Chapter 53: The Chemical Senses—Taste and Smel

Written By: Razaullah Khattak (DPT Medical Professional)

Student Source Reference: Guyton and Hall Textbook of Medical Physiology

Chapter: 53

To read more check out Chapter 37:

https://mednotesbyrazaullah.blogspot.com/2026/09/chapter-37-pulmonary-ventilation.html

Chemical Senses of Taste and Smell

The chemical senses of taste (gustation) and smell (olfaction) allow us to evaluate food, detect environmental hazards, and navigate nutritional needs. These senses trigger physiological responses involved in digestion, such as salivation, while also serving protective functions by prompting the rejection of toxic or spoiled food. Beyond survival, both gustatory and olfactory pathways connect to limbic and behavioral networks in the central nervous system, tying chemical signals directly to emotion, memory, and visceral reflexes.

Taste signals originate from specialized receptor cells located within taste buds on the tongue, palate, pharynx, and esophagus. Smell signals are captured by primary sensory neurons in the olfactory epithelium located at the top of the nasal cavity. Both systems transduce chemical interactions into electrical potentials through membrane channels or G-protein coupled receptors, transmitting visual information into distinct subcortical and cortical processing centers.

Primary Sensations and Chemical Receptors of Taste

• Taste perception relies on chemical receptors that respond to distinct chemical stimuli on the tongue.
• Taste and smell separate dangerous foods from pleasant ones.
• Chemical senses elicit digestive and metabolic physiological responses.
• Chemical senses connect strongly to emotional and behavioral functions.
• Taste perception combines smell, texture, and pain sensations.
• Taste allows selection of food based on metabolic needs.
• At least 13 chemical receptors exist in taste cells.
• Receptors include sodium, potassium, chloride, adenosine, and inosine.
• Receptors include sweet, bitter, glutamate, and hydrogen ion types.
• Receptors are grouped into five primary taste categories.
• Primary categories are sour, salty, sweet, bitter, umami.
• Sour taste intensity is proportional to hydrogen ion concentration.
• Salty taste is elicited mainly by sodium ion concentration.
• Sweet taste is caused primarily by organic chemical compounds.
• Small chemical changes alter sweet substances into bitter ones.
• Bitter taste is caused by long-chain nitrogen compounds and alkaloids.
• Alkaloid drugs like quinine, caffeine, and nicotine taste bitter.
• High-intensity bitter taste triggers food rejection to prevent poisoning.
• Umami designates a pleasant taste elicited by l-glutamate.
• Umami dominates in meat extracts and aging cheese.

Taste Thresholds, Indices, and Taste Blindness

• Sensory thresholds vary among primary tastes, with bitter taste exhibiting the highest sensitivity.
• Hydrochloric acid sour threshold averages 0.0009N.
• Sodium chloride salty threshold averages 0.01M.
• Sucrose sweet threshold averages 0.01M.
• Quinine bitter threshold averages 0.000008M.
• Bitter taste sense is significantly more sensitive than others.
• High bitter sensitivity protects the body against dangerous toxins.
• Relative taste index measures reciprocals of taste thresholds.
• Hydrochloric acid, quinine, sucrose, and NaCl have index 1.
• Brucine has a high bitter taste index of 11.
• 1-Propoxy-2-amino-4-nitrobenzene has a sweet index of 5000.
• Saccharin has a sweet taste index of 675.
• Fructose has a sweet taste index of 1.7.
• Glucose has a sweet taste index of 0.8.
• Taste blindness occurs for specific chemical thiourea compounds.
• Phenylthiocarbamide demonstrates taste blindness in 15 to 30 percent.

