Table of Contents
Chemical senses are those that detect substances dissolved in air, water, or body fluids. In animals, they mainly include smell (olfaction) and taste (gustation). In this chapter, the focus is on how these senses work as chemical detectors and how they differ and complement each other.
General Principles of Chemical Sensing
All chemical senses are based on a few shared principles:
- Chemical stimulus: Specific molecules (or ions) in the environment.
- Receptors: Specialized membrane proteins in sensory cells that bind these molecules.
- Signal transduction: Binding of a molecule changes the receptor, triggers intracellular signaling, and generates an electrical signal.
- Coding: The brain interprets the pattern and intensity of signals as a particular taste, smell, or other chemical quality.
Chemical senses are especially important for:
- Finding food
- Avoiding toxins or spoiled material
- Recognizing conspecifics (same species), mates, and offspring
- Detecting predators or prey
- Orientation and navigation (e.g., following odour trails)
Olfaction (Sense of Smell)
Location and Structure of Olfactory Organs
In vertebrates, smell is usually associated with olfactory epithelia:
- Mammals (including humans):
- Located high in the nasal cavity.
- Contain:
- Olfactory receptor neurons (ORNs): true nerve cells with cilia.
- Supporting cells: metabolic and structural support.
- Basal cells: stem cells that replace worn-out receptor neurons.
- Receptor neurons send axons through the skull bones to the olfactory bulb in the brain.
- Fish and many aquatic vertebrates:
- Have nasal sacs or pits through which water flows; olfactory epithelium lines these chambers.
- Insects:
- Olfactory receptors are on the antennae and sometimes on mouthparts.
- Receptors are embedded in hair-like structures called sensilla.
Odorant Molecules and Receptor Diversity
Odor is produced by volatile chemicals that can evaporate and reach the olfactory epithelium:
- Often small, organic molecules (e.g., esters in fruit aromas, sulfur compounds in garlic).
- Different species have different sets and numbers of odorant receptor genes:
- Humans: several hundred types.
- Many mammals (e.g., mice, dogs): more receptor types and higher sensitivity.
- Each olfactory receptor neuron usually expresses only one type of receptor protein.
- Each receptor type can be activated by a range of related odorant molecules, but with different strengths.
This creates a combinatorial code:
- A single odorant activates a characteristic combination of receptor types.
- Different odorants produce different activation patterns.
- The brain decodes these patterns as distinct smells.
Signal Transduction in Olfaction
Key steps in vertebrate olfactory transduction:
- Odorant dissolves in the mucus covering the epithelium.
- Odorant binds to a receptor on the cilia of the olfactory neuron.
- The receptor (a G‑protein‑coupled receptor) activates a signaling cascade.
- Ion channels open; ions flow in; the receptor neuron depolarizes.
- If depolarization reaches threshold, action potentials are generated.
- Action potentials travel along the axon to the olfactory bulb.
In the olfactory bulb:
- Axons of receptor neurons that express the same receptor type converge onto specific structures called glomeruli.
- This spatial pattern of active glomeruli further encodes which odorants are present.
- Signals are then relayed to higher brain regions involved in perception, memory, and emotion.
Sensitivity, Adaptation, and Mixtures
- Many animals can detect extremely low concentrations of odorants.
- Example: some moths detect female sex pheromones at a few molecules per cubic centimeter.
- Adaptation:
- Prolonged exposure to the same odor reduces receptor responsiveness.
- This is why a constant smell (e.g., your own perfume) quickly fades from awareness.
- Odor mixtures:
- Natural smells (food, flowers, body odours) are mixtures of many molecules.
- The brain integrates complex receptor patterns into a unified odour impression.
- Mixtures can mask or enhance individual components.
Specialized Olfactory Systems: Pheromones and Vomeronasal Organ
Besides general odours, many animals detect pheromones—chemicals released by individuals to influence others of the same species.
- Functions:
- Sexual attraction and readiness.
- Territorial marking.
- Alarm signals.
- Trail marking (e.g., ants).
In many vertebrates, pheromones are detected by a dedicated organ:
- Vomeronasal organ (VNO), or Jacobson’s organ:
- Paired structures in the nasal or oral cavity.
- Contains its own receptor types and nerve pathways, often distinct from the main olfactory system.
- Particularly important in many mammals (e.g., rodents, some reptiles).
- Often associated with specific behaviours (e.g., flehmen response in cats and horses).
- In insects, pheromones are typically detected by highly specialized sensilla on the antennae.
Humans:
- Have remnants of VNO structures, but their function is still debated and, at most, limited.
Gustation (Sense of Taste)
Basic Qualities of Taste
Humans and many other animals can distinguish several basic taste qualities:
- Sweet: indicates energy-rich nutrients (sugars).
- Salty: indicates essential minerals (e.g., Na⁺).
- Sour: signals acidity; often associated with unripe fruit or spoiled food.
- Bitter: many toxic plant substances taste bitter; protective warning.
- Umami: taste of certain amino acids (especially glutamate); signals protein-rich food.
Additional taste-like sensations (e.g., fat taste, metallic taste) are under discussion but not universally accepted as basic qualities.
Importantly, flavour is a combination of:
- Taste (from the tongue and oral cavity),
- Retronasal smell (odour from food reaching the olfactory epithelium from the back of the throat),
- Texture and temperature (touch),
- Trigeminal sensations (e.g., burning of chili, coolness of menthol).
