Gustaation

Anatomy of the taste buds and papillae;
physiology of gustation and gustatory pathway

1. GUSTATION

2. INTRODUCTION: Taste or gustation like olfaction is a chemical sense. Five primary tastes can be distinguished ; sour, sweet, bitter, salty, and umami (meaty or savory) Anatomy of Taste Buds and Papillae: The receptors for the sensation of taste are located in the taste buds Most of the nearly 10,000 taste buds of a young adults are found on the tongue, but some are found on the soft palate, pharynx and epiglottis. The number of taste buds decline with age Each taste bud is an oval body consisting of 3 kinds of epithelial cells

3. Supporting cells – surround about 50 gustatory receptor cells i n each taste bud. Gustatory hair projects from each gustatory receptor cell to external surface through taste pore. Basal cells – produce supporting cells which can develop into gustatory receptors. Each gustatory receptor has a life span of about 10 days. At their base gustatory receptor cells synapse with dendrites of the first order neurons that form the first part of the gustatory pathway.

4. Taste buds are found in elevations on the tongue called papillae – increase the surface area and provide the rough texture to the upper surface of the tongue Three type of papillae contains taste buds; Circumvallate papillae Fungiform papillae Foliate papillae Filiform papillae Circumvallate papillae about 12, very large Form an inverted V row at the back of the tongue Each of these papillae houses 100-300 taste buds.

5. Fungiform Papillae: Mushroom –shaped elevations s cattered over entire surface of the tongue Contains about 5 taste buds each. Foliate Papillae: Are located in small trenches in the lateral margins of the tongue most of their taste buds degenerate in early childhood . Filiform papillae: In addition, the entire surface of the tongue has filiform papillae These pointed thread like structures contain tactile receptors but no taste buds. They increase the friction between the tongue and f ood

6. Physiology of Gustation Chemicals that stimulate gustatory receptor cells are known as tastants Once a tastant is dissolved in saliva it can make contact with the plasma membrane of the gustatory hairs. The result is a receptor potential that stimulate exocytosis of synaptic vesicles from the gustatory receptor cell The liberated neuro transmitter molecules trigger nerve impulses in the first order sensory neurons that synapse with gustatory receptor cells

7. The receptor potential arises differently for different tastants . The sodium ions in a salty food enter gustatory receptor cells via sodium channels in the plasma membrane The accumulation of sodium inside causes depolarization which opens calcium channels In turn inflow of calcium triggers exocytosis of synaptic vesicles and liberation of neurotransmitters The Hydrogen ions in sour tastants may flow into the gustatory receptor cells via hydrogen channels. they also influence opening and closing of other types of ion channels Again the result is depolarization that leads to release of neurotransmitter

8. Other Tastants responsible for stimulating sweet, bitter and umami tastes do not themselves enter gustatory receptor cells. Rather they bind to receptors on the plasma membrane that are linked to G proteins The G proteins then activate several different channels known as second messengers cause depolarization in different ways Release of neurotransmitters

9. Gustatory Pathway The cranial nerves contain axons of the first order gustatory neurons that innervate the taste buds The facial nerve serves taste buds in the anterior 2/3 of tongue Glossopharyngeal nerve serves taste buds in the posterior 1/3 of tongue. The vagus nerve serves taste in the throat and epiglottis From the taste buds, nerve impulses propagate along these cranial nerves to the medulla oblongata

10. From medulla some axons carrying taste signals project to the limbic system and the hypothalamus. Others project to thalamus. The signals that project from the thalamus to the primary gustatory area in the parietal lobe of the cerebral cortex gives conscious perception of taste

11. Taste Threshold . Definition: The minimum concentration at which  taste  sensitivity to a particular substance or food can be perceived. T aste adaptation . a decrease in sensitivity to a stimulus that has been presented continuously to the gustatory system. The phenomenon can be complete, and the perception lost, after minutes of stimulation. https ://www.youtube.com/watch?v=xls2GJqZ5Rw