Introduction  Smell is one of the chemical senses  The sense of smell is also called Olfaction  the molecules we perceive as smells are called Odorants, Those molecules are generally light, volatile (easy to evaporate) chemicals that float through the air into your nose.

Nose Anatomy ( smell related)  Nose is the sense organ that detects smell. There are millions of smell receptors inside our nose.  Nasal cavity : A hollow portion of head behind nose warms air, filters out dust and houses the sense of smell

Nasal Epithelium  Olfactory epithelium - Area of nasal cavity with olfactory receptors  In the roof of each nostril is a region called the nasal mucosa. This region contains the olfactory epithelium covered by mucus.  The Olfactory epithelium is about the size of a dime or a postage stamp.  In humans, the olfactory epithelium is located about 7 cm up and into the nose from the nostrils.  Chemicals we breathe pass by the receptors found in the epithelium on their way to the lungs.

Cont…

Cont…  The nasal epithelium contains the receptor cells – about 10 million in humans  They possess a terminal enlargement a knob that projects above the epithelial surface, from which extend about 8-20 olfactory cilia.  These cilia contain the smell receptors. (They do not move like in the ear)

Nasal Epithelium

Olfactory Bulb  Olfactory bulb - organ which houses all the nerves which receive inputs from the olfactory receptors

Olfactory Bulb

Olfactory pathway  There are approximately 20 olfactory foramina on either side of the nose in the cribiform plate of the ethmoid bone passes 40 or so bundles of axons form right and left olfactory nerves  They terminate in the olfactory bulbs – below the frontal lobes of the cerebrum  Axons of the olfactory bulbs form the olfactory tract which projects to the primary olfactory area of the cerebral cortex.  Some project into the limbic system and hypothalamus (emotional and memory evoked responses).  Olfactory sensations are the only sensations that reach the cerebral cortex without first synapsing in the thalamus.  The primary olfactory area has axons that extend to the orbit frontal area (frontal lobe) – region for odor identification.

Olfactory pathway

Experiment  Objective To test the ability to identify familiar aromatic odours with one nostril at a time with eye closed  Equipment Vials of aromatic substance e.g. fresh instant coffee, orange extract, peppermint extract and clove oil

Cont…  Procedure: Before starting to check the sense of olfaction the examiner should ensure that both nasal passages are patent. Ask the subject to close one nostril, inhale and then exhale. The eye of the subject should be closed. For testing the sense of smell, the examiner should have four small bottles, one containing some oil of clove, second peppermint, third fresh instant coffee and the fourth extract of orange. First of all use the least irritating substance, after closing one nostril and open vial of the volatile substance is put close to the nostril and the subject is asked to inhale and identify the odour. The same procedure is repeated in the other nostril. Before starting with the other odour allow the subject to rest for one minute.

chemicals This is because when food is heated, more chemicals in food diffuse into the air. Do you Know? Why do warm foods have stronger flavour than cold foods?

Smell Adaptation We can get used to a smell. smell receptors After we smell something for some time, the smell receptors stop sending messages to the brain

Children and sense of smell  Although smell-identification ability increases during childhood, even newborn infants are highly sensitive to some important smells  recent research shows that newborn babies locate their mothers nipples by smell

Smell & Emotion  Our olfactory receptors are directly connected to the limbic system, the most ancient and primitive part of the brain, which is thought to be the seat of emotion.

Adding Points  There are more than different smells in the world, The human nose can detect 4000 different types of them  Olfactory sensitivity is determined by the number of receptors in the epithelium

Clinical points Anosmia – inability to smell Dysosmia – things smell different than they should Hyperosmia – an abnormally acute sense of smell. Hyposmia – decreased ability to smell Parosmia – things smell worse than they should Phantosmia – "hallucinated smell," often unpleasant in nature

What causes smell disorders?  Sinus and other upper respiratory infections  Polyps in the nasal cavities  Frontal head injuries  Dental problems  Exposure to certain chemicals, such as insecticides and solvents  Numerous medications, including some common antibiotics and antihistamines  Radiation associated with the treatment of head and neck cancers  Aging  Other health issues that affect the nervous system, such as Parkinson’s disease or Alzheimer’s disease