Getting information of any taste is the main work of tongue because receptors of tongue are situated in the tongue. Some receptors of taste are present in the mucus membrane of soft palate, tonsils, epiglottis, etc.
The tongue is a very active organ which completes such important works as mastication, swallowing, speech along with getting information of taste. In the mouth, the tongue is a sense organ made by covered voluntary muscles covered by mucus membrane. The muscles of the tongue are both types intrinsic and extrinsic. Intrinsic muscles make the main organ of the mouth and help in completing all kinds of sensitive movements whereas extrinsic muscles are situated at the middle portion of Styloid of tongue, hyoid bone, mandible and temporal bone and help in chewing, swallowing.
There are three parts of a tongue tip, body and base of root. Its base is attached to the hyoid bone where as its body as well as tip remain free. The upper surface of the tongue is called dorsum which is leveled by stratified squamous epithelium. When the tongue is folded upwardly, several layers of mucus membrane upwardly coming towards the middle line can be seen on its lower surface and they are called frenulum-linguae. It attaches to the posterior portion of the tongue to the base of mouth. The anterior portion of the tongue remains free. The frontal portion of the tongue become pointed when it is brought out but its frontal portion remains round when it is inside the mouth.
The mucus membrane of the tongue is moist and pink in healthy position. Its upper surface is velvety and is covered with many protuberances which are called papillae. Papillae are present on the upper portion of the tongue and there is no papillae on the lower surface of the tongue but they are found on the surface of the palate, throat and on the posterior surface of epiglottis. In the papillae, capillaries are spread in the form of net. The fibers of taste nerves –seventh, ninth and tenth of skull nerves terminate in these papillae. These papillae are called taste buds. There are one thousand taste buds in the human being. The number of these taste buds gets reduced in old age.
There are three kinds of papillae-
- Circumvallate papillae
- Fungiform papillae
- Filiform papillae
Circumvallate papillae: Such papillae are present at the back side of the tongue viz on the two third posterior portion of the tongue. In two parallel lines, they are ten to twenty in number in the form of English letter ‘V’. There are 90 to 250 taste buds in each circle or circumvalet papillae. They are very large in size and can be seen easily.
These papillae are the papillae which are singly scattered on the frontal portion and sides of the tongue and appear like mushroom. There are 1 to 8 taste buds in each of these papillae.
Such kinds of papillae are found on the two third frontal portion of the tongue and they are thread like pointed papillae. There is a lack of taste bud in such kinds of papillae. The work of such papillae is feeling the thing in place of getting information of taste. The sensation of taste is done by the circumvallate or fungiform pappilae.
The taste buds are the special end organs of taste. Lingual and glossopharyngeal nerve come in the each cell of it. each taste bud opens on the surface of papillae through tiny pore and its all the cells which reach on this place altogether terminate in several protuberances like minute hair. Food stuffs enter into these pores and stimulus these protuberances by their touch. Impulse of stimulus produced in it transmit in the taste centre of brain by the nerves (seventh, ninth and tenth skull cells) of taste sensation and there taste is analyzed and thereafter we know different kinds of tastes.
Each taste bud is in ovular form. Their axis is situated on the layer in straight and perpendicular direction. Blood pipes and nerves get entry in it from the lower side. There are chemoreceptor or gustatory cells and supporting cells in the taste buds.
Basic taste sensations:
There are four main sensations of taste-sweet, sour, bitter and sour. Most of food stuffs have flavor along with taste but it is related to the sensation of smell and stimulate the receptors of small not the receptors of taste.
Basic taste sensations don’t take birth on all the portions equally on the surface of the tongue. Some definite portions of the tongue get affected by the special taste. The sweet and salty taste is present on the tip of the tongue and the sour taste is present on the both sides of the tongue whereas the bitter taste is present on the back portion of the tongue viz near the throat. There is no taste sensation on the middle portion of the tongue. Thus, there are some special taste buds in the tongue for each definite special taste which get affected only by any special type of stimulus.
Pathways for taste impulses:
The impulses of taste sensations are transmitted into the brain by facial nerve from the two third frontal portion of tongue, by glossopharyngeal nerve from the one third back portion of the tongue and by vagus nerve from the palate and pharynx. Taste fibers of above mentioned skull nerves terminate into the nucleus solitarium of medulla oblongata. From this place, axons come out towards the thalamus and then reach into the taste center situated in the parietal lobe of cerebral cortex.