Chemical Senses: Taste (Gustation) and Smell (Olfaction)

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Chemical Senses

Introduction to Chemical Senses

The chemical senses, gustation (taste) and olfaction (smell), are mediated by chemoreceptors that detect chemical stimuli in the environment. These senses are essential for detecting food, environmental hazards, and play a role in flavor perception. While other chemoreceptors exist throughout the body (e.g., in skin, muscles, circulatory and digestive systems), this guide focuses on gustation and olfaction.

Gustation: Detection of dissolved substances by taste buds.
Olfaction: Detection of airborne chemicals by olfactory receptors.
Other sensory inputs (texture, temperature, cutaneous receptors) contribute to overall flavor perception.

Taste (Gustation)

Definition and Distinction

Taste is the sensory mechanism mediated by chemoreceptors sensitive to dissolved organic and inorganic compounds. Taste receptors are located on the tongue, mouth, pharynx, and other oral structures. Flavor is a broader concept, involving taste, olfactory, tactile, and thermal attributes.

Loss of taste: Ageusia

Qualities of Taste

There are five primary taste qualities, each associated with specific types of molecules and regions of the tongue:

Sweet: Produced by organic molecules (sugars, glycols, aldehydes); most sensitive at the tip of the tongue.
Bitter: Produced by alkaloids (e.g., quinine, caffeine); most sensitive at the back of the tongue.
Salt: Produced by ionizable salts; most sensitive at the front half of each side of the tongue.
Sour: Produced by acids; relates to pH; most sensitive at the back half of each side of the tongue.
Umami: Taste of glutamate; not localized.

All taste sensations are combinations of these primary qualities.

Threshold of Taste

The threshold of taste is the lowest concentration of a stimulus that can be detected. Increasing the area of the tongue exposed to a stimulus decreases the threshold concentration. Taste thresholds are genetically determined.

Adaptation

Continuous exposure to a taste stimulus leads to decreased intensity and afferent firing rate, resulting in adaptation (the solution becomes tasteless).

Taste Receptors and Papillae

Taste receptors are specialized structures on the tongue, organized into papillae:

Filiform papillae: Mechanical, nongustatory.
Fungiform papillae: 8-10 taste buds per papilla.
Circumvallate papillae: 7-12 in a V-shaped row, each with ~200 taste buds.
Foliate papillae: Located on lateral border, anterior to circumvallate, numerous taste buds.

Taste Buds and Cells

Each taste bud is a cluster of 40-60 cells, found on the tongue, palate, epiglottis, and pharynx.

Taste receptor cells: Chemoreceptors, elongated with microvilli at the apical surface.
Basal cells: Source of new taste receptors (regeneration).
Gustatory afferent fibers: Conduct action potentials to the brain.

Taste cells have a short lifespan and are continuously replaced.

Taste Coding

Afferent fibers respond to multiple taste stimuli but are most sensitive to one modality. Taste sensation depends on the pattern of nerve fiber activation.

Mechanisms of Taste Transduction

Different taste qualities use distinct cellular mechanisms:

Salty: Sodium enters via amiloride-sensitive sodium channels, causing depolarization and calcium influx, leading to neurotransmitter release.
Sour: Hydrogen ions enter and close potassium channels, causing depolarization and calcium influx.
Sweet: Sweet molecules bind G-protein coupled receptors, activating adenylyl cyclase, increasing cAMP, activating Protein Kinase A, closing potassium channels, causing depolarization and calcium influx.
Bitter I: Bitter molecules block potassium channels, causing depolarization and calcium influx.
Bitter II: Bitter molecules bind G-protein coupled receptors, activating phospholipase C, producing IP3, releasing internal calcium stores, causing neurotransmitter release (no change in membrane potential).
Umami: Glutamate binds transmitter-gated sodium channels, causing depolarization and calcium influx.

Nerve Supply to Taste

Taste cells are innervated by branches of the facial, glossopharyngeal, and vagus nerves:

Anterior two-thirds of tongue: Lingual nerve (branch of chorda tympani, facial nerve).
Posterior one-third: Glossopharyngeal nerve.
Pharyngeal portion, palate, epiglottis: Vagus nerve.
All afferent fibers end in the gustatory nucleus (medulla).
Gustatory nucleus neurons project to the ventral posterior medial nucleus (VPM) of the thalamus.
VPM projects to the primary gustatory cortex (Brodmann's area 43).
Gustatory nucleus also projects to brainstem regions for autonomic functions (swallowing, salivation, gagging, vomiting, digestion, respiration), hypothalamus, and amygdala (appetite, food preferences).

