The ability of a creature to survive and function depends on its perception – the physiological methods of perception are the senses. Traditionally humans are described as possessing five senses – sight, hearing, touch, smell and taste. However the definition and classification of senses can be relative, and in addition there are other senses – temperature, pain, balance, body awareness and internal senses, so we actually have more than five senses.

Although many other species have the same senses as humans, the capability of these senses may vary widely – for instance the sight of some species extends to different light frequencies – bees can perceive ultraviolet light and some snakes can perceive infra red light.  Many species have other senses that are not possessed by humans, including the ability to detect electric fields, the ability to detect magnetic fields, the ability to detect pressure and the ability to detect reflected sound.

The importance of each sense varies between species. Humans have evolved to depend on vision rather than smell, and consequently have a poor sense of smell compared to other primates and to many other species, yet our sense of smell still plays an important part in the way we perceive our surroundings.

Smell (olfaction) and taste (gustation) are activated by chemicals and are described as chemoreceptive senses. Our sense of smell is responsible for about 80% of what we can taste and enables us to detect flavours, our sense of taste can only detect 5 distinct sensations – sweet, salty, sour, bitter and umami (savoury).

Smells are perceived through the olfactory system. Individual odour receptor neurons in the olfactory system recognise a particular feature or class of odour molecules, these specific odour molecules if present “fit” into the specific receptors which then excite the olfactory receptor nerves and this stimulation produces signals which are sent to the brain. Air breathing and water dwelling species including mammals, birds, reptiles, fish and insects have olfactory systems and can detect chemicals in air and water respectively.

In humans, the chemoreceptors are located in the olfactory epithelium – a patch of tissue the size of a postage stamp located high in the nasal cavity. The sensory neurons are immersed in a layer of mucus, the airborne odour molecules enter the nasal cavity, then dissolve in the mucus and bind with the specific receptors, the response is conducted by the olfactory nerve to the brain.

The human sense of smell is quite feeble compared to some other species (eg we have about 5-6 million olfactory receptors, dogs have 220 million and have a sense of smell 1000 times more effective than ours) nevertheless we can still recognise an average of 4000 to10000 individual smells. Many scents are not consciously discerned, but may still effect moods eg pheromones. Each person has their own unique personal odour and their own  scent preference, but some scents are universally regarded as pleasant, for instance pure vanilla has a calming influence on humans and animals

People often remark that fragrances from their past can immediately trigger memories and evoke strong emotions, this is because the olfactory receptors are connected to the limbic system – the most ancient and primitive part of the brain which is thought to be the seat of emotion.

The role of human sense of smell tends to be undervalued; yet smells can effect our moods, well being, perceptions, performance and emotions, as well as enriching our appreciation of life around us.