Taste buds

Structure and location

Taste buds are microscopic sensory organs containing chemosensory cells which synapse with afferent fibers of gustatory nerves. The number of taste buds in the oral cavity and uppermost gastrointestinal tract is subject to a high degree of interindividual variation (500-5000) while the number of cells in one taste bud can be up to 150. Due to the abrasive environment of the oral cavity, gustatory cells are highly regenerative with their average turnover being 8-12 days, however, some of them remain much longer.

There are four types of cells found in taste buds:

• Type I (glial-like) gustatory epithelial cells: Supporting cells. Long spindle-shaped cells extend from the taste pore to the basal lamina. Their apical ends may be involved in salt taste transduction, however this is still debated.
• Type II (receptor) gustatory epithelial cells: express G protein receptors for bitter, sweet and umami taste. They secrete adenosine triphosphate (ATP) and acetylcholine (ACh) neurotransmitters.
• Type III (presynaptic) gustatory epithelial cells: Receptors for sour taste. They secrete serotonin, gamma-aminobutyric acid (GABA) and norepinephrine/noradrenaline neurotransmitters.
• Type IV gustatory epithelial/basal cells: Placed on the basal lamina of the epithelium. Thought to be undifferentiated or immature precursors to type I-III gustatory epithelial cells.

Gustatory cells are collectively organized into barrel-shaped epithelial structures that look like flower buds. At the top of each taste bud is an opening known as a taste pore: a fluid-filled funnel in which finger-like extensions (microvilli) of the gustatory cells called gustatory/taste hairs are located. Each of the sensory gustatory epithelial cells is in contact with terminals of nearby gustatory nerves (branches of the facial, glossopharyngeal, or vagus nerves).

Taste buds are largely located in the lingual papillae (except for the filiform variety) but they can also be found in the uppermost parts of the gastrointestinal tract i.e. soft palate, epiglottis, oropharynx and upper esophagus.

Learn more about the lingual papillae in this study unit on the structure of the tongue:

Innervation

Although widespread in the upper GI tract, the sensory supply of taste buds can be roughly classified by anatomic location according to the following nerves. Three cranial nerves provide sensory supply throughout their branches:

• Facial nerve (CN VII): the chorda tympani (via the lingual nerve) innervates taste buds in the anterior two-thirds of the tongue. Fibers of this nerve course to their cell bodies in the geniculate ganglion before continuing towards the brainstem via the sensory root of the facial nerve. Taste fibers of the chorda tympani may alternatively pass via the otic ganglion/greater petrosal nerve to reach the geniculate ganglion. The greater petrosal nerve also carries taste innervation from the soft palate.
• Glossopharyngeal nerve (CN IX): This nerve, predominantly innervates the posterior one-third of the tongue (containing the vallate papillae), palatoglossal arch of the soft palate, and oropharynx. The cell bodies of the glossopharyngeal nerve are located in the petrosal ganglion (a.k.a. inferior ganglion of glossopharyngeal nerve).
• Vagus nerve (CN X): Through its internal laryngeal branch, this nerve provides innervation for the epiglottic region and pharyngeal part of the tongue. The cell bodies are located in the inferior ganglion of the vagus nerve (nodose ganglion)

Expand your knowledge about the neuroanatomy of the taste pathway with the following study unit:

Function

Tastants (i.e. taste stimulating compounds) dissolved in oral saliva enter the taste pore after intake/mastication of liquids/food into the oral cavity. At the microscopic level, the microvillous apical surface of the gustatory sensory epithelial cells interacts with tastants through protein receptors or ion channels. From that point, the chemical stimuli are transduced through the sensory cell. The electric impulse is conducted via the afferent gustatory nerve fibers to the brainstem.

There are five gustatory sensations: sweet, salty, sour, bitter, and umami.

• Salty taste is the detection of high concentrations of Na+/sodium ions in the saliva which directly diffuse into the gustatory sensory epithelial cells causing them to depolarize and release neurotransmitters.
• Sour tastes occur via a similar means to that of salty (i.e. direct diffusion of ions), however this time in response to high H+/hydrogen ion concentrations (i.e. acids). As the concentrations increase, sour-detecting gustatory cells will depolarise.
• Sweet, bitter and umami tastes are transduced via specific G-protein coupled receptors rather than direct diffusion of ions as seen with salt/sour tastes. Sweet taste involves the detection of sugars (e.g. glucose and other monosaccharides). Bitter taste is detection of long-chain organic compounds known as alkaloids which contain basic (as in pH) nitrogen ions. Umami taste is action of receptors by certain amino acids e.g. glutamine, and therefore may be loosely considered as the ‘taste of proteins’.
• ’Hot and spicy’ flavors, even if often referred to as taste, are actually pain/temperature sensations caused by the substance called ‘capsaicin’ in food. This compound binds to thermoreceptors whose primary function is to detect hot foods/liquid in order to prevent burning of the lingual/oral mucosa. Such sensations are transmitted afferent nerve fibers of the trigeminal nerve.

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