Neural Coding for Taste

Labeled line hypothesis: Each receptor responds to a specific flavor; each axon represents a taste.
Population coding: Each receptor responds to many flavors, but differentially; the population activity represents a taste.
Gustation uses population coding.
Other sensory inputs contribute to overall taste perception.

Taste Perception

Subtlety of taste is largely interpreted through olfactory input. Blocking olfactory receptors limits taste range; strong odors can influence taste interpretation.

Olfaction (Smell)

Definition and Characteristics

Olfaction is the sensory function involving chemoreceptors sensitive to airborne chemicals dissolved in the nasal mucosa. Loss of smell is called anosmia.

Characteristics of Odorants

Volatile: Must transition from liquid to gas.
Water-soluble: Must penetrate the mucous layer.
Lipid-soluble: Must penetrate cell membranes of olfactory receptor cells.

Threshold and Discrimination

Olfactory receptors vary in sensitivity; some can be activated by a few molecules. The olfactory system discriminates compounds based on receptor site specificity, including stereochemistry (dextro/levo, cis/trans). Unlike taste, there are no primary olfactory qualities.

Adaptation

Olfactory sensation adapts rapidly with continued exposure to the same odorant.

Anatomy of Olfactory System

Located on the superior nasal concha, adjacent to the nasal septum.
Not in direct contact with inspired air; sniffing increases exposure.

Olfactory Pathway

Receptor cells → olfactory nerve → olfactory bulb → olfactory tract → olfactory cortex

Histology of Olfactory Epithelium

Receptor cells: Bipolar neurons with cell bodies in nasal mucosa; dendrites extend to epithelium; unmyelinated axons form olfactory nerve.
Bowman's glands/supporting cells: Tubular, microvilli, yellow-brown pigment.
Basal cells: Stem cells for regeneration.
Olfactory bulb: Contains mitral, granule, and tufted cells.
- Mitral cells: Major elements; dendrites synapse with olfactory nerve; axons form olfactory tract.
- Granule cells: Most numerous; dendrites synapse with mitral cells; no axon.
- Tufted cells: Receive olfactory nerve axons; send axons through olfactory tract.

Mechanisms of Olfactory Transduction

Odorant binds to G-protein coupled receptor.
500-1000 different olfactory receptor proteins (genetically coded).
Adenylyl cyclase produces cAMP, which binds to cation channels (Na+, Ca2+).
Ca2+ influx opens Ca2+-sensitive Cl- channels.
Ca2+ and Cl- cause membrane depolarization (receptor potential).
Action potentials generated in olfactory receptor fibers.

Central Olfactory Pathways

Olfactory receptor axons end in glomeruli of the olfactory bulb (precise mapping).
Glomeruli receive inputs from receptors with the same genetically specified molecule.
Olfactory bulb neurons project to the olfactory tubercle, then to the medial dorsal nucleus of the thalamus, and finally to the orbitofrontal cortex.
Olfactory bulb neurons also project to other brain regions (olfactory cortex, hypothalamus, amygdala).
Functions: odor discrimination, perception, motivation, emotions, reproduction, feeding, imprinting, memory.

Neural Coding in Olfactory System

Olfactory receptors respond to a variety of odorants.
Population coding is used; both spatial distribution and timing of action potentials are important.

Summary Table: Taste Papillae and Their Features

Papilla Type	Location	Number of Taste Buds	Function
Filiform	Entire tongue surface	None	Mechanical, nongustatory
Fungiform	Tip and sides of tongue	8-10 per papilla	Taste
Circumvallate	V-shaped row at back of tongue	~200 per papilla	Taste
Foliate	Lateral border, anterior to circumvallate	Numerous	Taste

Summary Table: Taste Modalities and Transduction Mechanisms

Taste Modality	Transduction Mechanism	Key Ion/Protein
Salty	Na+ influx via amiloride-sensitive channels	Na+, Ca2+
Sour	H+ influx, closes K+ channels	H+, Ca2+
Sweet	G-protein coupled receptor, cAMP pathway	cAMP, PKA, Ca2+
Bitter I	Blocks K+ channels	K+, Ca2+
Bitter II	G-protein coupled receptor, IP3 pathway	IP3, Ca2+ (internal stores)
Umami	Transmitter-gated Na+ channel	Na+, Ca2+

Key Equations and Concepts

Receptor Potential: Membrane depolarization due to ion influx (Na+, Ca2+, Cl-).
Threshold:
Population Coding:

Example

Example: Eating a sweet apple involves activation of sweet taste receptors (G-protein coupled, cAMP pathway) and olfactory receptors (G-protein coupled, cAMP pathway), as well as tactile and thermal receptors for texture and temperature. Blocking the nose reduces flavor perception to basic taste qualities.

Additional info: The notes have been expanded to clarify mechanisms, provide definitions, and organize tables for study purposes.