Physiology of Taste Sensation By Dr Abdel Aziz

Physiology of Taste Sensation By Dr. Abdel Aziz M. Hussein Lecturer of Physiology Member of American Society of Physiology

Taste Sense (Gustation) • Taste sensation is mainly the function of the taste buds present in the tongue and mouth.

Basic Taste Modalities • There are 5 established basic tastes: 1) Sour Taste: • Caused by acids • Tasted by edges of the tongue. 2) Salty Taste: • Elicited by ionized salts (Na. Cl) • Tasted by anterior part of the dorsum of the tongue. 3) Sweet Taste: • Elicited by organic chemicals e. g. sugars, glycols • Tasted by the tip of the tongue

Basic Taste Modalities

Basic Taste Modalities 4) Bitter Taste: • Caused by: a. Long-chain organic substances b. Alkaloids e. g. quinine, caffeine, strychnine, and nicotine. • Tasted by the back of the tongue • N. B. : Some substances that at first taste sweet have a bitter after taste e. g. saccharin. 5) Umami Taste: • Is the dominant taste of food containing L-glutamate (particularly monosodium glutamate (MSG), such as meat extracts and aging cheese. • Tasted by all parts of the tongue.

Taste buds or Receptors Site: • Taste buds are oval structures present in; a. Lateral walls of ; • Fungiform papillae → present in tip and edges of tongue • Circumvallate papillae → present in V-shaped area in back of tongue the b. Mucosa of palate, epiglottis and pharynx • Filliform papillae → present on the dorsum of the tongue don’t contain taste buds. No. • Adult tongue contains 10, 000 taste buds

Taste buds or Receptors Structure: • Each bud is formed of 3 types of cells; 1. Supporting or sustentacular cells. 2. Gustatory receptor cells: • Modified epithelial cells. • Each taste bud contains 50 -150 receptor cells. • Their upper poles containing cilia that project from the taste pore. • Sensory nerve endings synapse with their lower poles. • Their life span is 1 -2 week (10 days) 3. Basal cells: • They are stem cells for production of new receptor cells.

Mechanism of stimulation of Taste Receptors • For tastant to bind to the taste receptor, it must be in a solution or dissolved in saliva. • Taste binding protein (TBP) (produced by Ebner's glands) transports the tastant and concentrates it at taste buds. • Tastant binds to its specific receptors in the microvilli of gustatory cells. • Binding of the tastant to a taste receptor opens or closes ion channels, causing potential changes in the taste receptor cell and release of the chemical transmitter e. g. glutamate which excite the nerve fibers to generate nerve impulses. • For each tastant, there is a different mechanism

Mechanism of stimulation of Taste Receptors

Taste threshold and intensity discrimination • The ability of humans to discriminate differences in the intensity of taste is limited. • A 30% change in the intensity of the substance being tasted is necessary before an intensity difference can be detected. • The threshold for taste varies widely in different individuals. • It also depends on the substance tasted; the next table shows the taste threshold for some compounds

Taste Receptors Specificity • Each taste bud usually responds to only one of the 5 primary taste modalities when the concentration of tastant is low (adequate stimulus). • But at high concentration, most taste buds can be excited by 2 or more taste stimuli. • This explains the great number of tastes that a person can perceive.

Taste Pathway

Taste Pathway • 3 order neurons; 1) First order neurons: • It carries taste sensation from the taste buds to the gustatory nucleus (nucleus solitarius) as follow; • a) From anterior 2/3 of the tongue by facial nerve • b) From posterior 1/3 of the tongue by glossopharyneal nerve. • c) From the pharynx, hard and soft palate and epiglottis by vagus nerve. Note: • General sensations from the anterior 2/3 of the tongue by trigeminal nerve and posterior 1/3 are carried by glossophyangeal nerve.

Area 3, 1, 2 Sensory radiations PVNMT Medial leminiscus Facial N. Nucleus solitarius Ant. 2/3 Glossoph. N. Vagus N. Post. 2/3 epiglottis

Taste Pathway 2) Second order neurons: • From the gustatory nucleus (nucleus solitarius) axon of 2 nd order neuron ascend in the ipsilateral (same side) medial lemniscus and pass directly to the ventral posteromedial nucleus of the thalamus (VPMNT). 3) Third order neurons: • From the thalamus (VPMNT), the axons of the 3 rd order neurons pass in the sensory radiation to the face area of the somatosensory cortex in the ipsilateral postcentral gyrus. • They also pass to the anterior part of the insula which is anterior to the face area of the postcentral gyrus (conscious perception of taste and taste discrimination)

Taste Adaptation • Taste sensations adapt rapidly within minutes inspite of continuous stimulation. • It occurs in the CNS itself. • This is different from most other sensory systems which adapt mainly at the receptors.

Importance of Taste Sense • Taste sensation is important in: 1) Stimulation of GIT secretions e. g. salivary secretion through unconditioned reflexes. 2) Prevention of ingestion of most poisons which have strong bitter taste. 3) Determination of the flavor of food.

Taste Preference • This means the selection of certain types of food in preference to others. • Usually these types are needed to the body. In experimental animals it has been found that adrenalectomised animals show great preference to salty water (containing Nacl) while parathyroidectomised animals prefer Ca++ containing water. • Taste preference occurs in the CNS but the exact mechanism is not known.

Taste Disturbances 1. Ageusia (taste blindness): absence of taste sensation for certain substance. 2. Hypogeusia: diminished taste sensation 3. Gustatory hallucination: False sensation of certain tastes 4. Dysgeusia: disturbed taste sensation.

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