Como Funciona o Olfato: Explicação Nobel de Medicina

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http://www.nobelprize.org/nobel_prizes/medicine/laureates/2004/illpres/credits.html

The Nobel Assembly at Karolinska Institutet has awarded the Nobel Prize in Physiology or Medicine jointly to Richard Axel and Linda Buck for their discoveries of odorant receptors and the organization of the olfactory system. In a series of pioneering studies the laureates have clarified in molecular detail how our sense of smell works.
	Richard Axel Born 1946 Howard Hughes Medical Institute, Columbia University, New York, USA. Linda Buck Born 1947 Howard Hughes Medical Institute, Fred Hutchinson Cancer Research Center, University of Washington, Seattle, USA.

The vivid world of odours

The olfactory system is important for our quality of life. A unique odour can trigger distinct memories from our childhood or from emotional moments – positive or negative – later in life. When something tastes good it is mainly due to activation of the olfactory system.

To lose the sense of smell is a significant handicap; we no longer perceive the different qualities of food and we cannot detect warning signals, for example smoke from a fire.

The olfactory system

The olfactory epithelium contains millions of olfactory neurons, which send messages directly to the olfactory bulb of the brain.

The olfactory receptor cells are the only neurons in the nervous system exposed directly to the external environment.

A large family of odorant receptors

Richard Axel and Linda Buck published their fundamental paper in 1991, in which they described the genes coding for a large family of odorant receptors.

The odorant receptors are located on the olfactory receptor cells in the nasal cavity. Each olfactory receptor cell expresses only one type of odorant receptor, and each receptor can detect a limited number of odorant substances.

The olfactory receptor Each            receptor consists of a protein chain that traverses the cell membrane seven times. When an odorant substance attaches to an olfactory receptor, the shape of the receptor protein is altered, leading to a G protein activation. An electric signal is triggered in the olfactory receptor neuron and sent to the brain via nerve processes.
	Small variations All odorant receptors are related proteins and differ only in some amino acid residues (indicated in green, blue and red). The subtle differences in the protein chains explain why the receptors are triggered by different odorant molecules.

The organization of odorant receptor inputs in the olfactory cortex Signals derived from two different odorant receptors, M5 and M50, are targeted to different, but partially overlapping clusters of cortical neurons. These clusters have similar locations in the brains of different mice.

Receptor activation in the bulb

Receptor cells carrying the same type of receptor converge their processes on the same glomerulus.

Combinatorial receptor codes

The odorant receptor family is used in a combinatorial manner to detect odorants and encode their unique identities. Different odorants are detected by different combinations of receptors and thus have different receptor codes. These codes are translated by the brain into diverse odour perceptions.

The immense number of potential receptor combinations is the basis for our ability to distinguish and form memories of more than 10,000 different odorants.

Species differences

The area of the olfactory epithelium (red) in dogs is some forty times larger than in humans. Mice – the species Axel and Buck studied – have about one thousand different odorant receptor types.

Humans have a smaller number than mice; some of the genes have been lost during evolution. There are several millions of olfactory receptor cells in our olfactory epithelium.

Credits and References for the 2004 Nobel Poster for Physiology or Medicine

Scientific Advisers, Professors at Karolinska Institutet:Bertil Fredholm, Pharmacology Sten Grillner, Neurophysiology Göran K. Hansson, Medicine, Chair of the Nobel Committee Hans Jörnvall - Physiological Chemistry, Secretary of the Nobel Assembly Urban Lendahl, Genetics  Illustrations and layout: Annika Röhl, Bengt Gullbing Photo: EyeQnet: P O Eriksson/Naturfotograferna, Dag Ekelund/Windh, Ina Agency Press AB Medical Writer: Anders Nystrand Printed by: Alfa Print AB, Stockholm, Sweden 2004 Copyright © 2004: The Nobel Committee for Physiology or Medicine at Karolinska Institutet, SE-171 77 Stockholm, Sweden Web Adapted Version: Nobelprize.org