The Sense Of Taste (Gustation)
Introduction
Smell and taste are generally classified as visceral senses because of their close association with gastrointestinal function. Physiologically, they are related to each other. The flavors of foods are in large part a combination of their taste and smell. Food may taste different if one has a cold that depresses the sense of smell. Both taste and smell receptors are chemoreceptors. They are stimulated by molecules in solution in mucus in the nose and saliva in the mouth.
The Tongue
The tongue is a muscular organ on the floors of the mouths of most vertebrates which manipulates food for mastication. It is the primary organ of taste (gustation), as much of the upper surface of the tongue is covered in papillae and taste buds. It is sensitive and kept moist by saliva, and is richly supplied with nerves and blood vessels.
Taste Buds
The taste bud is composed of about 50 modified epithelial cells, some of which are supporting cells called sustentacular cells and others of which are taste cells. The taste cells are continually being replaced by mitotic division of surrounding epithelial cells. The outer tips of the taste cells are arranged around a minute taste pore. From the tip of each taste cell, several microvilli or taste hairs project outward into the taste pore to approach the cavity of the mouth. Interwoven around the bodies of the taste cells is a branching terminal network of taste nerve fibers that are stimulated by the taste receptor cells. Each taste bud is innervated by about 50 nerve fibers. Each nerve fiber receives input from 5 taste buds.
Location
A large number of taste buds surround the circumvallate papillae, which form a V-line on the surface of the posterior tongue. Moderate numbers of taste buds are on the fungiform papillae over the flat anterior surface of the tongue. Moderate numbers are on the foliate papillae located along the lateral surfaces of the tongue. Additional taste buds are located on the palate, and a few are found on the tonsillar pillars, on the epiglottis, and even in the proximal oesophagus. Adults have 3,000 to 10,000 taste buds, and children have a few more. Beyond the age of 45 years, many taste buds degenerate, causing the taste sensation to become progressively decreased in old age. Single taste buds show that each taste bud usually responds mostly to one of the 5 primary taste stimuli when the taste substance is in low concentration. But at high concentration, most buds can be excited by 2 or more of the primary taste stimuli.
Primary Sensations
There are 13 possible chemical receptors in the taste cells:
- 2 sodium receptors
- 2 potassium receptors
- 1 chloride receptor
- 1 adenosine receptor
- 1 inosine receptor
- 2 sweet receptors
- 2 bitter receptors
- 1 glutamate receptor
- 1 hydrogen ion receptor
The primary taste sensations are:
- Sour
- Salty
- Sweet
- Bitter
- Umami
Sour Taste
The sour taste is caused by acids, that is, by the hydrogen ion concentration. The intensity of this taste sensation is approximately proportional to the logarithm of the hydrogen ion concentration. The more acidic the food, the stronger the sour sensation becomes.
Salty Taste
The salty taste is elicited by ionized salts, mainly by the sodium ion concentration. The cations of the salts, especially sodium cations, are mainly responsible for the salty taste. The anions also contribute to a lesser extent.
Sweet Taste
Sweet taste is caused mainly by organic chemicals such as:
- Sugars
- Glycols
- Alcohols
- Aldehydes
- Ketones
- Amides
- Esters
- Some amino acids
- Some small proteins
- Sulfonic acids
- Halogenated acids; and
- Inorganic salts of lead and beryllium
Bitter Taste
It is caused by organic substances:
- Long-chain organic substances that contain nitrogen
- Alkaloids – quinine, caffeine, strychnine, and nicotine
The bitter taste, when it occurs in high intensity, usually causes the person or animal to reject the food (toxins).
Umami Taste
Umami is a Japanese word (meaning delicious) designating a pleasant taste sensation that is quantitatively different from sour, salty, sweet, or bitter. Umami is the dominant taste of food containing L-glutamate, such as meat extracts and aging cheese. The threshold for stimulation of the bitter taste is lowest. This sensation provides an important protective function against many dangerous toxins in food.