Anatomy and Location of Taste Buds

• Taste buds are microscopic sensory organs embedded in specialized lingual papillae and oral tissues.
• Taste buds measure 1/30 millimeter wide and 1/16 millimeter long.
• Each taste bud contains about 50 modified epithelial cells.
• Cells include sustentacular supporting cells and taste receptor cells.
• Taste cells continuously undergo replacement via mitotic cell division.
• Taste cell lifespan averages about 10 days in mammals.
• Taste cell tips surround a minute outer taste pore.
• Microvilli or taste hairs protrude into the taste pore.
• Microvilli provide the primary receptor surface for taste chemicals.
• Branching taste nerve fibers interweave around taste cell bodies.
• Vesicles beneath cell membranes release neurotransmitters to excite nerves.
• Circumvallate papillae walls contain large numbers of taste buds.
• Fungiform papillae on anterior tongue contain moderate taste buds.
• Foliate papillae along lateral tongue folds contain moderate buds.
• Palate, tonsillar pillars, epiglottis, and esophagus contain taste buds.
• Adults possess 3000 to 10,000 active taste buds.
• Taste buds degenerate after age 45, reducing taste sensitivity.
• Low chemical concentrations excite specific primary taste buds.
• High chemical concentrations excite multiple primary taste categories.

Mechanisms of Taste Bud Activation and Transduction

• Contact between taste chemicals and taste hairs depolarizes the cell to produce a receptor potential.
• Taste cell membranes carry a resting negative electrical potential.
• Applying taste chemicals causes loss of negative membrane potential.
• Depolarization change is called the taste receptor potential.
• Potential change is proportional to logarithm of chemical concentration.
• Taste chemicals bind protein receptors on taste villi membranes.
• Receptor binding opens ion channels to allow sodium or hydrogen influx.
• Saliva washes away taste chemicals to clear the stimulus.
• Receptor protein types determine the specific taste quality perceived.
• Salty and sour cations open specific apical ion channels directly.
• Sweet and bitter binding activates intracellular second-messenger systems.
• Initial stimulus application causes a peak nerve discharge rate.
• Discharge adapts within seconds to a lower continuous level.

Central Pathways, Reflexes, and Adaptation of Taste

• Taste signals travel via cranial nerves to the brainstem solitarius tract before projecting to the cortex.
• Anterior two-thirds tongue signals travel via lingual and facial nerves.
• Facial nerve fibers pass through chorda tympani to solitarius tract.
• Circumvallate papillae signals travel via the glossopharyngeal nerve.
• Posterior mouth and pharynx signals enter via the vagus nerve.
• All taste nerve fibers synapse in tractus solitarius nuclei.
• Second-order neurons ascend to thalamus ventral posterior medial nucleus.
• Third-order neurons project to postcentral gyrus and insular cortex.
• Taste pathways closely parallel tongue somatosensory neural pathways.
• Solitarius signals project to salivatory nuclei controlling salivation.
• Taste sensations adapt rapidly within a minute of stimulation.
• Taste bud adaptation accounts for only half of total adaptation.
• Final extreme adaptation occurs within central nervous system processing.

Taste Preference and Dietary Regulation

• Taste preferences change based on metabolic needs and central nervous system feedback.
• Taste preference leads animals to choose needed food types.
• Preferences change in accordance with body tissue metabolic needs.
• Salt-depleted animals automatically select high-sodium drinking water.
• Insulin-injected animals select sweetest foods due to low sugar.
• Calcium-depleted animals select water with high calcium chloride.
• Humans reject foods eliciting unpleasant affective taste sensations.
• Taste preference mechanisms reside within central nervous system circuits.
• Previous pleasant or unpleasant taste experiences determine taste preferences.
• Illness after eating induces negative taste aversion toward foods.

Morphology and Biology of the Olfactory Membrane

• The olfactory membrane is located in the upper nasal cavity and contains specialized sensory neurons.
• Olfactory membrane lies in the superior part of each nostril.
• Membrane covers superior septum, superior turbinate, and middle turbinate.
• Surface area of olfactory membrane measures 2.4 square centimeters.
• Olfactory receptor cells are bipolar neurons from central nervous origin.
• Epithelium contains roughly 100 million olfactory receptor cells.
• Sustentacular cells support bipolar olfactory cells in epithelium.
• Mucosal cell ends form knobs with 4 to 25 cilia.
• Olfactory cilia measure 0.3 micrometer wide and 200 micrometers long.
• Cilia project into nasal cavity mucus to react with odorants.
• Bowman's glands spaced in membrane secrete surface mucus layer.

