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Sense of smell

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Author Marion Richardson, BD, DipN, Cert Ed, RNT, RN, is principal lecturer, University of Hertfordshire.

This article continues our series on the senses and looks at the sense of smell.

Our sense of smell, or olfactory sense, allows us to detect small differences in odours. Smell may be an aid to survival as many noxious volatile materials have their own characteristic smell (Allan, 2005). It certainly appears to be involved in sexual attraction as we, like many other animals and insects, secrete chemicals when sexually excited. Although our sense of smell is far less acute than that of many animals, people do earn their living from using their sense of smell – for example as wine tasters or perfumiers. Smells are often linked with memories and a particular smell can trigger recollections of the past.

Keywords: Anatomy and Physiology, Olfactory, Smell

Author Marion Richardson, BD, DipN, Cert Ed, RNT, RN, is principal lecturer, University of Hertfordshire.

This article continues our series on the senses and looks at the sense of smell.

Our sense of smell, or olfactory sense, allows us to detect small differences in odours. Smell may be an aid to survival as many noxious volatile materials have their own characteristic smell (Allan, 2005). It certainly appears to be involved in sexual attraction as we, like many other animals and insects, secrete chemicals when sexually excited. Although our sense of smell is far less acute than that of many animals, people do earn their living from using their sense of smell – for example as wine tasters or perfumiers. Smells are often linked with memories and a particular smell can trigger recollections of the past.


In the nose
The ability to smell depends on the detection of chemicals in solution in the nose. The area concerned, the olfactory epithelium, is yellowish in colour, about 5cm2 and found in the roof of the nasal cavity where it extends to cover the superior nasal concha on each side of the septum (Fig 1).

The epithelium contains millions of olfactory receptor cells, shaped rather like bowling pins, which are supported and protected by columnar cells (it is these that contain the yellow-brown pigment that gives the epithelium its colour). Olfactory glands are also found here and these help to produce the thick mucus that covers the area in a thin layer. This mucus traps and dissolves airborne odour chemicals. It is constantly renewed and ‘old’ odour molecules are washed away.

Air drawn in through the nose swirls around the nasal cavity, bringing airborne chemicals to the olfactory area. Because of the position of the olfactory apparatus, a normal inspiration brings only about 2% of the inspired air in contact with the olfactory organs (Martini, 2005). Sniffing repeatedly draws more air to the area and so increases the stimulation of the receptors and thus the sense of smell.


Olfactory receptors
Between 10 and 20 million olfactory receptor cells are packed into the 5cm2 of the olfactory receptor area (Marieb, 2006). The olfactory receptor cells are unusual neurones that each end in a bulb from which protrude about 20 long cilia. The cilia vastly increase the receptive surface area and usually lie flat, covered by the mucus layer (this, too, is unusual as most of the cilia in other areas of the body beat rapidly and in a coordinated way). The receptor cells are further unique in that they last only about 60 days and are then replaced (Marieb, 2006).


Specificity of the olfactory receptors
Odours can be made up of hundreds of different chemicals and this makes it difficult to understand how we classify smell and determine whether different receptors detect different odours.

Humans can distinguish around 10,000 chemicals but our olfactory receptors seem to be stimulated by different combinations of a more limited number of ‘primary odours’ or olfactory qualities (Marieb, 2006). There is debate about the number of primary odours, with hypotheses ranging from seven (floral, musk, camphorous, pepperminty, ethereal, pungent stinging and putrid) to 50 different odour groups. It seems that we have at least 1,000 ‘smell genes’ that are active only in the nose and that each gene codes for a unique receptor protein. Odours seem to bind to several different receptors but the pattern of regional variation in the receptors is not clear (Marieb, 2006; Allan, 2005).

Some smells actually cause pain – pain receptors in the nose are stimulated by irritants such as the sharp smell of ammonia, the heat of chilli peppers and the ‘cold’ sensation of menthol (Marieb, 2006). The impulses from these receptors do not follow the ‘smell’ pathway to the brain but activate the trigeminal (Vth cranial) nerve.


