Taste and Smell

1

What is the purpose of the chemosensory system?

We detect chemicals through three different sensory systems: taste, olfaction, and somatosensory. Taste refers to the sensation arising from taste receptors and is used to evaluate the nutritious content of food and avoid ingestion of toxic substances. Smell is the detection of volatile odorants though olfactory and somatosensory systems. Olfaction is the perception of odorants through activation of odorant receptors and is mediated by cranial nerve (CN) I. Smell is also important for social interactions and memory. Trigeminal somatosensory fibers detect thermal, mechanical, and chemical stimuli and initiate protective respiratory reflexes.

2

What are the consequences of taste or smell dysfunction?

Chemosensation is an integral aspect of how we interact with the environment and guides our behavior. Loss of these senses can lead to hazardous situations, such as food poisoning and the inability to detect fire or gas. The disruption of appetitive cues can lead to weight changes and nutritional deficiencies. People without taste often lose the desire to eat and may require medical intervention to restore their appetite.

3

What is the impact of taste or smell dysfunction on quality of life?

Olfactory dysfunction negatively impacts quality of life through eating behaviors, social behaviors, environmental hazard exposures, and mental health. There is a well-established relationship between olfaction, emotion, and memory. The loss of chemosensation impairs the ability to feel motivated and engage in pleasurable activities and is associated with lower perception of quality of life, changes in mood, and depression.

Social chemical cues play a role in determining our social behavior. Odorants are reported to influence mate selection and cause females to synchronize their menstrual cycle, indicating a biological importance for olfactory cues.

4

What is the relationship between taste, smell, and flavor?

The flavor of our food is the combination of taste, smell, and the somatosensory-mediated sensations of temperature, texture, and pungency. Patients presenting with taste complaints may actually suffer from olfactory dysfunction as a result.

5

Describe the trigeminal (CN V) contribution to smell.

The trigeminal system mediates the perception of touch, pressure, temperature, and nociception (pain or irritation) and displays a limited spectrum of sensations compared to olfaction. The ophthalmic and maxillary branches of the trigeminal nerve innervate the nasal cavity. Most odorants can activate the trigeminal system, and individuals with impaired olfaction (CN I) may still be able to detect odors (often strong irritating odors such as gasoline or ammonia) through trigeminal sensations.

6

What are the five basic tastes?

Salty, sour, sweet, bitter, and umami. Umami is the detection of L-amino acids and is also described as savory. Sweet is indicative of energy-rich foods. The detection of salt allows us to control proper dietary electrolytic balance. Sour and bitter are used to warn against noxious/poisonous compounds. Fatty taste may be considered a new, sixth taste.

7

Where are taste receptors located?

Taste receptors are located on taste receptor cells. Taste buds are bundles of taste receptor cells. Taste receptors are also found on specialized chemosensory cells, ciliated cells, and smooth muscle cells in the airway and are thought to mediate the perception of irritants. Taste-sensing of food also occurs within the gastrointestinal tract as taste receptors are expressed by enteroendocrine cells.

8

Where are taste buds located?

Taste buds are located on a large portion of the tongue dorsum within small protrusions of epithelium called papillae. Taste buds are also found on the soft palate, larynx, pharynx, and epiglottis, and these taste buds are innervated by the vagus nerve (CN X).