The 2 Chemical Senses: Taste and Smell

The chemical senses—taste and smell—constitute fundamental sensory modalities that allow organisms to perceive—and interact with—the chemical composition of their environment. These senses are essential for survival, guiding behaviors such as feeding, mate selection, predator avoidance, and social communication. This article provides a comprehensive examination of the anatomical structures, transduction mechanisms, neural pathways, perceptual processes, and higher-order functions associated with gustation (taste) and olfaction (smell). By the end of this exposition, the reader should possess a coherent understanding of how chemical stimuli are detected, encoded, and integrated to produce complex percepts and behaviors.

Overview of Chemical Senses

• Definition: Chemical senses detect molecules in the external environment (exteroception) and within the body (interoception) via specialized receptor systems.
• Scope: The principal chemical senses in humans are gustation and olfaction; additional chemosensory modalities include chemesthesis (chemical irritation mediated by trigeminal nerve) and pheromonal signaling in many species.
• Functional roles: Nutrient detection and ingestion control, toxin avoidance, social communication, memory linkage, and modulation of emotion and appetite.

Anatomy and Peripheral Structures

Gustatory System (Taste)

• Taste buds: The primary peripheral organs for taste are taste buds, clustered within papillae on the tongue (fungiform, foliate, and circumvallate), as well as on the soft palate, pharynx, and upper esophagus.
  • Each taste bud contains 50–100 specialized epithelial taste receptor cells (TRCs) with a short lifespan (about 10–14 days) and basal progenitor cells for renewal.
  • TRCs have apical microvilli (taste pore) exposed to the oral cavity, permitting interaction with dissolved chemicals.
• Papillae types:
  • Fungiform: anterior tongue, few taste buds each, innervated by the facial nerve (VII).
  • Foliate: lateral posterior tongue, numerous buds, innervated by facial and glossopharyngeal nerves.
  • Circumvallate: large trenches at posterior tongue, many buds, innervated primarily by glossopharyngeal nerve (IX).
• Innervation:
  • Anterior two-thirds of tongue: chorda tympani branch of cranial nerve VII (facial).
  • Posterior one-third: glossopharyngeal nerve (IX).
  • Areas of the epiglottis and pharynx: vagus nerve (X).
• Peripheral transduction: Taste receptor cells transduce chemical stimuli into receptor potentials via ion channels and G protein–coupled receptors (GPCRs), depending on the taste quality.

Olfactory System (Smell)

• Olfactory epithelium: Located in the superior nasal cavity; houses olfactory receptor neurons (ORNs), supporting (sustentacular) cells, and basal stem cells.
  • ORNs are bipolar neurons with a dendrite bearing numerous cilia projecting into the mucus layer to contact odorant molecules.
  • ORNs regenerate throughout life from basal stem cells.
• Olfactory receptor proteins:
  • ORNs express odorant receptor (OR) proteins encoded by a large multigene family—approximately 400 functional OR genes in humans (and more in many mammals).
  • Each ORN typically expresses one OR allele, following a “one neuron–one receptor” rule that contributes to the receptor-level coding of odors.
• Olfactory bulb:
  • ORN axons converge on glomeruli within the olfactory bulb; axons expressing the same OR type converge onto the same glomeruli, creating a spatial map for odorant identity.
  • Mitral and tufted cells relay processed signals from glomeruli to higher brain regions.
• Trigeminal chemosensation:
  • Many odorants also stimulate trigeminal nerve endings in the nasal mucosa, producing sensations of irritation, cooling, burning, or tingling—important for detecting potentially harmful substances.

Molecular Mechanisms of Transduction

Taste Transduction Pathways

Gustatory transduction involves multiple receptor classes and pathways tailored to different taste modalities.

