, between the nose and the mouth. Snakes use it to smell prey, sticking their tongue out and touching it to the organ. Some mammals make a face called flehmen to direct air to this organ. In humans, the detection of pheromones is subliminal. These subliminal odor messages may transmit opposite immunological sexual compatibility. Finding a partner of non-similar immunological background may be evolutionarily advantageous because children born with a mixture of immunological systems are more likely to survive. It has been suggested that human females unconsciously use this process to recognize whom they find attractive.
Smell is extremely important for taste. The human tongue can only sense 4 different things (some studies say 5, 6 even 7, but still a limited number), while the nose can sense many thousands. This is the reason why you can taste very little when you have a blocked nose.
The importance and sensitivity of smell varies among different organisms: most mammals have a good sense of smell, whereas most birds do not. As an exception among birds, olfaction is important in the tubenoses. Among mammals it is well developed in the carnivores and ungulates, who 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 catarrhine primates (Catarrhini), and nonexistent in cetaceans, who in compensation have a sensitive and well-developed sense of taste. The lack of olfaction is called anosmia. In many species olfaction is highly tuned to pheromones; a male silkworm moth, for example, can smell a single molecule of bombykol.
Insects primarily use their antennae for olfaction. Sensory neurons in the antenna generate odor-specific electrical signals called spikes in response to odour. They process these signals from the sensory neurons in the antennal lobe followed by the mushroom bodies and lateral horn of the brain. The antennae have the sensory neurons in the sensilla and they have their axons terminating in the antennal lobes. These antennal lobes have two kinds of neurons, projection neurons (spiking) and local neurons (graded junction). The projection neurons send their axon terminals to mushroom body and lateral horn. Recordings from the projection neurons can accurately predict the odor presented to the animal. Processing beyond this level is not exactly known though some preliminary results are available.