Taste and Smell: How Your Chemical Senses Work

What Are Taste and Smell and How Do They Work?

Taste and smell are chemical senses that detect molecules in food and air. Taste receptors on the tongue identify five basic tastes (sweet, salty, sour, bitter, umami), while smell receptors in the nose detect thousands of different odor molecules. Together, they create the complex experience of flavor.

Taste and smell are often called the "chemical senses" because they work by detecting chemical molecules in your environment. While other senses like vision and hearing respond to physical phenomena (light waves and sound waves), taste and smell respond to the actual chemical composition of substances that enter your mouth and nose. This makes them fundamentally different from your other senses and uniquely suited to their roles in helping you interact with the world around you.

These two senses evolved primarily to help organisms find food and avoid danger. The ability to taste sweetness helped our ancestors identify energy-rich foods containing sugars and carbohydrates. Bitter taste sensitivity protected them from consuming potentially toxic plants, as many plant toxins taste bitter. The sense of smell allowed early humans to detect approaching predators, find ripe fruit, and identify whether food had spoiled before eating it. Today, while we may not face the same survival pressures, these senses continue to play crucial roles in our daily lives, from enjoying a delicious meal to detecting a gas leak or fire.

The remarkable integration between taste and smell creates what we experience as flavor. When you eat, volatile molecules from food travel through the back of your throat to your nasal cavity in a process called retronasal olfaction. This is why food seems to lose its flavor when you have a cold—your nose is blocked, preventing these odor molecules from reaching your smell receptors. Understanding how these senses work together helps explain many everyday experiences and can guide you in maintaining these important sensory functions.

The Chemical Detection Process

Both taste and smell work through a similar basic mechanism: specialized receptor cells detect specific chemical molecules and convert this information into electrical signals that travel to the brain. However, the details of how this happens differ significantly between the two senses. Taste receptors are located in structures called taste buds, which are primarily found on the tongue but also exist on the roof of the mouth, throat, and even the epiglottis. Smell receptors, on the other hand, are located in a specialized patch of tissue high in the nasal cavity called the olfactory epithelium.

When a chemical molecule binds to a receptor, it triggers a cascade of events inside the receptor cell. This leads to the generation of an electrical signal that travels along nerve fibers to the brain. The brain then interprets these signals as specific tastes or smells. This process happens remarkably quickly—you can identify a taste or smell within milliseconds of exposure, allowing for rapid responses to potentially dangerous substances.

How Does the Sense of Taste Work?

Taste works through specialized receptor cells in taste buds that detect five basic tastes: sweet (sugars, energy), salty (minerals, electrolytes), sour (acids, spoiled food), bitter (potential toxins), and umami (proteins, amino acids). These signals travel via cranial nerves to the brain's gustatory cortex for processing.

Your sense of taste, scientifically known as gustation, begins with taste buds—small sensory organs containing 50-100 taste receptor cells each. Adults typically have between 2,000 and 10,000 taste buds, with most located on the tongue. Contrary to the popular "tongue map" myth that suggests different areas of the tongue taste different flavors, research has definitively shown that all five basic tastes can be detected across the entire tongue surface. The distribution of taste receptors is relatively uniform, though sensitivity may vary slightly in different regions.

Taste buds are housed within small bumps on the tongue called papillae. There are four types of papillae, three of which contain taste buds. Fungiform papillae, shaped like small mushrooms, are scattered across the front two-thirds of the tongue and each contains 3-5 taste buds. Circumvallate papillae are large, dome-shaped structures arranged in a V-shape at the back of the tongue, with each one containing hundreds of taste buds. Foliate papillae are located on the sides of the tongue and contain several hundred taste buds. The fourth type, filiform papillae, are the most numerous but don't contain taste buds—instead, they provide texture sensation and help move food around the mouth.

When you eat, saliva helps dissolve food particles and carry them to taste receptors. Chemicals from the food enter small openings called taste pores and bind to receptor proteins on the surface of taste receptor cells. Different receptor cells are specialized for different tastes. Some use ion channels that open in response to specific ions (like sodium for salty taste or hydrogen ions for sour taste), while others use G protein-coupled receptors that trigger internal cellular signaling cascades (for sweet, bitter, and umami tastes).