• Salty (Na+):
  • Primarily mediated by epithelial sodium channels (ENaC). Na+ influx depolarizes TRCs, leading to neurotransmitter release and action potential generation in afferent fibers.
• Sour (H+):
  • Mediated by proton-sensitive mechanisms. Protons can enter cells via proton channels or block potassium channels, leading to depolarization.
• Sweet:
  • Detected by T1R2/T1R3 heterodimeric GPCRs. Binding activates G protein (gustducin) cascades, elevates second messengers (e.g., cAMP, IP3), modulates ion channels, and produces cellular depolarization and neurotransmitter release.
• Umami:
  • Mediated mainly by T1R1/T1R3 heterodimeric GPCRs responding to L-amino acids (notably L-glutamate) and by metabotropic glutamate receptors in some species.
• Bitter:
  • Detected by a diverse family of T2R GPCRs. Many bitter compounds bind to various T2Rs, often activating gustducin-mediated signaling to avoid ingesting toxins.
• Signal transduction commonalities:
  • Many GPCR-mediated taste signals result in intracellular Ca2+ mobilization and opening of transient receptor potential (TRP) channels (e.g., TRPM5), culminating in neurotransmitter (e.g., ATP) release onto gustatory afferent fibers.

Olfactory Transduction Pathways

Olfactory transduction relies heavily on GPCR mechanisms.

• Odorant receptor activation:
  • Odorant binding to ORs activates a G protein (Golf), stimulating adenylyl cyclase III, which increases cyclic AMP (cAMP).
• Ion channel activation:
  • Elevated cAMP opens cyclic nucleotide-gated (CNG) ion channels, allowing influx of Na+ and Ca2+, depolarizing the ORN.
  • Ca2+ opens Ca2+-activated Cl− channels, producing further depolarization via Cl− efflux (intracellular Cl− concentration is high in ORNs).
• Adaptation:
  • Desensitization mechanisms include Ca2+-calmodulin-mediated feedback decreasing CNG channel sensitivity and phosphorylation of receptors and downstream components, enabling adaptation to persistent odors.

Neural Pathways and Central Processing

Gustatory Pathway

• Primary afferents: TRCs synapse onto primary gustatory afferents within cranial nerve ganglia (geniculate ganglion for VII; petrosal ganglion for IX; nodose ganglion for X).
• Brainstem: Gustatory afferents project to the nucleus of the solitary tract (NTS) in the medulla—primary central relay for taste.
• Thalamus and cortex:
  • From NTS, secondary projections ascend (via the parabrachial nucleus in rodents; in primates some direct NTS-to-thalamus projections exist) to the ventroposteromedial nucleus (VPM) of the thalamus.
  • The primary gustatory cortex is in the insula and frontal operculum. Secondary processing occurs in the orbitofrontal cortex (OFC), which integrates taste, smell, and somatosensory information and represents reward value.
• Limbic connections:
  • Gustatory signals have rich connections to the amygdala and hypothalamus, mediating affective and homeostatic responses (e.g., appetite, satiety, conditioned taste aversion).

Olfactory Pathway

• Primary afferents: ORN axons form the olfactory nerve (CN I) and synapse in the olfactory bulb’s glomeruli.
• Bulb processing:
  • Local circuits involving periglomerular and granule cells perform lateral inhibition, contrast enhancement, and temporal patterning (oscillations) to refine odor representations.
• Direct cortical projections:
  • Unlike other senses, olfactory projections largely bypass the thalamus en route to primary cortex. Mitral/tufted cells project via the olfactory tract to primary olfactory (piriform) cortex, entorhinal cortex, and amygdala.
• Higher centers:
  • Piriform cortex performs pattern recognition and associative processing; orbitofrontal cortex integrates olfactory and gustatory inputs for flavor and reward assessment.
  • Connections to hippocampus and amygdala explain the close association between odors, memory, and emotion.
• Thalamic relay:
  • Although primary olfactory cortex receives direct input, olfactory information is also relayed to the mediodorsal thalamus and then to OFC for conscious olfactory perception and identification.

Coding Strategies

• Labelled-line vs. across-fiber pattern:
  • Taste: Evidence supports elements of both labelled-line (specific channels for basic tastes) and across-fiber pattern coding (combinatorial patterns across populations encode intensity and quality).
  • Smell: Predominantly combinatorial across-fiber pattern coding—odorants stimulate multiple OR types to varying degrees, producing characteristic spatial and temporal patterns across ORN populations and glomeruli.
• Temporal coding:
  • In both systems, timing (latency, oscillatory synchronization) can convey additional information about stimulus identity and concentration.
• Concentration coding:
  • Changes in firing rate and recruitment of additional receptor types or neurons can reflect increasing stimulus concentration.