Taste Organs and Taste Buds
In vertebrates, taste receptors are grouped in taste buds:
- Taste buds are small clusters of:
- Taste receptor cells (modified epithelial cells).
- Supporting cells.
- Basal (stem) cells.
- Each taste bud has a small opening, the taste pore, where microvilli of taste cells contact substances dissolved in saliva.
- Locations:
- Tongue (on papillae).
- Soft palate.
- Epiglottis and upper oesophagus.
- The classic “tongue map” (different regions for different tastes) is misleading:
- All regions with taste buds can detect all basic tastes.
- There may be small sensitivity differences but not strict separation.
Taste receptor cells are not neurons, but synapse onto sensory nerve fibers of cranial nerves that conduct signals to the brain.
Types of Taste Stimuli and Receptor Mechanisms
Different basic tastes use different molecular mechanisms:
- Salty:
- Mainly due to Na⁺ ions entering taste cells through specific ion channels.
- Leads directly to depolarization.
- Sour:
- Triggered by acids (H⁺ ions).
- H⁺ ions can enter via channels or block certain K⁺ channels, changing the membrane potential.
- Sweet, umami, and many bitter substances:
- Detected by G‑protein‑coupled receptors (GPCRs).
- Binding of the tastant molecule activates second messenger pathways.
- These pathways eventually open or close ion channels, depolarizing the cell.
Because bitter often indicates toxins:
- Animals are often extremely sensitive to bitter tastes.
- There are many different bitter receptor genes, each responsive to different bitter compounds.
Once a taste receptor cell is depolarized:
- It releases neurotransmitters at its synapse.
- Sensory nerve fibers generate action potentials.
- Signals travel to taste centers in the brainstem, then to higher centers where taste perception arises and is integrated with smell.
Protective and Regulatory Functions of Taste
Taste serves both protective and regulatory roles:
- Protective:
- Rejection of intensely bitter or very sour substances (possible toxins or spoiled food).
- Aversion learning: if a taste is followed by sickness, many animals (including humans) quickly develop a long-lasting avoidance of that taste.
- Regulatory:
- Attraction to sweet and umami promotes intake of energy and protein.
- Mild salt taste encourages intake of necessary minerals.
- Taste feedback from the mouth helps regulate saliva secretion, digestive enzyme release, and prepares the digestive tract.
Chemical Senses Beyond Taste and Smell
While olfaction and gustation are the main chemical senses, many organisms use additional forms of chemoreception:
Chemoreception in Aquatic Animals
In water, many dissolved substances are not volatile, so smell and taste partly overlap:
- Fish and aquatic invertebrates often have:
- Chemoreceptors in the mouth and on the head (taste-like).
- Chemoreceptors on skin, fins, or whiskers (barbels).
- Some fish (e.g., catfish) have taste receptors over large parts of their body surface, allowing them to detect food by contact with any body part.
Chemoreception in water is crucial for:
- Locating food (following chemical plumes).
- Locating mates (pheromones).
- Returning to spawning grounds (e.g., salmon homing to natal rivers by olfactory cues).
Insect Chemoreception
Insects rely heavily on chemical cues:
- Antennae:
- Rich in olfactory sensilla detecting odours and pheromones from long distances.
- Mouthparts and tarsi (feet):
- Have contact chemoreceptors functioning like taste.
- Example: butterflies “taste” leaves with their feet to decide where to lay eggs.
Different sensory neurons in a single sensillum may each respond to:
- Sugars (feeding).
- Salts.
- Bitter or toxic substances (avoidance).
Internal Chemoreception
Organisms also monitor internal chemical conditions:
- Vertebrates have receptors that sense:
- Blood CO₂ and pH (important for breathing regulation).
- Osmolarity (water and salt balance).
- Glucose levels (energy balance).
- These internal chemical sensors are usually not perceived as conscious tastes but are essential for homeostasis.
Integration of Chemical Senses with Behaviour
Chemical senses are tightly linked to behaviour:
- Feeding behaviour:
- Smell attracts animals toward food sources.
- Taste confirms edibility or warns against toxins at the last moment.
- Social and reproductive behaviour:
- Pheromones can trigger mating, territorial aggression, or social organization (e.g., in ants and bees).
- Orientation and navigation:
- Many animals follow odour trails (ants), scent marks (mammals), or chemical gradients (marine larvae) to find habitat, shelter, or hosts.
Because of their direct connection to survival and reproduction, chemical cues often elicit innate responses and are tightly connected to emotion and memory centers in the brain. This is one reason why certain smells can evoke strong, vivid memories in humans.
Plasticity and Disorders of Chemical Senses
Chemical senses are modifiable and can be impaired:
- Adaptation and learning:
- Repeated exposure can change sensitivity (e.g., increased tolerance to some bitter tastes).
- Preferences and aversions can be shaped by experience.
- Disorders:
- Anosmia: loss of smell.
- Hyposmia: reduced smell.
- Ageusia: loss of taste (often partial; complete loss is rare).
- Dysgeusia: distorted taste perceptions.
- Causes include infections, head injuries, nasal polyps, toxins, aging, or certain medications.
Changes in smell or taste can significantly affect:
- Appetite and food intake.
- Safety (e.g., detecting smoke or spoiled food).
- Quality of life.
These aspects illustrate how central chemical senses are—not only for information processing, but also for everyday functioning and well-being.