Taste Pathways
Taste impulses from the anterior 2/3 of the tongue pass first into the lingual nerve, chorda tympani, the facial nerve, and the tractus solitarius in the brain stem. The posterior 1/3 of the tongue uses the glossopharyngeal nerve to the tractus solitarius at a slightly more posterior level. Taste from the posteriormost part of the tongue and pharynx is transmitted via the vagus nerve to the tractus solitarius.
From the nucleus of the tractus solitarius, second-order neurons project to the ventral posterior medial nucleus of the thalamus. Third-order neurons then relay taste information from the thalamus to the lower tip of the postcentral gyrus in the parietal cerebral cortex.
From the tractus solitarius, many signals are transmitted within the brain stem itself directly into the superior and inferior salivatory nuclei. These areas transmit signals to the submandibular, sublingual, and parotid glands to help control the secretion of saliva during the ingestion and digestion of food.
Mechanism Of Stimulation
Binding of the taste chemical to a protein receptor molecule that lies on the outer surface of the taste receptor cell near to or protruding through a villus membrane opens ion channels. This allows positively charged sodium ions or hydrogen ions to enter and depolarize the normal negativity of the cell. Then the taste chemical itself is gradually washed away from the taste villus by the saliva, which removes the stimulus.
Factors Influencing Taste Sensation
- Area of stimulation: The threshold for taste decreases as the area of application of the stimulus increases.
- Temperature of substance: Maximum sensitivity to taste is obtained at 30-40 °C.
- Olfaction: Smell significantly influences the flavor of food.
- Individual variation: There is a decrease in taste sensitivity in older people due to atrophy of taste buds.
- Sex: Women are more sensitive to sweet and salt and less sensitive to sour than men.
- Adaptation: Taste buds adapt quickly to a particular taste.
- Interaction between taste-producing substances: The interaction between different taste-producing substances can affect taste sensation (for example, very sour lemon juice tastes good when mixed with sugar).
"Affective" Nature Of Taste
Pleasantness and unpleasantness are called affective attributes of a sensation. These attributes influence an individual’s selection or rejection of food. Sweet is unpleasant at a very low concentration but very pleasant at high concentration. Sour and bitter are pleasant at low concentrations but unpleasant at high concentrations.
Applied Physiology
Ageusia
Ageusia is the absence of taste sensation. It can be congenital or due to a lesion of the facial or glossopharyngeal nerve.
Hypogeusia
Hypogeusia is a diminished taste sensation due to some diseases.
Dysgeusia
Dysgeusia is an altered taste sensation. It is usually associated with temporal lobe syndrome.
Familial Dysautonomia
Familial dysautonomia is a very rare condition where a very high concentration of glucose fails to produce a sweet sensation.
Selective Taste Blindness
Selective taste blindness is an inherited Mendelian recessive trait. Individuals with this condition have a very high increase in the threshold to bitter taste, but all other taste sensations are normal to them.
Causes
Miscellaneous causes:
- Aging – difficulty detecting salty or bitter taste
- Anxiety disorder
- Cancer
- Renal failure
- Hepatic failure
- Endocrine disorders
- Cushing’s syndrome
- Hypothyroidism
- Diabetes Mellitus
- Deficiency
- Vitamin B3 deficiency
- Zinc deficiency
- Local injury or inflammation
- Radiation therapy
- Glossitis
- Tobacco abuse
- Denture use
Medications:
- Antirheumatic drugs (e.g., Penicillamine)
- Antiproliferative drugs (e.g., Cisplatin)
- ACE inhibitors
- Clarithromycin
- Zopiclone
Neurologic conditions:
- Bell’s palsy
- Familial dysautonomia
- Multiple sclerosis
Recent:
- Saccharin is 600 times as sweet as glucose.
- Miraculin is a taste-modifying protein obtained from miracle fruit in Africa. After eating this fruit, sour substances taste sweet (for example, lemon tastes like orange).
