SPECIAL SENSES TASTE GUSTATION LECTURER IN CHARGE BAMIDELE

SPECIAL SENSES TASTE (GUSTATION) LECTURER IN CHARGE BAMIDELE O.

INTRODUCTION • The sense of taste (gustation) affords an animal the ability to evaluate what it eats and drinks. • At the most basicl level, this evaluation is to promote ingestion of nutritious substances and prevent consumption of potential poisons or toxins. • The texture of food as detected by touch sensations in the mouth and in the fingers in many Africans who eats with their hands, visual and olfactory inputs and presence of condiments contribute to the overall taste and enjoyment of food.

TASTE RECEPTORS • The sense of taste is mediated by taste receptor cells which are bundled in clusters called taste buds. • They are located mainly on the papillae of the tongue, with a few on the epiglottis, soft palate and pharynx. • Each taste bud consists of an onion-like arrangement of about 100 receptor cells and supporting cells lying deep in thick epithelium of the tongue and opening onto surface through a small taste pore.

Figure 1: Taste buds

Figure 2: Papillae of Tongue Source: www. slideshare. net

TASTE RECEPTORS CONT'D • The sensory cells are taste cells while the supporting cells are called sustencular cells. • The receptor cells are constantly renewed by the surrounding supporting cells. • Each taste cell has a life span of about 10 days in lower animals. Life span in humans is unknown. • They have several microvilli or taste hair protuding from its outer surface through the taste pore into the oral cavity. • The microvilli provide the receptor surface for taste.

TASTE RECEPTORS CONT'D • Interwoven among the taste cells in a taste bud is a network of dendrite of sensory nerves called taste nerves. • When taste cells are stimulated by binding of chemical to their receptors, they depolarize the receptor cell directly or via second messenger and this depolarization form receptor potentials that is transmitted to the taste nerve fibres resulting in an action potential.

TASTE PATHWAY • Taste fibres are transmitted specifically along three major cranial nerves: 1. Cranial nerve VII (Facial nerve): It picks taste stimuli from anterior 2/3 of the tongue. 2. Cranial nerve IX (Glossopharygeal nerve): It picks stimuli from posterior 1/3 of the tongue. 3. Cranial nerve X (Vagus nerve): It picks stimuli from the epiglottis and the pharynx.

TASTE PATHWAY CONT'D • The cranial nerves VII, IX and X are united in the medulla oblongata and enter the Nucleus of the Tractus Solitarius (NTS). • Here, they synapse with second order neurons, the axons cross to the opposite side and join the medial lemniscus. • The fibres travel with fibres of touch, pain and temperature and relay to nuclei of the thalamus. • From thalamus, third order neurons relay to the insula of the cerebral cortex.

Figure 4: Taste pathways Source: www. slideshare. net

TASTE SENSATIONS • The sense of taste is equivalent to excitation of taste receptors for a large number of specific chemicals that have been identified to contribute to the reception of taste. • Despite this complexity, five types of taste are commonly recognised by humans: 1. Sweet: indicate energy rich nutients 2. Umami: taste of amino acids e. g meat broth 3. Salty: allows modulatory diet for electrolyte balance 4. Sour: typically the taste of acids 5. Bitter: allows sensing of diverse natural toxins

TASTE THRESHOLD • Taste threshold varies with different taste modalities and substances. For instance, bitter taste produce a response at a low concentration. • The bitter taste is much more sensitive than all other taste modalities. • None of the taste modalities is elicited by a single chemical. There are thresholds for detection of taste that differ among chemicals that taste the same. • For instance, sucrose, 1 -propyl-2 amino-4 nitrobenzene and lactose taste sweet to humans, but the sweet taste are elicited by these chemicals at concentrations of about 10 m. M, 2µM and 30 m. M.

Table 1: Examples of some human thresholds TASTE SUBSTANCE THRESHOLD FOR TASTING Salty Nacl 0. 01 M Sour Hcl 0. 0009 M Sweet Sucrose 0. 01 M Bitter Quinine 0. 000008 M Umami Glutamate 0. 0007 M

AFFECTIVE COMPONENTS OF TASTE • This can be either pleasant or unpleasant. • Sweet taste is pleasnt, but very low concentration of the sweet substance can be unpleasant e. g. • those who used to taking many cubes of sugar in their cup of tea before they can enjoy its taste can find the tea unpleasant to taste if only one cube of sugar is added. • Bitter, sour or salt taste are unpeasant in high concentrations. In a very low concentration they can be pleasant e. g. • a little quantity of bitter leaf can make a vegetable soup taste more pleasant.

TASTE REFLEX • A large number of impulses are transmitted from the tractus solitarius to the superior and inferior salivatory nuclei. • From here, impulses are relayed to the submaxillary and parotid glands and promote the secretion of saliva from these glands during ingestion of food. • The sensation of taste initiates the flow of gastric juice in the cephalic phase.

ABNORMALITIES OF TASTE SENSATIONS • Ageusia: This is absence of taste. • The causes include: q Lesion of facial nerves q Lesion of glossopharyngeal nerves q Lesion of mandibular division of trigeminal nerve. q Drugs such as penicillamine, captopril

ABNORMALITIES OF TASTE SENSATIONS • Hypogeusia: Diminished taste sensitivity. • Dysgeusia: Disturbed sense of taste in temporal lobe syndrome. • Selective taste blindness * Inherited autosomal recessive trait * Increased threshold for phenylthiocarbamide (bitter taste)

Refrences • www. slideshare. net • Oyebola D. O. (2002)Essential Physiology for students of Medicin e, dentistry, Pharmacy and Related disciplines. Volume 2.

THANK YOU