Signal Transduction and Excitation in Olfactory Cells

• Odorant molecules activate a G-protein second-messenger cascade within olfactory cilia.
• Olfactory cilia respond directly to airborne olfactory chemical stimuli.
• Odorants diffuses into mucus and bind ciliary receptor proteins.
• Receptor proteins thread through ciliary membrane seven times.
• Odorant binding activates a three-subunit G-protein complex.
• Alpha subunit breaks away to activate adenylyl cyclase enzyme.
• Activated adenylyl cyclase converts intracellular ATP into cAMP.
• Intracellular cAMP opens gated sodium channels in ciliary membrane.
• Inward sodium ion flux increases positive internal cell potential.
• Depolarization excites olfactory neuron to transmit action potentials.
• Second-messenger cascade greatly multiplies excitatory odorant effects.
• Cascading chemical steps allow detection of minute odorant concentrations.
• Volatile substances sniffed into nostrils elicit smell sensations.
• Odorants must be water soluble to pass through mucus.
• Lipid solubility helps odorants cross the lipid ciliary barrier.
• Unstimulated olfactory cell membrane potential averages −55 millivolts.
• Unstimulated cells generate continuous action potentials at slow rates.
• Odorants depolarize cells from −55 millivolts toward −30 millivolts.
• Depolarization increases action potential firing to 20–30 per second.
• Impulse firing rate matches stimulus strength logarithm.
• Olfactory receptors adapt 50 percent in the first second.
• Complete smell adaptation occurs via central nervous system inhibition.
• Centrifugal fibers activate inhibitory granule cells in olfactory bulb.

Primary Sensations, Affective Nature, and Thresholds of Smell

• Smell sensitivity is characterized by low detection thresholds and diverse receptor categories.
• Past classifications proposed primary sensations like camphoraceous and musky.
• Classifications included floral, pepperminty, ethereal, pungent, and putrid.
• Modern genetic studies indicate at least 100 primary smell sensations.
• Up to 1000 different odorant receptor genes may exist.
• Discrete odor blindness exists for over 50 different substances.
• Odor blindness reflects lack of specific ciliary receptor proteins.
• Smell carries strong affective qualities of pleasantness or unpleasantness.
• Foul smells from previously offensive food trigger immediate nausea.
• Odors act as primary excitants of sexual drive in animals.
• Methylmercaptan is smelled at one 25 trillionth of a gram per mL.
• Low thresholds allow adding methylmercaptan to detect gas leaks.
• Maximum smell intensity occurs at 10 to 50 times threshold.
• Compressed intensity scales detect presence rather than exact quantity.

Transmission of Olfactory Signals into the Central Nervous System

• Olfactory pathways send axons directly through the olfactory bulb into primitive and modern brain structures.
• Olfactory bulb and tract form an anterior brain outgrowth.
• Olfactory bulb lies over perforated cribriform plate.
• Short olfactory cell axons pass upward through cribriform perforations.
• Axons terminate in globular structures called glomeruli in bulb.
• Each glomerulus receives 25,000 olfactory cell axons.
• Glomeruli synapse with 25 mitral cells and 60 tufted cells.
• Mitral and tufted cell axons form the olfactory tract.
• Olfactory tract divides into medial and lateral olfactory areas.
• Medial olfactory area represents a very old olfactory system.
• Medial area septal nuclei project into primitive limbic structures.
• Medial system mediates basic feeding reflexes like salivation and licking.
• Lateral olfactory area includes prepyriform, pyriform, and amygdaloid cortex.
• Lateral system projects into hippocampus for learned food preferences.
• Lateral area causes absolute food aversion after nausea experiences.
• Lateral signals enter paleocortex without passing first through thalamus.
• Newer pathway passes via dorsomedial thalamus to orbitofrontal cortex.
• Newer system assists in conscious analysis and perception of odors.
• Centrifugal brain fibers send inhibitory feedback via granule cells.
• Inhibitory granule cell feedback sharpens specific odor discrimination.

Disclaimer: This content is for educational purposes only and not a substitute for professional medical advice.