Taste bud
The taste buds or taste buds ( Caliculi gustatorii ) are onion-shaped structures in the oral mucosa of vertebrates . In addition to other cell types, they house the sensory cells of the sense of taste .
At the tip of every taste bud, the surrounding epithelium of the mucous membrane forms an opening ( porus gustatorius ) through which saliva and food components dissolved in it can reach the taste sensory cells. Protrusions of the sensory cell membrane protruding into the taste pore, their apical microvilli , carry the majority of the molecular taste receptors . Chemical contact with these irritates the sensory cell. Their signal is picked up by dendrites of afferent nerve cells and passed on to the central nervous system. These nerve fibers leave a taste bud at the base, in which there are mostly chemoreceptors for several taste qualities (→ gustatory perception ).
The taste buds of the tongue are assigned to surface structures, the taste papillae ( papillae gustatoriae ). In mammals, about 75% of the taste buds are located in papillae on the tongue, most of them on the back third towards the base of the tongue. The rest of the taste buds are distributed across the soft palate , nasopharynx , larynx, and upper esophagus .
The taste buds of the papillae on the base of the tongue are cleaned by small serous flushing glands, taste glands ( Glandulae gustatoriae ) or - after their discoverer (1872) Viktor von Ebner-Rofenstein - called Ebner flushing glands . Their secretion also contains unspecific lipases with an optimum in the acidic range (tongue base lipases ), the effect of which is to release fatty acids from dietary fats .
Taste papillae
There are up to a hundred sense cells in one taste bud . In turn, more than a hundred taste buds can lie in a taste papilla. The adult person usually has fewer than a hundred taste papillae on the tongue and a total of around 10,000 taste buds, most of them in the papillae.
According to their shape, the actual taste papillae are divided into three types:
- Wall papillae ( papillae vallatae ): In humans, about a dozen wall papillae are located in the back third of the tongue. They are much larger than mushroom papillae and can contain several hundred taste buds.
- Leaf papillae ( papillae foliatae ): The leaf papillae are in the form of folds lying close behind one another. They're on the side of the back third of the tongue. Each papilla contains around 50 taste buds.
- Fungal papillae ( Papillae fungiformes ): They are mainly distributed on the front two thirds of the back of the tongue. In humans, they carry around 3–5 taste buds. They are clearly visible after drinking milk.
In addition to these taste papillae, there are also mechanical papillae ( papillae mechanicae ) that do not absorb any taste stimuli and have keratin structures . Filiform papillae ( papillae filiformes ) are distributed over the entire back of the tongue , the sensory cells of which respond to mechanical stimuli and thus convey a tactile sensation on the tongue. Very thick and keratinized papillae are called conical papillae ( papillae conicae ), and very flat and thick papillae are called lens-shaped papillae ( papillae lentiformes ).
Innervation
Taste sensory cells are secondary sensory cells ; as specialized epithelial cells , they do not have their own axon . To transmit the signals to the central nervous system , they are innervated by afferent nerves, the taste fibers. The major petrosal nerve , a branch of the facial nerve (VII), supplies the taste buds of the palate. Another branch of the facial nerve , the chorda tympani, supplies the fungal papillae in the anterior two thirds of the tongue and parts of the taste buds in the anterior leaf papillae. The rest of the leaf papillae and the wall papillae are innervated by the tongue branches of the glossopharyngeal nerve (IX). The taste buds of the epiglottis are supplied by the superior laryngeal nerve , a branch of the vagus nerve (X). The nerve through which the taste information of the buds of the esophagus and the nasopharynx is transmitted has not yet been fully clarified; it is assumed that the glossopharyngeal and vagus nerves are also involved here.
Cell types
In every human taste bud there are around 40–60 sensory cells of the sense of taste.
It has long been known that taste buds are made up of several cell types. The most common division today includes type I to type III cells and basal cells, sometimes referred to as type IV cells. The classification is originally based on observations of tissue sections under the electron microscope and was later supported by molecular biological methods.
- Type I cells are smaller than Type II and Type III cells, typically electron dense, have multiple microvilli at their apex and have membrane protuberances that envelop adjacent Type II and Type III cells. For this reason, a supporting function of these cells is assumed. This assumption is supported by the fact that type I cells express GLAST ( glutamate-aspartate transporter ), among other things , which is also found in glial cells , the supporting cells of the nervous system.
- Type II cells are less electron dense and have only a single microvillus at their tip. The exact role of type II cells is not yet fully understood. Based on the expression pattern, it can be assumed that they contain a large part of the taste receptors. Type II cells express α-gustducin, a subunit of the G protein involved in taste perception , the phospholipase subtype PLCβ2 and IP 3 R3, a subtype of the inositol trisphosphate receptor . The protein synaptobrevin , which is important for the transmission of stimuli , could also be detected.
- Type III cells are also poorly electron-dense and form the synapses with afferent nerve cells of the three cranial nerves . This is reflected in their protein expression profile: Synaptobrevin, SNAP-25 - which are involved in the transmitter release in the presynapse - and NCAM (neuronal cell adhesion molecule) could be detected. However, the proteins PLCβ2 and IP 3 R3 , which are important for taste perception, are also found in type III cells.
- New cells are constantly emerging from the basal cells, which replace and renew the short-lived taste sensory cells.
distribution
The distribution and number of taste buds differ within mammals. In birds , the tongue has no taste buds, here they are located in the throat . The catfish Ictalurus furcatus even has taste buds all over its body.
In 2010 it became known that mammals apparently also have receptors for bitter substances in their lungs. If bitter substances are inhaled , they should relax the smooth muscles of the bronchi so that the airways become wider. These receptors are, however, formed by cells of the smooth muscles of the respiratory tract, which can react to bitter substances with a dilatation; One should not speak of (neuronally integrated) sensory cells in this context.
Individual evidence
- ↑ Also in the outer skin of fish, e.g. on the fin rays , see Osphronemidae .
- ↑ a b c d e f g D. V. Smith, JD Boughter jr: Neurochemistry of the Gustatory System . In: A. Lajtha, DA Johnson (Ed.): Handbook of Neurochemistry and Molecular Neurobiology . Springer US, 2007, pp. 109-135, ISBN 978-0-387-30349-9 .
- ↑ Stefan Silbernagl , Agamemnon Despopoulos : Pocket Atlas Physiology. 8th edition, Thieme, Stuttgart 2012, ISBN 978-3-13-567708-8 , p. 360.
- ^ B. Lindemann: Receptors and transduction in taste . In: Nature . No. 413, 2001, pp. 219-225, PMID 11557991 , ISSN 0028-0836
- ↑ Bitter taste receptors on airway smooth muscle bronchodilate by localized calcium signaling and reverse obstruction . In: Nature Medicine , October 24, 2010. Secondarily presented as: Newly discovered sensory cells: bitter substances could help against asthma . Spiegel Online , October 25, 2010.
