36.3 Taste and Smell

Taste and Smell

Taste and smell are the body's chemical senses, meaning they work by detecting molecules rather than physical stimuli like light or sound waves. These two senses are tightly linked to each other and have an unusually direct connection to the brain's emotional and memory centers. Understanding how they work reveals a lot about how organisms interact with their chemical environment.

Smell and Taste vs. Other Senses

• Chemical senses detect molecules dissolved in mucus (smell) or saliva (taste), while physical senses detect stimuli such as light (vision), sound waves (hearing), and pressure (touch).
• Smell and taste are closely linked because flavor perception combines both senses. Other senses operate more independently. This is why losing your sense of smell dramatically changes how food tastes.
• These senses have a stronger connection to emotions and memories than other senses. The olfactory bulb connects directly to the limbic system (the brain region involved in emotion and memory), which is why a particular smell can trigger a vivid memory almost instantly.
• Sensory adaptation occurs more quickly with smell and taste than with other senses. Prolonged exposure to a particular odor or taste decreases your sensitivity to it, freeing you up to detect new or changing chemical stimuli in the environment.

Five Primary Human Tastes

Each of the five tastes corresponds to a different type of molecule and serves a distinct evolutionary purpose.

• Sweet
  • Receptors respond to sugars and other sweet substances
  • Evolutionarily advantageous because it helps identify high-calorie, energy-rich foods like fruits and honey
• Salty
  • Receptors respond to sodium ions ($Na^+$)
  • Drives consumption of salt, which is critical for maintaining electrolyte balance and hydration
• Sour
  • Receptors respond to acidic substances ($H^+$ ions)
  • Helps identify spoiled or unripe foods; also plays a role in maintaining pH balance
• Bitter
  • Receptors respond to a wide range of bitter compounds
  • Humans have many different bitter receptor types because detecting potentially toxic substances was a major survival advantage
• Umami
  • Receptors respond to glutamate and other amino acids
  • Signals the presence of protein-rich foods (mushrooms, aged cheeses, meat broths) and enhances overall flavor perception

Canine vs. Human Olfactory Systems

Dogs are often cited as having a far superior sense of smell compared to humans. Several structural differences explain why.

• Size of olfactory epithelium
  • The olfactory epithelium is the tissue lining the nasal cavity that contains odor receptors
  • Dogs have a much larger olfactory epithelium than humans, providing greater surface area for more odor receptors and increased sensitivity
• Number of olfactory receptors
  • Dogs have roughly 300 million olfactory receptors, while humans have only about 6 million
  • More receptors allow dogs to detect a wider range of odors at much lower concentrations
• Size of olfactory bulb
  • The olfactory bulb is the brain structure that processes smell information
  • In dogs, the olfactory bulb is proportionally much larger relative to total brain size than in humans, allowing more complex processing of odor information
• Nasal cavity structure
  • Dogs have a more complex nasal cavity with scroll-like structures called turbinates
  • Turbinates increase the internal surface area of the nasal cavity, making odor absorption more efficient
  • Human nasal cavities are simpler in structure and less efficient at capturing odor molecules

Neural Processing of Taste and Smell

Once chemical signals are detected by receptors, the brain has to interpret them. Here are the key structures and concepts involved in that processing:

• Gustatory cortex: the brain region responsible for processing taste information. Taste signals travel from taste buds on the tongue through cranial nerves to reach this area.
• Olfactory cortex: the brain region that processes smell information, receiving input from the olfactory bulb.
• Olfactory tract: the nerve pathway that transmits signals from the olfactory bulb to the olfactory cortex.
• Retronasal olfaction: the perception of odors that travel from the mouth up through the back of the nasal cavity. This is a major reason why flavor feels like more than just taste alone. When you chew food, volatile molecules rise into the nasal passage and stimulate olfactory receptors.
• Pheromones: chemical signals released by an organism that can influence the behavior or physiology of other members of the same species. Many animals rely on pheromones for mating, territory marking, and alarm signaling. Their role in human behavior is still debated.