Taste – Organs, Receptor, Mechanisms, Central Taste Pathways, Neural Coding

What is Taste?

• Taste is a sensory system essential to human survival, having evolved as a mechanism to identify both nourishing and harmful substances. Humans, as omnivores, consume a wide range of foods from plants and animals. Taste plays a crucial role in distinguishing edible substances from potential toxins. From an evolutionary standpoint, taste allowed early humans to navigate their environment, identifying new food sources while avoiding harmful substances. Inborn preferences, such as an affinity for sweetness and an aversion to bitterness, reflect this survival mechanism. Sweetness, for example, is associated with energy-rich foods like mother’s milk, while bitterness often indicates toxicity, such as with certain poisons.
• The basic tastes humans can recognize fall into five categories: saltiness, sourness, sweetness, bitterness, and umami. Saltiness and sourness are chemically linked to substances like salts and acids. Sweetness spans a wide variety of chemicals, from sugars like fructose and sucrose to certain proteins and artificial sweeteners. Notably, artificial sweeteners and specific proteins can be thousands of times sweeter than natural sugars. Bitter substances range from simple ions, such as potassium (K+) and magnesium (Mg2+), to complex organic molecules like quinine and caffeine, which can be detected at extremely low concentrations, further enhancing their protective role.
• The fifth taste, umami, represents the savory flavor of amino acids like glutamate, commonly found in foods containing monosodium glutamate (MSG). Umami highlights the body’s ability to recognize proteins, a critical nutrient. While these five basic tastes are universally recognized across cultures, ongoing research suggests that other taste qualities may exist.
• Despite the limited number of basic taste categories, the range of flavors experienced is vast. Each food engages a distinct combination of these basic tastes, creating a unique flavor profile. Furthermore, the perception of flavor is not limited to taste alone. Smell, texture, and even temperature contribute significantly to the overall food experience.
• For instance, without the ability to smell, differentiating between certain foods, such as onions and apples, becomes challenging. Texture and the sensation of heat, as felt with spicy foods containing capsaicin, also influence flavor perception. Therefore, the brain integrates information from multiple sensory systems to create a complete and nuanced experience of food, combining taste, smell, and feel.

The Organs of Taste

The organs of taste are essential in helping humans detect and differentiate between various flavors. While the tongue is the primary organ associated with taste, other areas such as the palate, pharynx, and epiglottis also play a role in the tasting process. Additionally, odors from food contribute to flavor perception through olfactory receptors in the nasal cavity. Below is a detailed explanation of the organs of taste:

• Tongue Regions and Sensitivity:
  • The tongue has specific regions that are more sensitive to certain tastes: the tip detects sweetness, the back is sensitive to bitterness, and the sides respond to saltiness and sourness.
  • However, the entire tongue is capable of perceiving all basic tastes to varying degrees.
• Papillae:
  • Small projections called papillae are scattered across the tongue. These come in different forms:
    • Foliate papillae (ridge-shaped)
    • Vallate papillae (pimple-shaped)
    • Fungiform papillae (mushroom-shaped)
  • These papillae are visible to the naked eye, with smaller papillae located at the front and sides, and larger ones near the back of the tongue.
• Taste Buds:
  • Each papilla contains one to several hundred taste buds, which are only visible under a microscope.
  • A taste bud contains 50–150 taste receptor cells (taste cells) arranged in sections, similar to the segments of an orange.
  • Taste buds make up only 1% of the tongue’s epithelium.
  • The total number of taste buds varies widely between individuals, ranging from 2,000 to 5,000, but can be as few as 500 or as many as 20,000.
• Taste Receptor Cells:
  • Taste receptor cells are specialized cells within the taste buds, each designed to detect specific taste stimuli (sweet, sour, salty, bitter, or umami).
  • Surrounding the taste cells are basal cells, which support the function of taste cells.
  • Gustatory afferent axons are connected to the taste buds and send taste information to the brain.
• Threshold Concentration:
  • Each taste receptor cell is activated only when the taste stimulus reaches a certain concentration known as the “threshold concentration.”
  • At concentrations just above this threshold, a papilla may respond to only one type of taste (e.g., sweet or sour).
  • However, as the concentration increases, papillae can respond to multiple tastes, becoming less selective.
• Papillae Sensitivity:
  • At low concentrations of taste stimuli, papillae are highly selective, with each papilla responding primarily to one basic taste.
  • As concentrations rise, papillae may respond to several different tastes, indicating the versatility of taste receptor cells within the same papilla.