Perception and Psychophysics

• Qualities of taste: The canonical basic tastes are sweet, salty, sour, bitter, and umami. Other proposed modalities include fat (oleogustus), metallic, and kokumi (mouthfulness) though their status varies.
• Olfactory perception:
  • Humans can discriminate thousands of odors despite a limited set of receptor types due to combinatorial coding.
  • Odor identification is often difficult without context; recognition improves with experience and semantic labels.
• Flavor:
  • Flavor arises from multisensory integration—taste, smell (especially retronasal olfaction), somatosensory (texture), temperature, and vision. Retronasal olfaction (odorants released in the mouth during eating and traveling to the olfactory epithelium via the nasopharynx) is critical for flavor.
• Adaptation and habituation:
  • Continuous exposure to a taste or odor leads to decreased perceived intensity (peripheral adaptation and central habituation), enabling detection of novel stimuli.

Development and Plasticity

• Ontogeny:
  • Both taste and olfactory receptor neurons originate from embryonic ectodermal tissues and undergo maturation and refinement during development.
  • Early life exposure to flavors (via amniotic fluid and breast milk) shapes later food preferences and acceptance.
• Regeneration:
  • ORNs and TRCs exhibit ongoing turnover, requiring continual regeneration to maintain sensory function. Basal stem cells replenish receptor populations.
• Plasticity:
  • Sensory receptor expression and central representations can be modulated by experience, learning (e.g., conditioned taste aversion or preference), injury, and disease.

Clinical Considerations and Disorders

• Anosmia and hyposmia:
  • Complete loss (anosmia) or partial loss (hyposmia) of smell can result from viral infections (including post-viral olfactory dysfunction), head trauma (shearing of ORN axons), sinonasal disease (obstruction), neurodegenerative disorders (e.g., Parkinson’s, Alzheimer’s), and congenital anosmia.
  • Olfactory dysfunction has significant impact on quality of life, safety (inability to detect smoke or gas leaks), and nutrition.
• Ageusia and hypogeusia:
  • Loss or impairment of taste may arise from medications, radiation therapy, nutritional deficiencies (e.g., zinc), systemic disease, or nerve injury.
  • Often, reported taste loss is actually olfactory dysfunction, since flavor perception depends heavily on retronasal olfaction.
• Phantosmia and parosmia:
  • Phantosmia: perception of odors that are not present (phantom smells).
  • Parosmia: distorted odor perception—commonly reported during recovery from post-viral olfactory loss and often distressing as pleasant odors become unpleasant.
• Assessment:
  • Clinical testing includes odor identification, discrimination, and threshold tests (e.g., Sniffin’ Sticks, UPSIT). Gustatory testing uses taste strips, whole-mouth tests, and electrogustometry.
• Rehabilitation:
  • Olfactory training (repeated sniffing of a set of odorants over months) can improve function in some cases of post-viral and post-traumatic olfactory loss.
  • Management of taste disturbances focuses on treating underlying causes, nutritional counseling, and flavor enhancement strategies.

Evolutionary and Comparative Perspectives

• Diversity across species:
  • Olfactory receptor gene repertoires vary widely across species—terrestrial mammals often have expansive OR gene families, reflecting ecological reliance on smell.
  • Aquatic species possess specialized chemosensory systems adapted to dissolved molecules, including pheromone detection and waterborne cues.
• Pheromones and chemical communication:
  • Many animals use pheromones to coordinate reproductive, territorial, and social behaviors via accessory olfactory systems (e.g., vomeronasal organ, VNO). The role of pheromones in humans remains debated and less clear.
• Adaptive significance:
  • The ability to detect specific tastants (sweet for energy-rich carbohydrates, umami for amino acids/protein, bitter for potential toxins) and odors (food sources, predators, mates) has important fitness consequences.

