The Olfaction reference article from the English Wikipedia on 24-Jul-2004
(provided by Fixed Reference: snapshots of Wikipedia from wikipedia.org)

Olfaction

You can make a difference by sponsoring a child
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.

How Smell Works

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 can not. Each olfactory receptor neuron in the nose expresses only one functional odor receptor which 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. 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.

To detect pheromones many vertebrates have an auxiliary olfactory sense organ called Jacobson's organ or 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, the detection of pheromones is subliminal. These subliminal odor messages may transmit opposite immulogical sexual compatibility. Children born with a mixture of immulogical systems are more likely to survive. It has been suggested that human females unconsciously use this process to recognize who they find attractive.

Olfaction in the Animal Kingdom

The importance and sensitivity of smell varies among different organisms: most mammals have a good sense of smell, whereas most birds don't. Among birds, it 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 moless, 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.

Insects sense smell with their antennae and process smells in the mushroom bodies of the brain. In many species olfaction is highly tuned to pheromones; a male silkworm moth, for example, can smell a single molecule of bombykol.

External links


Nervous system - Sensory system Edit
Visual system - Auditory system - Olfactory system - Gustatory system - Somatosensory system

Sensory system - Olfactory system Edit
Olfactory bulb - Olfactory nerve - Olfactory epithelium - Glomeruli - Olfactory mucosa - Olfactory receptor neurons - Mitral cells - Piriform cortex