Taste Anatomy and Function

• The lecture is about the anatomy and function of taste.
• The learning objectives are to understand the key functions of the Tongue, how specific anatomical features and cell types relate to its function (Taste), and conditions of the tongue and how taste is effected in relation to the anatomical features and cell types.

Biological Function of the Tongue

• The tongue's primary functions include taste (gustation), speech, and food manipulation.
• The ability to taste is important for human survival.
• An inability to sense taste can lead to failure to recognize poisons or toxins resulting in death, failure to recognize food with high caloric content resulting in malnourishment, and failure to experience pleasure/enjoyment resulting in malnourishment.
• Taste and smell are the last lines of defense in the acceptance or rejection of food.
• Primary functions of taste are to inform the selection of food, detect specific chemicals, aid digestion, and provide information to the brain, including the intensity and duration of stimuli as well as the quality/edibility of food.

Primary Anatomical Sections of the Tongue

• Papillae are small bumps or protrusions containing taste buds.
• Taste buds are clusters of cells containing taste receptor cells (gustatory cells).
• Four types of papillae found in the tongue are filiform, fungiform, circumvallate and foliate.

Taste Anatomy - Papillae

• The human tongue contains four individual types of papillae.
• Each type of papillae has a distinct role in the sensation of taste and texture.
• They are visible to the naked eye.

Taste Anatomy - Foliate Papillae

• Foliate papillae are found on the sides of the tongue.
• There are usually around 20, with hundreds of taste buds.
• Histologically, they are characterized by short vertical folds and are covered by non-keratinized stratified squamous epithelium.
• They are functionally characterized by increased sensitivity to sour tastes.

Taste Anatomy - Fungiform Papillae

• Fungiform papillae are located within the dorsal surface of the tongue.
• The tongue contains 200-400 papillae, but only a few which are taste buds (3-5).
• They are distinguished histologically by their "mushroom" shape.
• Characterized functionally by increased sensitivity to salty tastes.

Taste Anatomy - Circumvallate Papillae

• The circumvallate papillae are the largest type of papillae found towards the back of the tongue near the throat.
• Nine circumvallate papillae are arranged in a chevron formation and contain approximately 250 taste buds.
• Characterized by increased sensitivity to bitter tastes and important regarding the gag reflex.

Taste Anatomy - Filiform Papillae

• Filiform papillae are characterized histologically by cone-shaped projections found primarily in the dorsal surface.
• They do not contain taste buds.
• Friction generated informs manipulation and perception of texture.

Taste Bud Anatomy

• Key anatomical features are taste receptor cells (TRC), microvilli and basal cells.
• Taste receptor cells (TRC) detect taste through contact with specific chemicals (tastants) from food and drink.
• Each taste receptor contains gustatory cells.
• Microvilli have an increased surface area for taste.
• Basal cells rise to taste receptor cells.
• Replacement of TRCs takes approximately 10 days.
• Supporting cells provide structure and support and clear neurotransmitters, but are not directly involved in taste.

Taste Receptor Cells (TRCs)

• TRCs can also be referred to as gustatory cells (hence gustation!).
• Each taste bud contains between 50-150 taste receptor cells, the composition of which is not fixed.
• Nerve endings connect TRCs to the Central nervous system (CNS).
• Signals are passed to the brain, resulting in the sensation of taste.

Taste Receptor Cell Types

• Type I cells- glial-like cells, help NT clearance
• Type II cells- taste receptor cells
• Type III cells- presynaptic cells, basal cells

Factors Effecting Taste

• Variation in the number of papillae influences sensitivity.
• More papillae result in more taste buds and more TRCs.

Genetic Influence of Taste Sensitivity

• Individuals are genetically predisposed to variation in sensitivity to tastes.
• The ability to taste phenylthiocarbamide (PTC) is genetically encoded.
• The TAS2R38 gene, discovered in 2003, has two common alleles.
• Variation in shape dictates affinity to bind PTC.
• PTC recognition correlates with sensitivity to bitter substances (some of which are toxins!).
• On average, 75% of individuals can taste PTC while 25% cannot which varies between different populations.

