Gustation

Olfactory component

The sense of taste is only partly conveyed by the tongue. The sense of smell also has a significant role to play. Odorants, airborne odor molecules, are inhaled through the nose and make contact with the olfactory epithelium that is coated with a range of olfactory receptors found on olfactory cilia of sensory cells. These chemical signals are then transduced into electrical ones within the olfactory nerve.

The olfactory nerve (cranial nerve I) enters the skull in small nerve bundles (olfactory fila) via foramina in the cribriform plate of the ethmoid bone . These small unmyelinated nerve bundles form the olfactory nerves which synapse in the olfactory bulb. At this point, the olfactory nerve forms the olfactory tract and passes posteriorly to widen into a triangular shaped structure known as the olfactory trigone.

Tongue

Taste buds

Taste buds are microscopic gustatory organs which contain chemosensory cells synapsing with afferent fibers of gustatory nerves. The cells are organized into a barrel-shaped epithelial structure resembling a flower bud. At the top of the taste bud is an opening called the taste pore, which is a fluid-filled funnel where the microvilli (a.k.a. gustatory hairs) of gustatory epithelial cells can be found. Food molecules, also known as tastants, are typically dissolved in saliva, and then bind with and stimulate the receptors on the microvilli. Taste signals are passed through the gustatory epithelial cells which are in turn connected to the terminals of gustatory nerves. These nerves send afferent information (special sensory) to the brain. The brain decodes the afferent information sent by both the taste buds and the olfactory receptors and creates the sensation of the taste of food.

There are four types of cells found in taste buds:

• Type I (glial-like) gustatory epithelial cells: These are long spindle-shaped supporting cells extending 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: These cells are involved in the expression of G protein receptors for bitter, sweet and umami taste.
• Type III (presynaptic) gustatory epithelial cells: They are involved in sour taste transduction.
• Type IV gustatory epithelial/basal cells: These cells are found on the basal lamina of the epithelium and are thought to be undifferentiated or immature precursors to type I-III gustatory epithelial cells.

The number of taste buds in the oral cavity and uppermost gastrointestinal tract is subject to a high degree of interindividual variation, ranging from 500 to 5000, while the total number of gustatory cells in one single taste bud can reach up to 150. As the oral cavity has an abrasive environment, due to mechanical traction and high/low temperatures of ingested food, chemosensory cells in the taste buds are highly regenerative and get replaced every 8-12 days.

Taste sensation can be categorized as sweetness, sourness, saltiness, bitterness, and umami. These basic tastes can either have a stimulatory effect and encourage intake (sweet, salty, and savory) or inhibit it (bitter and perhaps sour).

Lingual papillae

Taste perception is mediated by gustatory receptors, also known as taste buds. There are approximately 4000 taste buds in humans which are distributed throughout the oral cavity and upper gastrointestinal tract. About three quarters of all taste buds are found on the dorsal surface of the tongue in small raised structures called papillae. There are four different types of papillae which are further discussed below:

• Vallate papillae: These papillae lie in a V-shaped row immediately anterior to the terminal sulcus, which divides the dorsum of the tongue into its anterior two-thirds and a posterior third. Vallate papillae
  • Vallate papillae are round in shape. Their apex is coated with stratified squamous epithelium. About 50% of all taste buds are found in the circumvallate papillae. This type of lingual papilla is sensitive to bitter taste and can be involved in the gag reflex. Surrounding each papilla is a depression in which gustatory glands (of Von Ebner) empty a serous secretion. It is believed that this circular depression of epithelium infolding acts as a moat around each papilla, and works to remove stimuli from the base of the circumvallate papillae, ensuring that they are always primed for new taste stimuli. These papillae are innervated by the glossopharyngeal nerve (cranial nerve IX) (despite the fact that they lie anterior to the terminal sulcus).

• Fungiform papillae: As their name suggests, fungiform papillae resemble mushrooms. They occur singly and are fairly evenly spaced between the filiform papillae. Most are found on the anterior part of the dorsum of the tongue with around half located near the tip and none in the midline. They contain numerous taste buds on their surface that are able to discern sweetness, sourness, saltiness, bitterness and umami. About 25% of all taste buds are found in the fungiform papillae.
  • Histologically, lying at their core is connective tissue which is highly vascularized. Unlike filiform papillae, fungiform papillae are not keratinized.
  • These papillae are innervated by the facial nerve (cranial nerve VII), with nerve fibers leaving the tongue via the submandibular ganglion, lingual nerve, chorda tympani and geniculate ganglion in order to reach the solitary nucleus of the brainstem.