Taste Receptor Cells

Taste receptor cells are the specialized sensory cells responsible for detecting different taste stimuli. These cells, located in the taste buds across the tongue and other areas of the mouth, function as the interface between the chemical environment in the mouth and the nervous system. Below is a detailed breakdown of the structure and function of taste receptor cells:

• Apical End and Microvilli:
  • The apical end of a taste receptor cell is the chemically sensitive region.
  • Microvilli, thin extensions from the apical end, project into the taste pore, an opening on the tongue’s surface where taste chemicals interact with the taste receptor cells.
• Non-Neuronal Nature:
  • Taste receptor cells, though they form synapses with gustatory afferent axons, are not classified as neurons by traditional standards.
  • These cells also create electrical and chemical synapses with basal cells in the taste bud.
• Regeneration and Lifespan:
  • Taste receptor cells have a short lifespan of about two weeks, undergoing a continuous cycle of growth, death, and regeneration.
  • This regenerative process is dependent on sensory nerve activity; if the nerve connected to a taste bud is severed, the taste bud will degenerate.
• Receptor Potential:
  • When a chemical substance activates a taste receptor cell, it causes a change in the membrane potential, typically depolarizing the cell.
  • This change in membrane potential is known as the receptor potential.
  • In some cases, if the depolarization is large enough, the taste receptor cell can fire action potentials similar to neurons.
• Calcium Channels and Transmitter Release:
  • Depolarization of the receptor cell’s membrane opens voltage-gated calcium channels.
  • Calcium (Ca2+) enters the cytoplasm, triggering the release of neurotransmitters from the taste receptor cell.
• Taste-Specific Neurotransmitters:
  • Different types of taste cells release different neurotransmitters depending on the taste they detect:
    • Sour and salty taste cells release serotonin onto gustatory axons.
    • Sweet, bitter, and umami taste cells release adenosine triphosphate (ATP) as their primary neurotransmitter.
  • These neurotransmitters excite the sensory axon, causing it to fire action potentials that send taste information to the brainstem.
• Additional Neurotransmitters:
  • Other neurotransmitters, such as acetylcholine, GABA, and glutamate, may also be involved in the transmission process, though their specific roles in taste signaling are not yet fully understood.
• Taste Selectivity:
  • Research suggests that most taste receptor cells respond primarily to one of the five basic tastes (sweet, sour, salty, bitter, or umami).
  • For example, some taste receptor cells may show strong responses to salt (NaCl), while others may be more sensitive to sweet (sucrose) stimuli.
  • However, some gustatory axons can respond to multiple basic tastes, though each axon has a distinct preference or bias for certain taste categories.
• Variability in Response:
  • The selectivity of taste receptor cells depends on the specific transduction mechanisms they use, explaining why some cells respond to one chemical type while others respond to multiple categories of chemicals.
  • In experiments, certain gustatory axons show strong responses to specific taste stimuli, while others are influenced by a broader range of tastes.

Mechanisms of Taste Transduction

The mechanisms of taste transduction refer to the processes by which tastants (taste stimuli) cause an electrical response in taste receptor cells. Each basic taste—salty, sour, sweet, bitter, and umami—utilizes specific molecular pathways to convert chemical signals into electrical impulses that can be interpreted by the brain. Here is a detailed breakdown of how different tastes are transduced:

