Olfactory Sensation

            Olfactory system is responsible for our sense of smell. This sense, also known as olfaction, is one of our five main senses and involves the detection and identification of molecules in the air. Once detected by sensory organs, nerve signals are sent to the brain where the signals are processed. Our sense of smell is closely linked our sense of taste as both rely on the perception of molecules. It is our sense of smell that allows us to detect the flavors in the foods we eat. Olfaction is one of our most powerful senses. Our sense of smell can ignite memories as well as influence our mood and behavior.

Our sense of smell is a complex process that depends on sensory organs, nerves, and the brain. Structures of the olfactory system include:

·         Nose: opening containing nasal passages that allows outside air to flow into the nasal cavity. Also a component of the respiratory system, it humidifies, filters, and warms the air inside the nose.

·         Nasal cavity: cavity divided by the nasal septum into left and right passages. It is lined with mucosa.

·         Olfactory epithelium: specialized type of epithelial tissue in nasal cavities that contains olfactory nerve cells and receptor nerve cells. These cells send impulses to the olfactory bulb.

·         Cribriform plate: a porous extension of the ethmoid bone, which separates the nasal cavity from the brain. Olfactory nerve fibers extend through the holes in the cribriform to reach the olfactory bulbs.

·         Olfactory nerve: nerve (first cranial nerve) involved in olfaction. Olfactory nerve fibers extend from the mucous membrane, through the cribriform plate, to the olfactory bulbs.

·         Olfactory bulbs: bulb-shaped structures in the forebrain where olfactory nerves end and the olfactory tract begins.

·         Olfactory tract: band of nerve fibers that extend from each olfactory bulb to the olfactory cortex of the brain.

·         Olfactory cortex: area of the cerebral cortex that processes information about odours and receives nerve signals from the olfactory bulbs.

Our Sense of Smell:

Our sense of smell works by the detection of odours. Olfactory epithelium located in the nose contains millions of chemical receptors that detect odours. When we sniff, chemicals in the air are dissolved in mucus. Odour receptor neurons in olfactory epithelium detect these odours and send the signals on to the olfactory bulbs. These signals are then sent along olfactory tracts to the olfactory cortex of the brain through sensory transduction. The olfactory cortex is vital for the processing and perception of odour. It is located in the temporal lobe of the brain, which is involved in organizing sensory input. The olfactory cortex is also a component of the limbic system. This system is involved in the processing of our emotions, survival instincts, and memory formation. The olfactory cortex has connections with other limbic system structures such as the amygdala, hippocampus, and hypothalamus. The amygdala is involved in forming emotional responses (particularly fear responses) and memories, the hippocampus indexes and stores memories, and the hypothalamus regulates emotional responses. It is the limbic system that connects senses, such as odours, to our memories and emotions.

Sense of Smell and Emotions:

The connection between our sense of smell and emotions is unlike that of the other senses because olfactory system nerves connect directly to brain structures of the limbic system. Odours can trigger both positive and negative emotions as aromas are associated with specific memories. Additionally, studies have demonstrated that the emotional expressions of others can influence our olfactory sense. This is due to activity of an area of the brain known as the piriform cortex which is activated prior to odour sensation. The piriform cortex processes visual information and creates an expectation that a particular fragrance will smell pleasant or unpleasant. Therefore, when we see a person with a disgusted facial expression before sensing an odour, there is an expectation that the odour is unpleasant. This expectation influences how we perceive the odour.

Odour Pathways:

Odours are detected through two pathways. The first is the orthonasal pathway which involves odours that are sniffed in through the nose. The second is the retronasal pathway which is a pathway that connects the top of the throat to the nasal cavity. In the orthonasal pathway, odours that enter the nasal passages and are detected by chemical receptors in the nose. The retronasal pathway involves aromas that are contained within the foods we eat. As we chew food, odours are released that travel through the retronasal pathway connecting the throat to the nasal cavity. Once in the nasal cavity, these chemicals are detected by olfactory receptor cells in the nose. Should the retronasal pathway become blocked, the aromas in foods we eat cannot reach odour detecting cells in the nose. As such, the flavors in the food cannot be detected. This often happens when a person has a cold or sinus infection.

Smell Disorders:

Individuals with smell disorders have difficulty detecting or perceiving odours. These difficulties may result from factors such as smoking, aging, upper respiratory infection, head injury, and exposure to chemicals or radiation. Anosmia is a condition defined by the inability to detect odours. Other types of smell defects include parosmia (a distorted perception of odours) and phantosmia (odours are hallucinated.) Hyposmia, the diminished sense of smell, is also linked to the development of neurodegenerative diseases such as Parkinson's and Alzheimer's disease.