Activation of the olfactory receptors
For us to be able to smell a particular odour, the chemicals that make up the odour must be volatile, that is, in a gaseous state, so they can be carried into the nose on the airstream. The chemicals must also be soluble so they can dissolve in the mucus covering the olfactory receptors. The dissolved chemicals bind to protein receptors on the surface of the olfactory membrane and set off discharges along the neurone system.

Our olfactory system is very sensitive and as few as four molecules of an odiferous substance can activate an olfactory receptor, although we are not necessarily aware of this happening (Martini, 2005).


Pathways to the brain
The fine, unmyelinated axons of the olfactory receptor cells gather into small fascicles that group together and form the olfactory nerve (Ist cranial nerve).

The olfactory nerves transmit impulses to two small, oval-shaped olfactory bulbs that lie under the frontal poles of the brain (Fig 2). In the olfactory bulbs, the neurones synapse with and pass information to mitral cells which are found in complex glomeruli in the bulbs. These mitral cells then collate the incoming information and relay it along the olfactory tracts to the brain.

The way in which we code smells is unclear but it seems that each glomerulus receives only one type of odour signal. A complex odour would activate several glomeruli and the pattern of nerve stimulation to the brain would be different for each smell. But this is still largely supposition.

Cells in the olfactory bulbs also receive information from the brain, which modify our reaction to smell in certain situations. For example, how we perceive food smells depends on whether we are hungry or full.

Once the mitral cells are activated, impulses flow along the olfactory tracts to two main areas of the brain. They go via the thalamus to the olfactory cortex (in the piriform and frontal lobes) where smells can be consciously recognised and interpreted.

They also go via the subcortical route to the hypothalamus, amygdala and other regions of the limbic system, where we analyse and respond to the emotional aspects of food (Marieb, 2006). It is these connections that account for emotional and behavioural responses to some smells:
- Smells associated with danger, such as smoke, trigger the sympathetic fight or flight response.
- Appetising odours cause increased salivation and stimulation of the digestive tract.
- Unpleasant odours can trigger protective mechanisms such as sneezing and choking.
- Substances with a sharp smell, such as ammonia, cause reflex cessation of breathing.
- Some odours trigger sexual responses. The perfume industry invests heavily developing such odours (Martini, 2005).

Adaptation to smell occurs very quickly (within a minute or two) in the brain, rather than in the nose. When a smell is constant, we soon stop noticing it, though new smells will stimulate the nerves and reach the brain.


Problems with smell
Anosmia (no sense of smell) can have a number of causes Marieb (2006):
- Genetic factors;
- Head injuries that tear olfactory nerves;
- After-effects of nasal cavity inflammation due to cold, allergy or smoking;
- Obstruction in the nasal cavity (for example polyps);
- Age;
- Zinc deficiency – this accounts for one-third of all anosmias and is soon cured with zinc supplements. Zinc is a growth factor for the receptors of both smell and taste.

Changes in our ability to smell are usually a sign of an underlying disorder in the brain.
Uncinate fits are olfactory hallucinations during which people experience a particular and usually unpleasant odour, such as rotting meat. Many of these fits are caused by irritation of the olfactory pathway by head injury or brain surgery (Marieb, 2006). Some people with epilepsy experience olfactory auras prior to a fit, which are a type of uncinate fit.

- Part two of this series, which looks at the sense of taste, will be published in next week’s issue.


References

Allan, D. (2005) The special senses.
In: Montague, S. et al, Physiology for Nursing Practice. Edinburgh: Elsevier.

Marieb, E, (2006) Essentials of Human Anatomy and Physiology. San Francisco, CA: Pearson Benjamin Cummings.

Martini, F.H. (2005) Fundamentals of Anatomy and Physiology. New Jersey, NJ: Benjamin Cummings.

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