Olfactory System

Learning Outcomes:

  • Analyse the function of the olfactory system in terms of its use in everyday life.
  • Analyse the development of the olfactory system and its impact on performance in everyday life.
  • Describe the structure and function of the olfactory system, including the receptors, pathways and the olfactory cortex.

A bit of background to how we know about smell…

Much of what we know about the olfactory system comes from research and studies from the perfume, food and more recently marketing and branding industry.

Companies that pump the smell of warm bread and cakes onto their shop floor, before the bakery is even open, know that scent can make us feel happy and hungry and that content and hungry shoppers will buy.

Memories, imagination, old sentiments, and associations are more readily reached through the sense of smell than through any other channel. – Oliver Wendell Holmes

Physician and Poet

The sense of smell is a powerful chemical sense which is increasingly used to exploit its close connections and links to both our emotional and memory brain areas. Fragrances can be used to not only make us salivate but also to make us smile, increase or decrease our heart rate and smell can make us smile. Smells can quickly remind us about special times and unique people, whisking us up and away to sunny places and exotic food smells.

Smells, scent marketing and branding, is a growing trend in advertising. (Lindstrom, 2010). Brand Sense 2012 describes how despite us not always being aware that smell and scents can persuade us to buy products and how this is exploited and used to successfully market some of the world’s most successful brands. The power of smell to sell utilised widely in marketing, not just in the food industry.

While it is not fully understood why smell is such a powerful motivator and persuader of sales, perfumes and scents have been applied with success to marketing virtually every type of product. Abercrombie and Fitches use their particular new jeans smell to lure avid teens into shopping in their stores, while new car “upholstery and carpet” in-car smell canisters used to enhance cheaper car brands clench many deals.

Some smells are used to infer luxury and comfort. The exclusive leather smell used to promote Gucci and the luxury distinctive smell of a 1965 Rolls Royce. Luxury airlines who have patented their inflight fragrance and rely on the power to smell to relax and comfort anxious fliers and welcome back returning customers, reminding them of why they have chosen this particular airline.

Lindstrom goes as far as to say that fragrance is much more of a marketing tool these days than visual logos and flashing billboards, which can overwhelm the already overloaded visual systems of today. He argues persuasively that the is true of our auditory systems with theme tunes and advertising jingles and slogans competing with buzzes and blips of modern technology.

Read this excerpt from a study “The truth about Youth…” (http://mccann.com/wp-content/uploads/2012/06/McCann_Truth_About_Youth.pdf) “

“Given a list of things (including cosmetics, their car, their passport, their phone and their sense of smell) and told they could only save two, 53% of those aged 16-22 and 48% of those aged 23-30 would give up their own sense of smell if it meant they could keep an item of technology (most often their phone or laptop).

The youth’s top three highest rated global motivations were commendably commune, justice and authenticity however, worryingly all three of these motivations were underpinned and fueled by their relationship with technology. The study goes on to point out that technology is “now so intrinsic and fundamental that half of the young people would sooner give up one of their human senses than give up their technology. We all know how important technology is to young people, but a willingness to sacrifice one of their human senses to keep it shows just how intrinsic it has become.”

Despite compelling evidence from research that supports the fact that the olfactory system is such a unique and essential sense which can evoke strong memories and emotions, it has been under-represented in research to improve health and wellbeing, remaining a poor relative to visual, auditory even the use of proprioception in studies about alternative pain management. Aromatherapy and the use of smell in clinical settings remains largely relegated the realms of alternative rather than complementary therapies.

The Development of Our Olfactory System

To understand the functions of smell, it is vital to know something about how the sense of smell develops. In utero, a baby’s nose starts to develop after about seven weeks. By 9 weeks, small nostrils have begun to form and a week later the smell receptors are fully formed.

