Olfactory perception

Olfaction, the sense of odor (smell), is the detection of chemicals dissolved in air (or in water, by animals that live under water). In vertebrates smells are sensed by the olfactory epithelium located in the nasal cavity and first processed by the olfactory bulb in the olfactory system. In insects smells are sensed by sensilia located on the antenna and first processed by the antennal lobe.

Receptors
As discovered by Linda B. Buck and Richard Axel (who were awarded the Nobel Prize in 2004), 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, not all people can smell androstenone, a component of male sweat.

Each olfactory receptor neuron in the nose expresses only one functional odor receptor. Odor receptor nerve cells may function like a key-lock system: if the odor molecules can fit into the lock the nerve cell will respond. According to shape theory, each receptor detects a feature of the odor molecule. Weak-shape theory, known as odotope theory, suggests that different receptors detect only small pieces of molecules, and these minimal inputs are combined to create a larger olfactory perception (similar to the way visual perception is built up of smaller, information-poor sensations, combined and refined to create a detailed overall perception). An alternative theory, the vibration theory proposed by Luca Turin (1996, 2002), posits that odor receptors detect the frequencies of vibrations of odor molecules in the infrared range by electron tunnelling. The major predictions of this theory have been found lacking (Keller and Vosshall, 2004), though other studies disagree (citation needed).

In the brain
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.

Pheromonal olfaction
Some pheromones are detected by the olfactory system, although in many vertebrates pheromones are also detected by the 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, it is unknown whether or not pheromones exist.

Olfaction and taste
Olfaction, taste and trigeminal receptors together contribute to flavor. The human tongue can only distinguish among 7-8 distinct types of taste, while the nose can distinguish among hundreds of substances. This is the reason why food has little flavor when your nose is blocked, as from a cold.

Olfaction in the other alive kingdoms
The importance and sensitivity of smell varies among different organisms: most mammals have a good sense of smell, whereas most birds do not, with the exceptions being the tubenoses (i.e. petrels and albatrosses) and the kiwis. 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
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 where they synapse with other neurons there in semidelineated (with membrane boundaries) called glomeruli. These antennal lobes have two kinds of neurons, projection neurons (excitatory) and local neurons (inhibitory). The projection neurons send their axon terminals to mushroom body and lateral horn (both of which are part of the protocerebrum of the insects) and local neurons have no axons. Recordings from projection neurons show in some insects strong specialization and discrimination for the odors presented (especially for the projection neurons of the macroglomeruli, a specialized complex of glomeruli responsible for the pheromones detection). Processing beyond this level is not exactly known though some preliminary results are available.