• Foliate papillae: These are vertical, relatively short papillae which can be found on either side and posterior aspect of the tongue. They can be located anterior to the palatoglossal arch. They are coated with non-keratinized epithelium and are therefore softer than other papillae. They contain numerous taste buds. The remaining 25% of all taste buds are found in the foliate papillae.

• Filiform papillae: These are the smallest and most common papillae on the tongue. They are conical and cover the majority of the anterior two thirds of the tongue. They do not contain taste buds and are thus not involved in taste sensation.
  • Histologically, they comprise a core of irregular connective tissue, which is covered by a keratinized epithelium featuring secondary processes, which contain elastic fibers and are hence flexible.
  • Filiform papillae appear white on the tongue and are overlapped into a brush-like dense layer of processes. These papillae are believed to be involved in texture perception of food.

Facial nerve

The chorda tympani of the facial nerve (cranial nerve VII) provides fibers to the anterior two-thirds of the tongue (fungiform and foliate papillae). Additionally, the greater petrosal branch of the facial nerve supplies the taste buds on the soft palate.

A variable degree of taste information can bypass the path of the chorda tympani in the middle ear and travel along the greater petrosal nerve. The chorda tympani and greater petrosal nerve converge at the geniculate ganglion after which the afferent fibers form the intermediate nerve. Its afferent gustatory fibers synapse in the rostral solitary nucleus, which is also known as the gustatory nucleus.

Glossopharyngeal nerve

The glossopharyngeal nerve (cranial nerve IX) provides fibers to the posterior third of the tongue. The nerve leaves the skull through the jugular foramen along with the vagus nerve (cranial nerve X) and the descending part of the spinal accessory nerve (cranial nerve XI). From the taste buds, nerve signals are transmitted in the lingual branches traveling towards the jugular foramen.

The inferior glossopharyngeal ganglion, also known as the petrosal or petrous ganglion contains the sensory cell bodies and is situated just below the jugular foramen. The glossopharyngeal nerve enters the cranium through the jugular foramen with the vagus nerve (cranial nerve X) and accessory nerve (cranial nerve XI). The afferent fibres travel through the superior glossopharyngeal, or lesser petrosal, ganglion. They carry on into the medulla through the cerebellopontine angle to synapse in the rostral solitary nucleus.

Vagus nerve

The vagus nerve (cranial nerve X) provides fibers to the region of the palate and epiglottis. Taste information is carried by the superior laryngeal branch of the vagus nerve. Afferent fibers from this branch join afferent fibers of the main vagus nerve from thoracic and abdominal internal organs. Their sensory cell bodies form the inferior vagal ganglion. The afferent fibres enter the cranium through the jugular foramen with the glossopharyngeal nerve and the accessory nerve and then pass through the superior vagal ganglion. They synapse in the rostral solitary nucleus.

Other projections of the vagus nerve, such as those responsible for saliva secretion; and gastric secretion and motility, synapse in the solitary nucleus. This explains why taste increases salivation and gastric activity.

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At the rostral solitary nucleus (gustatory nucleus), the paths of the taste afferents converge. The fibers from each gustatory nerve mix and split into 3 pathways:

• The first is projected to the ventral posteromedial nucleus of the thalamus then on to the taste sensory cortex where we become aware of the sensation.
• In the second, fibers travel to synapse in the pontine taste area before going on to terminate in the lateral hypothalamic area.
• The third pathway also synapses in the pontine taste area and then runs to the amygdala.

The taste sensory cortex communicates with the lateral hypothalamic area and amygdala. It is generally accepted that the lateral hypothalamic area and amygdaloid body are responsible for appetite, satiety and other homeostatic mechanisms. The fact that the sensory cortex sends signals to these areas could be the reason we feel more satiated after experiencing tastes we desire. The amygdala is involved in emotion and memory formation, amongst other functions, explaining why we attach strong emotions to food and perhaps why we crave certain foods when in certain emotional states.