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Olfaction is the sense of smell. The sense of smell is how a human or animal notices a smell (or odour or odor) by using the nose. Many animals have better noses than people. Some animals can detect small particles in the air or sometimes water that people cannot.

People have special cells in the nose that can detect some chemicals. These are special nerve cells attached to the olfactory epithelium. All vertebrates have these cells. The smell is first processed by the olfactory system. The information is given to the olfactory bulb in the front of the forebrain.

In insects, smells are sensed by sensilia on the antennae and first processed by the antennal lobe.

Olfactory reception cells[change | change source]

Olfactory epithelium and neurons. The end cilia of the neurons stick out into the mucus (not shown here)

The olfactory reception (OR) cells are neurons (nerve cells). Many tiny hair-like cilia stick out of these cells into the mucus covering the surface of the epithelium.[1] The surface of these cilia is covered with olfactory receptors, a kind of protein.[2]

There are about 1000 different genes which code for the ORs, though only about a third are functional.[3] The rest are pseudogenes. The OR genes are the largest gene family. An odor molecule dissolves into the mucus of the olfactory epithelium and then binds to an OR. Various odor molecules bind to various ORs. The basis of the sense of smell is that different groups of scent molecules bind to different receptor cells and so fire different groups of neurons.[4] Inside the olfactory region of the brain, the firing of neurons produces the perceived smell.

When the OR is activated, changes start in the cells. Positive ions come in and negative ions go out of the cells. This causes the neuron to fire an impulse (generate an action potential).[5][6]

References[change | change source]

  1. Rinaldi A (2007). "The scent of life. The exquisite complexity of the sense of smell in animals and humans". EMBO Reports 8 (7): 629–33. doi:10.1038/sj.embor.7401029. PMC 1905909. PMID 17603536. See especially figure 1 in this review.
  2. Touhara, Kazushige (2009). "Insect olfactory receptor complex functions as a ligand-gated ionotropic channel". Annals of the New York Academy of Sciences 1170: 177–80. doi:10.1111/j.1749-6632.2009.03935.x. PMID 19686133.
  3. Buck L. & Axel R. 1991. A novel multigene family may encode odorant receptors: a molecular basis for odor recognition. Cell 65 (1): 175–187. [1]
  4. Niimura Y. & Nei M. 2003.. "Evolution of olfactory receptor genes in the human genome". Proceedings of the National Academy of Sciences of the United States of America 100 (21): 12235–40. doi:10.1073/pnas.1635157100. PMC 218742. PMID 14507991.
  5. Bieri, S.; Monastyrskaia, K; Schilling, B (2004). "Olfactory receptor neuron profiling using sandalwood odorants". Chemical Senses 29 (6): 483–7. doi:10.1093/chemse/bjh050. PMID 15269120.
  6. Fan, Jinhong; Ngai, John (2001). "Onset of odorant receptor gene expression during olfactory sensory neuron regeneration". Developmental Biology 229 (1): 119–27. doi:10.1006/dbio.2000.9972. PMID 11133158.

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