At the same time, the olfactory system is also beginning to develop. After 2 months the olfactory bulb has differentiated from the forebrain and the olfactory receptors reach maturity by the 29th – 30th weeks of gestation. Nasal plugs dissolve by the 36th week of pregnancy and within the womb, babies can then start to use their sense of smell alongside the closely linked sense of taste.  As they take in amniotic fluid through the mouth and nose, they are getting to know the smell and taste of mum, including it is now believed flavours from the mother’s diet. This may explain why some food preferences are familial or culturally and geographically driven.

Listen here…

http://www.npr.org/2011/08/08/139033757/babys-palate-and-food-memories-shaped-before-birth

This development of smell and taste receptors is essential to helping a newborn baby recognise its mother. Studies have shown that babies can recognise their mother from a panel of other mums by the smell of her breast milk alone.  Smell and its association with being contained and safe the womb, then being held and cuddled close while being fed may help explain why for us as humans smell, food, feeling safe, comforted and secure are so closely linked.

Neonates have a demonstrable capacity to learn the odour signature of their mother (Cernoch and Porter 1985; Schleidt and Genzel 1990; Schaal et al. 1998). When a parent cuddles their baby, and they both smell each other’s scent, the hormone oxytocin is released in higher levels. Galbally (2007) reports how studies have found correlations between oxytocin levels and attachment and bonding. Galbally is clear to point out this phenomenon is not restricted to only mother-baby interactions.

Very young infants are likely to have a small range of specific and familiar smells and may sneeze and wriggle when exposed to intense aromas and unfamiliar scents.  As the baby starts to move and explore more, the smell is likely to help the baby to know and recognise people alongside visual and auditory clues.

Weaning onto solid food and finger food means that the baby and later toddler, will start to use their senses of smell and taste to decide if they like something or not, and new food preferences start to appear. This increased ability to choose and refuse food can be seen in the happy responses to seeing but importantly smelling food being made. It is now that parents might notice how their child’s smell (and taste) preferences are similar to their own.

The sense of smell will continue to develop into later childhood and even adolescence.

Experiments conducted in the 70s and replicated in 90’s have shown that some odour sensitivity does not develop until they puberty. The 9-year-old subjects in both studies were insensitive to two musk odours, while ably detecting other odours in the same way as adolescents and adults, suggesting a possible link between these odours becoming discernible as reproductive maturity is reached.

What is the Function of Smell…

Stevenson 2009 suggests smell serves three primary functions

  • Eating and ingestion
  • Avoiding environmental hazards
  • Social communication

We will now explore these in further detail…

  1. Eating and Ingestion

The need to eat and ingest food begins in utero, you will learn more about this later. Young babies rely on smell, in conjunction with the tactile system, to help drive and elicit the adaptive behaviours necessary for survival. For example, this helps the baby find the breast or bottle to begin feeding. The rooting reflex is integral to the establishment of breastfeeding whether you are a baby or a kitten. Babies move to turn towards the smell of their Mum, a scent they know and recognise from in utero. This movement of the face towards the smell of mum facilitates the human rooting reflex, which is triggered when the corner of the baby’s mouth is stroked. The baby will turn his or her head and open his or her mouth and begins to suckle.

pexels-photo-895515

Smell can assist in our location of food from afar – we can all follow the wafting aroma of newly baked bread to a bakery, or across the dunes to a barbeque. Humans have the ability to follow an odour trail and the rising popularity of the street food experience on foreign holidays may attest to humans having retained and innate ability to do this. Once the food source is reached, the smell of the foodstuff can provide additional information about it’s suitability to be eaten.

Rotten meat, dairy, eggs and fruit have very distinctive smells that alert us to the food having spoiled long before it reaches the mouth. We can use smell in a more refined and discriminative way, sniffing a melon or pineapple to choose the ripest’ just right one’ to pick or purchase. Once in the mouth, odour molecules stimulated in the back of the throat must match the perception of what is being tasted. When a mismatch occurs, we will spit out the food substance. Have you ever bought or been given a coffee expecting tea…what is your immediate reflexive response?

