Chapter 2: The chemical senses
Smell (pp. 39—44)
- What is the function of sniffing to detect odours?
- Why are smells more noticeable after a rainstorm on a hot day?
- Does olfaction play an unconscious role in human mate selection?
What you need to know
- The Anatomy and Physiology of Smell (pp. 40—42)
- Sensory pathways
- Cortical processing
- The Perception of Smell (pp. 42—44)
- Detection and identification
- Theories of odour identification
The Anatomy and Physiology of Smell
The human olfactory epithelium contains about 6 million olfactory receptor cells, each projecting 10-20 cilia (hair cells) into the olfactory mucosa.
Airborne molecules dissolve in the mucosa, arrive at the cilia, and allow olfactory receptor neurons to neurally encode the chemical composition of odours.
Humans are thought to have 500-1000 different types of olfactory receptors, each sensitive to different odour molecules.
The lifetime of an olfactory receptor cell is approximately 60 days.
Free nerve endings are also found in the mucosa, and these probably signal coolness, tingling, and burning sensations.
Olfactory receptors synapse with mitral cells in the olfactory bulb. The synapses between receptor cell axons and mitral cell dendrites accumulate to form olfactory glomeruli. Unlike the random distribution of olfactory receptors, the spatial arrangement of mitral cell dendrites is identical within species.
Each mitral cell receives approximately 200 signals, but only from the same type of olfactory receptor.
Olfaction is unique among the senses in that:
- Mitral cell projections to the primary olfactory cortexdo not go via the thalamus.
- Mitral cells connect directly to the amygdala (associated with generating emotional responses).
The Perception of Smell
The concentration for threshold detection of a smell is odour-dependent, and can vary by a factor of 1 million. At this strength, identification is poor.
Humans can recognise up to 10,000 different smells. Cain (1982; see FP p. 42) showed that women outperform men in approximately 80% of odour identification tasks. The Howard Hughes Medical Institute (external link), gives information about the possible social and sexual chemical signals in smells.
A simple linking proposition for odour identification suggests that each odour activates one type of olfactory receptor, and that activity in different receptors corresponds to different perceived smells. This view is too simplistic because:
- There are insufficient types of olfactory receptors (500-1000) to match the number of recognisable odours (10,000).
- Sicard and Holley (1984; see FP p. 43) demonstrated that a specific odour activates not just one but a variety of different receptor types.
A more sophisticated population-coding theory suggests each odour is identified by matching the 'signature of activity' it causes in a population of olfactory receptors, against stored 'signatures' of known odours. This is consistent with:
- Sicard and Holley's (1984; see FP p. 43) finding that a specific odour excites different types of olfactory receptors by different amounts.
- The fact that mitral cells have a consistent anatomical arrangement. This simplifies the task of odour identification to the recognition of a certain spatial distribution of activity.
Refer to the research study by Malnic et al. (1999) for a more recent and comprehensive interpretation of odour identification.
Research Study: Combinatorial receptor codes for odours, Malnic et al. (1999)
Continuous exposure to a smell causes temporary, odour-specific adaptation.
Anosmia can result from a head injury that damages the olfactory nerve where it passes through the cribriform plate. Partial anosmia is an inability to detect a particular odour, and probably reflects a genetic deficiency in a specific olfactory receptor molecule.
So What Does This Mean?
Olfaction is mediated by up to 1000 different types of chemoreceptors, each sensitive to different odour molecules. Since a specific smell activates a variety of different types of olfactory receptors, odour identification is best considered in terms of a population coding theory that recognises the 'signature of activity' across a population of sensory neurons.