The Five Basic Tastes

For many years, scientists recognized only four basic tastes: sweet, salty, sour, and bitter. In 1908, Japanese scientist Kikunae Ikeda identified a fifth taste he called "umami," meaning "pleasant savory taste" in Japanese. It took nearly a century for umami to be widely accepted by the scientific community, but it is now recognized as a distinct basic taste with its own dedicated receptors. Recent research suggests there may be additional basic tastes, including a fat taste called oleogustus and possibly tastes for calcium, carbon dioxide, and water, though these are not yet universally accepted.

How Taste Signals Reach the Brain

Once taste receptor cells are activated, they release neurotransmitters that stimulate sensory neurons. These signals travel along three cranial nerves to reach the brain. The facial nerve (cranial nerve VII) carries taste information from the front two-thirds of the tongue. The glossopharyngeal nerve (cranial nerve IX) serves the back one-third of the tongue. The vagus nerve (cranial nerve X) carries taste signals from the throat and epiglottis. These nerves converge in the brainstem and then project to the thalamus, which relays the information to the gustatory cortex in the insula and frontal operculum regions of the brain.

The brain doesn't just passively receive taste information—it actively integrates signals from multiple sources to create your perception of flavor. Information about food temperature, texture, and appearance from other senses combines with taste and smell signals. Additionally, your expectations, memories, and emotional state all influence how you perceive taste. This is why the same food can taste different depending on your mood, your hunger level, or even the color of the plate it's served on.

How Does the Sense of Smell Work?

Smell works through approximately 400 types of olfactory receptor neurons in the nasal cavity that can detect over one trillion different odors. Odor molecules bind to these receptors, generating signals that travel directly to the olfactory bulb in the brain, then to areas controlling memory, emotion, and conscious smell perception.

Your sense of smell, scientifically known as olfaction, is remarkably sophisticated. Humans possess approximately 400 different types of olfactory receptors, each encoded by a different gene. These receptors work in combination to detect an estimated one trillion different odors—far more than the roughly 10 million colors we can see or the 340,000 tones we can hear. Each odor molecule activates a specific pattern of receptors, and the brain interprets this pattern as a particular smell. This combinatorial coding system allows a relatively small number of receptor types to distinguish an enormous variety of odors.

The olfactory epithelium, where smell receptors are located, is a specialized tissue about the size of a postage stamp located high in the nasal cavity. This tissue contains millions of olfactory receptor neurons, each extending a single dendrite to the surface where it branches into cilia covered with receptor proteins. When you breathe in, air carrying odor molecules flows past this tissue. Some molecules dissolve in the mucus layer covering the epithelium and bind to receptors on the cilia, triggering electrical signals in the receptor neurons.

Unlike other sensory neurons, olfactory receptor neurons have a direct connection to the brain without passing through the thalamus first. Their axons pass through small holes in the cribriform plate (a thin bone at the base of the skull) and enter the olfactory bulb, a structure at the front of the brain that processes smell information. This direct pathway may explain why smells can trigger such immediate and powerful responses. From the olfactory bulb, signals travel to several brain regions including the piriform cortex (for conscious smell perception), the amygdala (for emotional responses), and the hippocampus (for memory formation).

Orthonasal vs Retronasal Smell

There are two ways odor molecules can reach your smell receptors. Orthonasal olfaction occurs when you sniff through your nose—this is how you smell a flower or detect smoke in the air. Retronasal olfaction occurs when odor molecules from food in your mouth travel through the back of your throat and up into the nasal cavity. This second pathway is crucial for experiencing flavor while eating, as it allows your brain to integrate smell information with taste information in real-time as you chew and swallow.

Interestingly, the brain processes orthonasal and retronasal smells somewhat differently. When you detect an odor through sniffing, you tend to perceive it as coming from the external environment. When the same odor reaches your nose retronasally during eating, you perceive it as part of the food's taste rather than its smell. This is why you might describe chocolate as "tasting" rich and complex, even though much of that complexity comes from volatile molecules detected by your nose rather than your tongue.

Adaptation and Recovery

Both smell and taste show a phenomenon called sensory adaptation, where continuous exposure to a stimulus causes decreased sensitivity to it. If you've ever noticed that you stop smelling a scent after being in a room for a while, or that food tastes less intense as you continue eating, you've experienced this adaptation. This mechanism helps your sensory systems remain sensitive to changes in your environment rather than being overwhelmed by constant stimulation.