Integrative and Cognitive Aspects

• Emotional and memory links:
  • Olfactory pathways to the amygdala and hippocampus yield strong odor-evoked memories and affective responses. A single odor can evoke vivid autobiographical recollections—phenomena sometimes called the “Proustian effect.”
• Decision-making and reward:
  • Taste and smell inputs modulate feeding decisions via OFC, hypothalamic circuits, and dopaminergic reward pathways. Palatability and anticipated reward influence ingestion beyond homeostatic needs.
• Multisensory integration:
  • The brain synthesizes chemical senses with visual, somatosensory, and auditory cues to form unified percepts (e.g., flavor). Context, expectation, and attention shape hedonic ratings and identification.
• Cognitive modulation:
  • Attention, learning, and expectation can enhance or attenuate taste and odor perception. Top-down influences from prefrontal areas alter sensory processing to align perception with goals and context.

Experimental Methods and Research Techniques

• Molecular and genetic tools:
  • Gene knockout/knock-in models (e.g., OR knockouts, TRP channel manipulations) reveal receptor function and coding strategies.
  • Transgenic reporters (e.g., OR promoter–driven fluorescent markers) map ORN projections and convergence patterns.
• Electrophysiology:
  • Single-unit recordings from ORNs, mitral/tufted cells, and gustatory neurons elucidate response properties and temporal patterns.
• Imaging:
  • Functional MRI and PET map central processing in humans; optical imaging (e.g., calcium imaging, voltage-sensitive dyes) visualizes activity patterns in animal models.
• Behavioral assays:
  • Preference tests, conditioned taste aversion, odor discrimination, and operant conditioning paradigms assess perceptual thresholds and behavioral relevance.
• Psychophysics:
  • Human psychophysical methods quantify detection thresholds, identification accuracy, intensity scaling, and cross-adaptation effects.

Applications and Technological Interfaces

• Flavor industry:
  • Understanding chemical senses informs the design of foods and beverages, flavor enhancers, and strategies to reduce sugar or salt while maintaining palatability.
• Diagnostics and biomarkers:
  • Olfactory dysfunction is an early biomarker for several neurodegenerative diseases; standardized testing can aid in early detection.
• Electronic noses and taste sensors:
  • Bioinspired sensors aim to mimic biological olfactory and gustatory systems for applications in quality control, environmental monitoring, and medical diagnostics.
• Therapeutic strategies:
  • Modulating chemosensory receptors or central circuits may offer interventions for appetite disorders, obesity, or taste disorders resulting from cancer therapies.

Summary and Conclusions

The chemical senses of taste and smell are intricate systems that transduce chemical stimuli into rich perceptual experiences, guide essential behaviors, and influence emotion and memory. They rely on specialized peripheral receptor cells, diverse molecular transduction mechanisms, and hierarchical neural processing from brainstem nuclei to cortical and limbic structures.

Coding strategies combine specificity and combinatorial patterns, permitting discrimination among vast repertoires of chemicals. Aging, disease, and injury can impair these senses, with considerable impact on nutrition and quality of life; however, regenerative capacity and plasticity offer avenues for recovery and rehabilitation. Continued research—spanning molecular biology, systems neuroscience, psychophysics, and computational modeling—will deepen our understanding of how chemical information is represented and utilized by the brain, with broad implications for health, industry, and technology.

Key References for Further Reading

• Buck, L., & Axel, R. (1991). A novel multigene family may encode odorant receptors: A molecular basis for odor recognition. Cell, 65(1), 175–187. https://doi.org/10.1016/0092-8674(91)90418-X
• Lindemann, B. (2001). Receptors and transduction in taste. Nature, 413(6852), 219–225. https://doi.org/10.1038/35093032
• Shepherd, G. M. (2004). The human sense of smell: Are we better than we think? PLoS Biology, 2(5), e146. https://doi.org/10.1371/journal.pbio.0020146
• Ache, B. W., & Young, J. M. (2005). Olfaction: Diverse species, conserved principles. Neuron, 48(3), 417–430. https://doi.org/10.1016/j.neuron.2005.10.022
• Rolls, E. T. (2015). Taste, olfactory, and food reward value processing in the brain. Progress in Neurobiology, 127–128, 64–90. https://doi.org/10.1016/j.pneurobio.2015.03.002