Taste Anatomy - Function

• Taste buds are responsive to tastants (chemicals) which trigger taste sensation by activating TRCs.
• TRCs have specialized receptors and ion channels on their apical microvilli which interact with chemicals for the sense of taste.
• Different ion channels and receptors are responsible for the sense of different tastes.
• Ions channels depolarize membranes.
• Types 1 receptors (GPCRs) are dependent on the specific receptor.
• T1R2/T1R3 is responsible for sweet taste via sucrose/fructose.
• T1R1/T1R3 is responsible for umami taste via glutamate (MSG).
• T1R2 is responsible for bitter taste via caffeine, quinine, and nicotine.

Neurotransmission

• Ion channels such as Sodium (ENaC) – influx of ions leads to depolarization
• GPCRs- tastants bind to the GPCR causing a conformational change in the receptor, leading to the activation of G protein.
• Generation of action potential in sensory nerve fibers.
• Signals are passed to the brain via cranial nerves VII/XI/X to the solitary nucleus in brain stem.
• Second-order neurons project to either the thalamus or the hypothalamus/amygdala resulting in a signal through the solitary nucleus.

Biological Function Taste

• Taste informs the selection of food through neurotransmission, leading to pleasure/enjoyment.
• Tasting aids in detecting specific chemicals as different chemicals bind to different receptors/ion channels.
• Tasting aids in digestion as neurotransmission leads to salivation.
• Tasting provides information to the brain, the intensity and duration of stimuli, and the quality/edibility of food.

Taste Sensitivity and Neural Coding

• The taste map of regional sensitivity to different taste modalities is oversimplified.
• Taste buds in specific areas contain TRCs that respond to specific tastants.
• All taste buds are sensitive to each taste modality including sweet, salt, sour, bitter, and umami.
• Each taste receptor cell is responsive to one modality.
• Overall taste is the sum of the activity across all cells.

Taste Anatomy - Conditions

• Taste disorders encompass a range of conditions including dysgeusia, hypogeusia, and ageusia.
• Dysgeusia is a distortion of taste.
• Hypogeusia is a reduced taste.
• Ageusia is a loss of taste, which is commonly due to viruses.
• More than 200,000 people visit a doctor each year for problems with their ability to taste and smell.

Taste Abnormality - Viral infections

• Are one of the most common causes of taste loss.
• Mechanisms of viral-associated taste loss include direct damage of nerve cells and inflammation within the oral cavity.
• Cytokine storm and Proinflammatory cytokines also lead to taste loss.

Taste Abnormality - Bacterial infections

• Infections such as gingivitis can also lead to a loss of taste.
• Similar mechanisms to those of viral infection are involved.
• Inflammation leads to the production of cytokines which can directly damage taste buds and other associated cells and nerves.

Taste Abnormality - Nasal polyps

• Nasal polyps affect up to 40% of the general population and impact the sense of taste independently of anatomical features of the tongue.
• Nasal polyps block the nasal cavity which leads to decrease transmission and reduced taste.
• The sense of taste is intertwined with the sense of smell.
• When food is eaten, odors travel through the nasal cavity to olfactory receptors.
• Combined neurotransmission from TRCs contributes to the overall taste.

Taste Abnormality - Cancer

• Cancer can directly cause damage to the sense of taste.
• Chemotherapy and radiotherapy that are used to destroy cancerous tissues destroys healthy tissues.

Taste Abnormality - Fungal infections

• Fungal infections (primarily Candida albicans) can lead to a disruption of the sense of taste.
• They are characterized by white patches or lesions on the tongue, inner cheeks, and throat, as well as a burning or painful sensation.
• The destruction of taste buds and taste receptor cells occurs via inflammation.