Olfactory receptor neurons are unusual among nerve cells in that they are replaced throughout life. New neurons are generated from stem cells in the olfactory epithelium, mature over several weeks, and then replace older neurons that have died. This ongoing regeneration allows the olfactory system to recover from some types of damage, though the process becomes less efficient with age. Taste receptor cells also regenerate, with a turnover period of about 10-14 days.

Why Are Taste and Smell So Closely Connected?

Taste and smell are connected because they work together to create flavor perception. While taste identifies basic categories (sweet, salty, sour, bitter, umami), smell provides the detailed information that distinguishes between specific foods. About 80% of what we perceive as "taste" actually comes from smell through retronasal olfaction.

The intimate connection between taste and smell becomes obvious when one of these senses is impaired. People with anosmia (complete loss of smell) often report that food has become bland and uninteresting, even though their sense of taste remains intact. This occurs because so much of what we experience as flavor comes from odor molecules that reach the nose during eating. Without this olfactory input, we're left with only the basic taste sensations—sweet, salty, sour, bitter, and umami—which provide much less information about what we're eating.

Consider the difference between eating an apple and an onion while holding your nose. Both are crunchy, slightly sweet, and mildly acidic—their basic taste profiles are surprisingly similar. It's the volatile molecules that give each food its distinctive character, and these can only be detected by your nose. When you chew food, you release these volatile compounds, which travel from your mouth through the nasopharynx to your olfactory receptors. This is why chewing food thoroughly enhances its flavor—more volatiles are released and more reach your nose.

The brain integrates taste and smell signals in several regions, including the orbitofrontal cortex, which is considered the primary site for flavor perception. This integration is so seamless that we're usually unaware that our perception of flavor involves multiple senses. We experience flavor as a unified sensation that we attribute entirely to our mouth, even though much of the information is coming from our nose. This perceptual binding demonstrates the brain's remarkable ability to combine information from different sensory systems into a coherent experience.

The Role of Other Senses in Flavor

While taste and smell are the primary contributors to flavor, other senses also play important roles. The trigeminal nerve, which provides sensation to the face, detects irritating compounds like capsaicin in chili peppers, menthol in mint, and carbonation in fizzy drinks. These sensations—burning, cooling, tingling—add another dimension to flavor. Temperature and texture detected through touch receptors in the mouth also contribute, which is why the same soup can seem more satisfying when served hot and why crispy foods are often preferred over soggy ones.

Vision and hearing surprisingly influence flavor perception as well. Studies have shown that people rate wine as tasting better when served in expensive-looking glasses, that the color of food affects how sweet or flavorful it seems, and that the sound of a chip crunching affects how fresh it tastes. These multisensory interactions remind us that perception is constructed by the brain from many sources of information, not simply received from a single sense organ.

Why Does Smell Trigger Powerful Memories and Emotions?

Smell triggers powerful memories because the olfactory system connects directly to the limbic system—the brain's emotional and memory centers—bypassing the thalamus that other senses use. This direct connection to the amygdala (emotion) and hippocampus (memory) explains why smells can instantly evoke vivid, emotional memories, a phenomenon called the Proust effect.

Few experiences are as striking as catching a whiff of something—perhaps a particular perfume, a type of food, or the smell of rain on pavement—and being instantly transported back to a specific moment from your past, complete with vivid emotions. This phenomenon, sometimes called the Proust effect (after author Marcel Proust, who wrote famously about a madeleine cookie triggering childhood memories), reflects the unique neural architecture connecting smell to memory and emotion.

The olfactory bulb, where smell information first reaches the brain, has direct connections to both the amygdala (which processes emotions) and the hippocampus (which is crucial for memory formation). This is unusual—other senses must relay through the thalamus before reaching these structures. Because of this direct pathway, smell information reaches emotional and memory centers very quickly and without the filtering that thalamic processing provides. This may explain why smell-triggered memories tend to be more emotional and vivid than memories triggered by sights or sounds.

Research has shown that memories associated with smells are often among our oldest and most emotionally evocative. When people are asked to recall autobiographical memories in response to smells versus words or pictures, the smell-triggered memories tend to come from earlier in life (often from the first decade) and are rated as more emotional and more vivid. This suggests that the smell-memory connection is established early and remains powerful throughout life.

