Overview
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:
- Respiration.
- 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.
The olfactory receptors are extremely sensitive. Only a few molecules are required to activate them. Olfactory neurons tend to adapt rather quickly when they are exposed to unchanging stimulus of odor. This is the main reason why a woman stops smelling her own perfume but quickly picks up the scent of another person’s perfume.
TASTE BUDS AND SENSE OF TASTE
The tongue is a versatile organ with specialized functions like taste and speech. Beneath a cover of taste buds, the tongue is almost entirely made up of muscle. The muscles of the tongue are essential for its bodily movement and intrinsic manipulations, required for actions like speech, articulation, deglutition or swallowing, whistling, licking, kissing and even cleaning teeth.
Taste is the brain’s interpretation of chemicals that trigger receptors on the tongue, which are housed in the taste buds. The basic chemical components, are found in foods, toxins, and other ingested matter. Unappealing tastes are usually associated with toxins, as this is a defense mechanism preventing consumption. The chemicals bind their particular receptors and initiate signaling that travels through the nerves to the brain, where they are interpreted.
Taste Buds
The taste buds are specific receptors for the sense of taste which are widely distributed in the oral cavity. Of the 10,000 taste buds that humans have, most are located in the tongue. Few are found on the soft palate and inner surface of the cheeks.
Papillae – small peg-like projections that covers the dorsal surface of the tongue. These peg-like projections are of three types namely:
- Sharp filiform papillae
- Rounded fungiform papillae
- Circumvallate papillae
Taste buds are found on the sides of the circumvallate papillae but are more numerous on the fungiform papillae. When a person eats something, the specific cells that respond to the chemical dissolved in the saliva are epithelial cells called gustatory cells. Gustatory cells are surrounded by supporting cells in the taste bud. Their long microvilli, the gustatory hairs, protrude through the tastepore and when they are stimulated, they depolarize and impulses are transmitted to the brain. To carry the taste impulses three cranial nerves transports it to the gustatory cortex namely:
- Cranial nerve VII
- Cranial nerve IX
- Cranial nerve X
image courtesy of medicalook.com, health.howstuffworks.com
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