- Explain taste and smell as chemical senses
- Describe the receptors that respond to touch
- Discuss the experience of pain
- Describe the basic functions of the vestibular, proprioceptive, and kinesthetic sensory systems
Chemical Senses
Our senses of taste (gustation) and smell (olfaction) are called chemical senses because they respond to molecules in the food we eat and the air we breathe. These two systems are deeply connected—when you describe the flavor of chocolate or pizza, you’re actually combining information from both taste and smell.
Taste (Gustation)
Humans have taste buds, or gustatory cells, that can detect at least five well-established tastes:
- Sweet – signals sugars and energy-rich nutrients.
- Salty – detects essential electrolytes like sodium.
- Sour – detects acidity (protons/H⁺ ions).
- Bitter – warns of potentially toxic substances.
- Umami – detects amino acids and proteins (notably glutamate).
These five are widely accepted because each has clearly identified receptor mechanisms and consistent cross-species evidence, though additional research shows that humans can detect even more distinct taste categories (Kinnamon & Vandenbeuch, 2009; Mizushige et al., 2007). These include fatty, starchy, and ammonium. Evidence suggests humans can taste fatty acids using receptors such as CD36 and GPR120, which detect the chemical makeup of fats rather than just their texture (Mizushige et al., 2007; Running, 2014). Some scientists have also proposed emerging categories like starchy (for complex carbohydrates) and ammonium chloride (more of a sour taste detected via the OTOP1 channel), though these are still under study.
Molecules from the food and beverages we consume dissolve in our saliva and interact with taste receptors on our tongue and in our mouth and throat. Taste buds are formed by groupings of taste receptor cells with hair-like extensions that protrude into the central pore of the taste bud (Figure 1). Taste buds have a life cycle of ten days to two weeks, so even destroying some by burning your tongue won’t have any long-term effect; they just grow right back.
Taste molecules bind to receptors on this extension and cause chemical changes within the sensory cell that result in neural impulses being transmitted to the brain via different nerves, depending on where the receptor is located. Taste information is transmitted to the medulla, thalamus, and limbic system, and to the gustatory cortex, which is tucked underneath the overlap between the frontal and temporal lobes (Maffei et al., 2012; Roper, 2013). Taste buds are not mapped to any specific region of the tongue.
Smell (Olfaction)
Our sense of smell allows us to detect and identify thousands of chemical compounds in the environment—from the aroma of freshly baked bread to the scent of rain or smoke.
Olfaction begins when airborne molecules are inhaled and come into contact with specialized receptor cells high inside the nasal cavity.
How Smell Works
Olfactory receptor cells are located within a mucous membrane at the top of the nose. Each receptor cell has hair-like extensions called cilia, which trap odor molecules that have dissolved in the mucus (Figure 2).
When an odor molecule binds to a receptor, it triggers chemical changes inside the cell that generate electrical signals. These signals travel to the olfactory bulb, a bulb-shaped structure at the base of the frontal lobe where olfactory nerves begin. From the olfactory bulb, the information is relayed to:
- The limbic system, which helps connect smells with emotions and memories, and
- The primary olfactory cortex, located near the gustatory cortex, where smell and taste information combine (Lodovichi & Belluscio, 2012; Spors et al., 2013).
Olfactory Receptors and Coding Smells
Olfactory receptors belong to a large family of proteins called G protein-coupled receptors (GPCRs). Each receptor weaves across the cell membrane seven times, forming:
- An outer region that binds to specific parts of odor molecules, and
- An inner region that activates the neuron’s signaling pathway.
Humans have about 350 functional olfactory receptor genes, and each gene produces a receptor that responds to particular molecular features (like carbon chains or ring structures).
Receptors of the same type send their signals to the same clusters of neurons in the olfactory bulb called glomeruli.
When you smell something, the pattern of activation across glomeruli creates a kind of “neural fingerprint” that represents the odor’s chemical structure (Shepherd, 2005). This system allows humans to recognize thousands of different smells—even though most real-world odors, such as coffee or bacon, are actually mixtures of many molecules. The brain stores these combined scent patterns in memory, allowing instant recognition the next time you encounter them.
Comparing Olfaction Across Species
Different species vary dramatically in their sense of smell. For example, dogs have between 800 and 1,200 olfactory receptor genes, compared to fewer than 400 in humans (Niimura & Nei, 2007). This helps explain their remarkable ability to detect scents at concentrations 10,000 to 100,000 times lower than humans can.
Dogs have been trained to detect drops in blood glucose, cancerous tumors, and even COVID-19 infections—demonstrating just how sensitive their olfactory systems are (Wells, 2010).
Pheromones and Chemical Communication
Many animals use chemical signals called pheromones to communicate with others of their species (Wysocki & Preti, 2004). Pheromones can convey important social or reproductive information. For example, when a female rat is ready to mate, she releases pheromones that trigger sexual behavior in nearby males (Furlow, 1996, 2012; Purvis & Haynes, 1972; Sachs, 1997).
Although human pheromones remain a controversial topic, some research suggests that subtle body odors may influence attraction, mood, or even menstrual synchrony among women (Comfort, 1971; Russell, 1976; Wolfgang-Kimball, 1992; Weller, 1998). However, the evidence is mixed, and scientists continue to debate whether humans possess a true pheromonal communication system.