Anatomy and Physiology: Special Senses – TASTE and SMELL
The nose has two primary functions. The first is olfaction – the sense of smell. However, the second function is of primary interest to this discussion – filtration, heating and humidification of the inhaled air. To accomplish the second task, the nasal cavity contains a convoluted set of passageways called the turbinates on the lateral wall of each nasal cavity.
The nose performs other various functions such as:
- Air conditioning of the inspired air.
- Filtration of the inspired air.
- Olfaction or sense of smell.
- Resonating the spoken voice.
- Draining the para nasal sinuses and the nasolacrimal duct.
ANATOMY OF THE NOSE
The nose consists of the following:
- External nose – triangular-shaped projection in the center of the face.
- Nostrils – The two openings into the nasal passages.
- Nasal passages – passages that are lined with mucous membranes and tiny hairs (cilia) that help to filter the air and move nasal and sinus mucous to the back of the throat. Nasal passages are separated by the nasal septum.
- Septum – made up of cartilage and bone and covered by mucous membranes. The cartilage also gives support to the lower part of the nose and divides the nasal passages into right and left sides.
- Sinuses – four-paired, air-filled cavities.
OLFACTORY RECEPTORS AND THE SENSE OF SMELL
Chemoreceptors are those that respond to chemicals in solution for taste and olfaction. The olfactory receptors are believed to be sensitive to a much wider range of chemicals. Thousands of olfactory receptors occupy a postage stamp-sized area in the roof of each nasal cavity. These are the receptors for the sense of smell.
Air entering the nasal cavities must make a hairpin turn to enter the respiratory passageway below which causes more air to flow superiorly across the olfactory receptors, thus, intensifying the sense of smell. Olfactory receptor cells are special neurons that are equipped with olfactory hairs. Olfactory hairs are long cilia that protrude from the nasal epithelium and are continually bathed by a layer of mucus secreted by underlying glands.
When the receptors are stimulated by chemicals that are dissolved in the mucus, they transmit impulses along the olfactory filaments, which collectively make up the olfactory nerve (cranial nerve I) to the olfactory complex of the brain. It is in the brain that the interpretation of the odor occurs. The olfactory pathways are closely tied into the limbic system, the emotional-visceral part of the brain. Hence, olfactory impressions are long lasting and are very much part of our memories and emotions.