Evolutionary Significance

The strong connection between smell, emotion, and memory likely reflects evolutionary history. For our ancestors, quickly learning and remembering which foods were safe versus dangerous, which places harbored predators, and which individuals were family members would have been matters of survival. Smell provided crucial information for all these assessments, and the ability to form rapid, emotionally tagged memories about smells would have been highly advantageous.

Even today, smell continues to influence our emotions and behavior in subtle ways. Studies have shown that pleasant ambient odors can improve mood, increase prosocial behavior, and enhance cognitive performance. Certain scents are reliably associated with specific emotional states across cultures—lavender tends to be calming, citrus tends to be energizing. While some of these associations may be learned, others may reflect more fundamental properties of how our olfactory system is wired to emotional circuits.

What Causes Loss of Taste or Smell?

Loss of taste (ageusia) or smell (anosmia) can result from viral infections (including COVID-19), head injuries, nasal polyps, sinus conditions, aging, neurological diseases, medications, or exposure to toxic chemicals. Most cases from upper respiratory infections are temporary, but some causes lead to permanent impairment.

The COVID-19 pandemic brought unprecedented attention to disorders of smell and taste. Many people infected with SARS-CoV-2 experienced sudden loss or distortion of these senses, sometimes as their only symptom. This heightened awareness of chemosensory disorders led to increased research and public understanding of conditions that have long affected millions of people worldwide. Understanding the causes of taste and smell loss is important for prevention, treatment, and knowing when to seek medical attention.

Viral upper respiratory infections are the most common cause of temporary smell loss. Viruses can damage olfactory receptor neurons or cause inflammation that blocks odor molecules from reaching them. In most cases, smell returns within weeks as the infection resolves and damaged neurons regenerate. However, some people experience prolonged or permanent impairment, especially after severe infections. COVID-19 appears to have a particular affinity for attacking supporting cells in the olfactory epithelium, which may explain the high rate and sometimes prolonged nature of smell loss with this virus.

Head trauma is another significant cause of chemosensory loss. The delicate olfactory nerve fibers that pass through the cribriform plate can be sheared by sudden head movements, as in car accidents or falls. Even mild traumatic brain injury can cause smell loss, and recovery is often incomplete. The severity and permanence of post-traumatic smell loss depends on the extent of nerve damage and whether the olfactory epithelium itself was affected.

Common Causes of Chemosensory Loss

• Viral infections: Common cold, flu, COVID-19, and other respiratory viruses can temporarily or permanently damage smell and taste function
• Nasal and sinus conditions: Chronic sinusitis, nasal polyps, and deviated septum can block airflow to olfactory receptors
• Head injury: Trauma can damage olfactory nerves or brain regions involved in processing smell and taste
• Aging: Both smell and taste sensitivity naturally decline with age, particularly after age 60
• Neurological conditions: Parkinson's disease, Alzheimer's disease, and multiple sclerosis often involve early smell loss
• Medications: Some antibiotics, blood pressure medications, and chemotherapy drugs can affect taste and smell
• Exposure to toxins: Cigarette smoke, industrial chemicals, and certain solvents can damage chemosensory tissues
• Dental problems: Oral infections and poor dental hygiene can affect taste perception

Treatment and Recovery

Treatment for taste and smell disorders depends on the underlying cause. When nasal obstruction is responsible, treating the obstruction with medications, nasal steroids, or surgery may restore function. For post-viral smell loss, a promising approach called smell training has shown effectiveness. This involves repeatedly sniffing specific odors (typically rose, eucalyptus, lemon, and clove) for several months to stimulate olfactory nerve regeneration and brain plasticity. Studies suggest this can accelerate and improve recovery for many patients.

Unfortunately, some cases of chemosensory loss are not reversible with current treatments. When olfactory neurons have been severely damaged or when brain regions processing smell are affected, recovery may be limited. Research into stem cell therapies, neural regeneration, and other approaches continues to offer hope for future treatments. In the meantime, people with permanent smell loss can benefit from safety measures (like installing smoke and gas detectors) and from working with specialists to maximize their remaining sensory function and quality of life.

How Do Taste and Smell Change with Age?

Both taste and smell decline with age, particularly after 60. Smell sensitivity may decrease by 50% by age 80. Taste changes less dramatically but taste bud numbers decrease and sensitivity to certain tastes diminishes. These changes can affect appetite, nutrition, and safety (detecting spoiled food or gas leaks).