• Saltiness:
  • The primary salt stimulus is sodium (Na+), typically from table salt (NaCl).
  • Low concentrations of sodium enter taste cells through amiloride-sensitive sodium channels, which are distinct from voltage-gated sodium channels.
  • The influx of Na+ depolarizes the taste receptor cell, triggering the opening of voltage-gated sodium and calcium channels.
  • The result is the release of neurotransmitters onto the gustatory afferent axon, sending a signal of saltiness to the brain.
  • At high concentrations, NaCl may activate bitter and sour receptors, triggering avoidance behaviors due to their unpleasant taste.
• Sourness:
  • Sour tastes arise from the presence of acids, which produce hydrogen ions (H+).
  • Protons from acidic substances can enter taste cells through ion channels or block potassium (K+) channels, reducing the membrane’s permeability to K+.
  • This blockage leads to depolarization of the taste receptor cell, similar to the process in salt transduction.
  • Additionally, protons may activate transient receptor potential (TRP) channels, allowing more ions to enter the cell and further contributing to depolarization.
  • The complexity of proton interaction with cellular mechanisms suggests multiple pathways are involved in sour taste perception.
• Bitterness:
  • Bitter compounds are detected by around 25 different types of T2R receptors, which are G-protein-coupled receptors.
  • Bitter receptors activate G-proteins, which in turn stimulate the enzyme phospholipase C.
  • This enzyme increases the production of inositol triphosphate (IP3), a second messenger.
  • IP3 opens taste-specific ion channels, causing depolarization and release of calcium from intracellular stores.
  • The increase in intracellular calcium triggers the release of ATP as a neurotransmitter, activating gustatory afferent neurons.
  • The brain detects bitterness as a warning signal, often associated with potentially harmful substances.
• Sweetness:
  • Sweet substances are detected by T1R2 and T1R3 receptor dimers, both of which are G-protein-coupled receptors.
  • When a sweet molecule binds to these receptors, the same second messenger system used in bitter transduction is activated.
  • This involves the stimulation of phospholipase C, leading to IP3 production, which then opens ion channels and releases intracellular calcium.
  • The increased calcium causes ATP release, similar to the mechanism in bitter taste transduction, but different gustatory axons are involved, preventing confusion between sweet and bitter tastes.
• Umami (Amino Acids):
  • Umami, the taste of amino acids (such as glutamate), is detected by a receptor composed of T1R1 and T1R3 proteins.
  • The umami receptor shares the T1R3 subunit with the sweet receptor, but the T1R1 subunit makes it sensitive to amino acids rather than sugars.
  • Like sweet and bitter tastes, umami transduction uses the same G-protein-coupled pathway involving phospholipase C, IP3 production, ion channel opening, and calcium release.
  • The release of ATP as a neurotransmitter follows, and umami-specific axons carry this signal to the brain, indicating the presence of savory amino acids in the food.

Central Taste Pathways

The central taste pathways refer to the route by which taste information is transmitted from the taste receptors to various parts of the brain, leading to both conscious perception and physiological responses. Here’s a detailed breakdown of the central taste pathways:

• Initial Pathway:
  • Taste information originates from taste buds on the tongue, palate, pharynx, and epiglottis.
  • This information is transmitted by three cranial nerves:
    1. Facial nerve (Cranial Nerve VII): Carries signals from the anterior two-thirds of the tongue and the palate.
    2. Glossopharyngeal nerve (Cranial Nerve IX): Conducts signals from the posterior third of the tongue.
    3. Vagus nerve (Cranial Nerve X): Relays taste information from regions such as the glottis, epiglottis, and parts of the pharynx.
• Transmission to the Brainstem:
  • Taste axons from these cranial nerves converge in the brainstem and synapse in the gustatory nucleus, a part of the solitary nucleus in the medulla.
  • The gustatory nucleus plays a key role in processing this sensory input before the signals diverge to other brain regions.
• Conscious Taste Perception:
  • For conscious perception of taste, signals are transmitted from the gustatory nucleus to the ventral posterior medial (VPM) nucleus of the thalamus, which handles sensory information from the head.
  • From the VPM nucleus, the taste signals are sent to the primary gustatory cortex, located in Brodmann’s area 36 and the insula-operculum region of the cerebral cortex.
  • This pathway ensures that taste information is processed primarily on the ipsilateral side of the brain relative to the cranial nerves that carry it.
  • Damage to the VPM or gustatory cortex (such as from a stroke) can lead to ageusia, the loss of taste perception.
• Taste and Basic Behaviors:
  • Beyond conscious perception, taste plays a critical role in feeding and digestive behaviors.
  • Projections from the gustatory nucleus extend to other regions in the brainstem, particularly in the medulla, that regulate essential functions such as swallowing, salivation, gagging, vomiting, and digestion.
  • These pathways are essential for maintaining normal physiological responses during eating.
• Involvement of the Hypothalamus and Limbic System:
  • Taste information is also distributed to the hypothalamus and parts of the basal telencephalon (limbic system structures).
  • The hypothalamus and limbic system are involved in the emotional and motivational aspects of eating, influencing food palatability and drive.
  • Lesions in the hypothalamus or the amygdala (a basal telencephalon structure) can drastically affect eating behavior, leading to conditions such as overeating, food aversion, or changes in food preferences.