The smell of a curry or roast dinner is appetising when we are hungry and research suggests that this response stimulates appetite and need to eat and that once we are full the smell is less pleasing and can even be unpleasant.  Research by Cabanac (1971) and Duclaux et al. (1973) has suggested that the process is restricted to only food-related odours, further evidence for the importance of smell to adequate food intake. The regulation of our food intake through smell does not stop here, it would appear that the smell of very palatable foods and energy-rich foods being prepared will encourage appetite and even craving, insuring increased intake. Probably a survival function, stockpiling calories for when times are less plentiful.

Humans and animals also appear to be able to learn about how much is enough of a particular flavour (taste and smell receptors are required to identify flavour), ensuring the just right amount of intake on the next occasion a portion of food is eaten.  The mechanisms behind this mechanism are complex and are not yet as well understood as a physiological need for certain substances e.g. salt might affect this need or craving for certain foods at particular times.

  1. Avoiding Environmental Hazards

The use of smell to ensure survival continues throughout life. The smell is protective, it acts as a general advance warning alarm system. We are wired to smell fire and damp rot and to retreat. Smell as a sense is a far sense, allowing for a retreat from the danger before contact with the potential hazard is more imminent.

Research recognises that responses to adverse smells can be divided into two basic human emotions; fear and disgust. Each emotion relates very clearly to a distinct hazard.

pexels-photo-3214347

Disgust:  Threats from exposure to bacteria or fermentation which could threaten survival including from urine, faeces, vomit and organic decay and deterioration. Anyone who has smelt gangrene or putifying meat will not easily forget the smell.

Emotion Fear (insert emoji)

Fear: These threats are more diverse and include things which may result in not just disgust, but also activation of the Autonomic nervous system (ANS) flight and fight responses. This consists of the smell of our enemies or predators, fire, musty or poor quality air and poisons.

Primitive man used smell to choose a dry cave over a damp one where illness and disease might fester more easily. Today most people will instinctively know and be put off buying a house that smells damp or musty – Is it possible that we have inherited unconsciously primitive memories perhaps reminding us that damp ‘caves’ might not be suitable for our health?

Throughout history, smell has been used to identify life and death, good and evil. Historical texts from across the globe are littered with examples;

  • The use of spices and perfumes to keep predators and evil spirits at bay or alternatively to encourage good fortune
  • The appreciation of the pure perfumes of the Greek gods
  • The story of Helen of Troy who became beautiful after receiving a recipe from the gods for a secret perfume recipe
  • The measurement of stench to identify witches
  • Sweet smells confirming the sanctity of saints

All stand testimony to the power of smell. Ancient medicine and medications have used smell to identify but also defeat disease and illness. Identification of diabetes mellitus (Mellitus means honey) by its sweet smell led to its name. Smelling salts were used to revive the fainting and herbed posies and garlic have battled plagues and smallpox.

  1. Social communication

We are unlikely to be attracted to someone who doesn’t have a pleasing/pleasant smell, a way to avoid disease and illness, perhaps helping us find the healthiest partner with which to have our babies.  However several independent studies point to evidence that smell is useful in assisting humans in avoid inbreeding; which carries with it an increased risk of genetic abnormalities and infant mortality. Studies by Shepher in 1983 and Wolf in 1995 have found that children raised together are unlikely to marry or have sex with each other in adulthood. The possible mechanism for this is early exposure to olfactory signatures of the other person during childhood is olfactory cues (Porter et al. 1986; Weisfeld et al. 2003; Olsson et al. 2006).

You may well be wondering how it is humans each have their own unique smell signature and how distinctive these are? Distinct odour profiles are probably determined by a person’s genes; human leukocyte antigens, meaning that close relatives may have a similar but a little different pattern. Some but not all studies have shown we may be more likely to choose a partner with a different HLA code.