As we age, our chemical senses undergo gradual but significant changes. These alterations can affect quality of life, nutritional status, and safety in older adults. Understanding normal age-related changes versus pathological losses is important for both individuals and their healthcare providers. While some decline is inevitable, awareness of these changes can help people adapt and maintain the ability to enjoy food and detect environmental hazards.

Smell sensitivity typically peaks in early adulthood and remains relatively stable through middle age before declining more noticeably after about age 60. By age 80, many people have lost about half of their smell sensitivity compared to young adults. This decline occurs gradually and often goes unnoticed because people adapt to their diminished sense. The olfactory epithelium thins with age, contains fewer receptor neurons, and regenerates more slowly when damaged. Additionally, age-related changes in the brain's olfactory processing regions contribute to reduced smell perception.

Taste is generally more resilient to aging than smell, though changes do occur. The number of taste buds decreases with age, and the remaining taste buds may contain fewer receptor cells. Saliva production often decreases in older adults, which can affect how well food is dissolved and presented to taste receptors. Some medications commonly taken by older adults can also alter taste perception. However, the basic ability to detect the five tastes usually remains largely intact even into advanced age, though higher concentrations may be needed for detection.

Practical Implications

The decline in taste and smell with age has important practical implications. Many older adults report that food no longer tastes as good as it once did, which can lead to decreased appetite and poor nutrition. Some people compensate by adding excessive salt or sugar to food, which can exacerbate conditions like hypertension or diabetes. Others may lose interest in eating altogether, contributing to malnutrition and frailty.

Reduced chemosensory function also poses safety risks. The ability to detect spoiled food, smoke, gas leaks, or other environmental hazards diminishes. Older adults living alone may be at particular risk if they cannot smell a fire or a gas leak. Using smoke detectors, gas detectors, and carefully checking food expiration dates become especially important as these senses decline.

Healthcare providers and families can help older adults maintain nutrition and safety despite sensory changes. Using herbs and spices instead of excess salt to enhance flavor, presenting food attractively to engage vision, varying textures to provide interest, and ensuring adequate lighting for eating can all help. Regular dental care to maintain taste function and attention to medications that might affect taste are also important. For safety, home environment modifications and regular checks on detector batteries should be routine.

How Can You Protect Your Sense of Taste and Smell?

Protect your taste and smell by not smoking, avoiding exposure to toxic chemicals, treating nasal and sinus conditions promptly, maintaining good oral hygiene, staying hydrated, protecting your head from injury, and eating a varied diet. If you lose these senses, smell training may help recovery.

While some decline in chemosensory function is normal with aging, there are many steps you can take to protect these important senses throughout life. Prevention is key, as some types of damage are irreversible. By understanding the factors that can harm taste and smell, you can make informed choices to preserve these senses for as long as possible.

Smoking is one of the most damaging habits for both taste and smell. Cigarette smoke contains thousands of chemicals that can directly damage olfactory epithelium and taste buds. Smokers often have significantly reduced sensitivity to both taste and smell compared to non-smokers. The good news is that much of this damage is reversible—former smokers typically show improvement in chemosensory function within months to years of quitting, though full recovery may take time and may not be complete in long-term heavy smokers.

Exposure to environmental toxins can also harm these senses. Workers in certain industries—such as those involving solvents, metals, or dust—may be at elevated risk for chemosensory damage. Using appropriate protective equipment and following safety guidelines is essential. At home, avoiding prolonged exposure to strong chemical odors and ensuring adequate ventilation when using cleaning products or paints can help protect your senses.

Protective Strategies

• Don't smoke: Smoking damages both taste and smell; quitting allows partial recovery
• Treat sinus conditions: Chronic sinusitis and allergies can impair smell; seek treatment to maintain clear nasal passages
• Maintain oral hygiene: Brush and floss regularly, and see a dentist for check-ups to keep taste function optimal
• Stay hydrated: Adequate fluid intake maintains saliva production, which is essential for taste
• Protect your head: Wear helmets during activities with head injury risk; olfactory nerves are vulnerable to trauma
• Eat a varied diet: Regular exposure to diverse flavors may help maintain taste sensitivity
• Manage chronic conditions: Diabetes, hypertension, and other conditions can affect taste and smell if poorly controlled
• Review medications: Ask your doctor about taste/smell side effects of your medications