The Olfactory System

Olfaction is the sensory system of smell. Olfaction, along with taste, is a form of chemoreception. The chemicals that activate the olfactory system, in general at very low concentrations, are called odorants.

Odorants

Odorants are small chemical compounds with an associated odor. They're small enough to be volatile, enabling signaling over long distances (i.e. can be transported into the olfactory system). Humans are sensitive to molecules between about 20 and 400 molecular weight. (E.g. alcohol is 46, sugar 342).

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Basic Anatomy

Each nostril contains, in its roof, a region known as the nasal mucosa. This contains the olfactory epithelium (type of tissue), which is covered in mucus.

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Odorants are carried along the mucosa until they hit the sensory neurons - the epithelium is kind of like the retina for the nose. It's 5-10cm2 in humans (much bigger in cats, and again in dogs).

Chemical receptors regenerate themselves continuously; olfactory receptors regenerate every 5-7 weeks.

Olfactory sensory neurons (olfactory receptor cells) are activated by their receptors and pass signals to the mitral cells of the olfactory bulb.

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From the olfactory bulb, neural signals get sent to the:

  • Piriform cortex (Primary Olfactory) in the temporal lobe
  • Oribitofrontal cortex (Secondary Olfactory) in the frontal lobe
  • Amygdala (Limbic system) - important for emotional/associative properties of olfactory cognition.

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Olfaction is unusual because it is not “gated” by the thalamus before the sensory signal enters the cortex.

Linking Chemical Shapes to Perceived Smell

Odor receptor nerve cells function like a key-lock system; If the airborne molecules of a certain chemical can fit into the 'lock', the nerve cell will respond. There are, at present, a number of competing theories regarding the mechanism of odor coding and perception.

Vibrations Theory

The original theory was Vibration Theory; the concept that the vibrations of a molecule caused receptors to fire and hence we perceive a scent. It is not the mainstream theory, but does explain why some odours are similar/different (but not intensity of odours).

Shape Theory

Moncrieff's 1951 Shape Theory suggests that the shape of a compound determines its scent. Problems with this include the lack of a convincing way to predict scent, and the existence of similar molecules with different smells, and distinct molecules with similar smells (e.g. pineapple).

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Amoore built upon this theory and proposed that 10,000+ smells could be composed of seven primary odors, including: sweaty, spermous, fishy, malty, urinous and musky.

Shape-Pattern Encoding

To deal with the problem of similar/different molecules/scents, we have the theory of coding across receptors. I.e. oderants are combinations of patterns that a receptor will recognise, meaning we need multiple receptors to each take a section of the smell and activate in various combinations. This combination is called a recognition profile - sometimes time order of activation is important, and sometimes specific receptors may be required.

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Identification of Odours

Human Odour Detection Thresholds

Our recognition threshold (the concentration of an amount needed to determine the quality of an odorant) is 3 times higher for recognition as opposed to detection of a scent, and discrimination between scents.

We can discriminate between 100,000 odours, but cannot label them all.

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We are better at recognising (and we prefer these) familiar smells; babies recognise their mother's scent within a few weeks, and will suck more if he/she smells their own mother rather than a stranger - Macfarlane, 1975

Odour Preferences

Odour preferences start very young. Steiner (1977, 1979) and Soussignan et al (1997) measured the facial expressions of 3-8 hour old infants in response to some food related odours and found averse facial responses to unpleasant smells (and concluded that some responses to odours are hard-wired).

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Human Odour Identification

Russel's 1976 study of undershirts worn for 24 hours without showring/deoderants revealed that 75% of the time people can identify their own odour, and identify the sex of an odour.

Fear vs Happiness

Chen & Haviland-Jones (2000) experiment on fear revealed that humans can identify the difference between odours of fear and happiness.

Scent-Tracking

Porter's 2007 Scent-Tracking Experiment revealed that the zig-zag pattern found in animals is also spontaneously exhibited in humans, presumably to counteract the quick adaptation to a scent.

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