The Neural Coding of Taste

The neural coding of taste refers to how the brain processes and interprets the variety of taste sensations we experience. This complex process involves the integration of signals from different types of taste receptors and neurons to produce a clear and specific perception of taste. Here’s a detailed explanation of how the neural coding of taste works:

• Labeled Line Hypothesis:
  • The concept of neural coding could theoretically follow the labeled line hypothesis, where each taste receptor would be highly specific to certain tastes (like sweet, salty, bitter, or sour).
  • In this model, the signal from each receptor would travel along its own specific pathway to neurons in the brain, where it would stimulate only those neurons associated with that particular taste.
  • This hypothesis, while initially appealing, does not fully explain the complexity of how taste is processed, as taste receptor cells and neurons are not always strictly specific to just one type of stimulus.
• Selective Sensitivity of Receptor Cells:
  • Taste receptor cells are often selectively sensitive to particular stimuli, such as sweet or bitter.
  • However, some taste receptor cells are more broadly tuned, meaning they can respond to more than one type of tastant (for example, both salty and sour stimuli).
  • This variability in sensitivity at the level of receptor cells introduces some ambiguity in taste coding from the very start.
• Broad Tuning of Primary Taste Axons:
  • Primary taste axons, which carry signals from the taste receptor cells to the brain, are often less specific than the receptor cells themselves.
  • These axons can receive input from multiple receptor cells, and this convergence causes them to be responsive to more than one taste type.
  • As a result, a single axon might relay signals related to both salt and sour stimuli, making it harder to pinpoint the exact taste from one signal alone.
• Broad Tuning of Central Taste Neurons:
  • As the taste signals progress deeper into the brain, neurons in the gustatory nucleus receive input from numerous taste axons, each carrying signals from multiple receptor cells.
  • This continued broad tuning means that neurons in the brain are often less selective for specific tastes than the primary axons.
  • The result is that taste signals are increasingly mixed, further complicating the clear labeling of any one taste.
• Limitations of Specific Coding:
  • If the brain relied on specific taste cells that were highly tuned to only one type of stimulus, it would require an enormous variety of receptor types to handle the wide range of flavors encountered in the environment.
  • Even with a large number of receptor types, such a system would struggle to adapt to new tastes that it has not encountered before.
• Population Coding as a Solution:
  • The likely solution to this coding challenge is population coding, a system in which the brain interprets the signals from a large group of broadly tuned neurons rather than a small number of highly specific ones.
  • In population coding, different taste stimuli activate overlapping but distinct patterns of neurons.
    • For example, one set of neurons may respond strongly to bitter tastes, while others respond more moderately to sour and salt stimuli.
    • This pattern of activation allows the brain to differentiate between multiple taste stimuli based on the combined responses of the population of neurons involved.
• Discrimination of Specific Tastes:
  • To accurately distinguish between tastes, the brain relies on a population of taste cells with different response patterns.
  • For instance, when tasting two different foods, some neurons may fire strongly for one food and moderately for another, while others may be inhibited.
  • The overall pattern of activity across a large number of neurons allows the brain to discern the specific taste of each food.
• Integration with Other Sensory Inputs:
  • Taste perception is not limited to taste receptors alone; it also involves inputs from other senses, such as olfaction (smell), temperature, and texture.
  • For instance, the coldness and creaminess of ice cream contribute to the perception of its flavor, helping to differentiate it from other chocolate-based foods like chocolate cake.