The Special Senses

Taste (Gustation)

Taste, or gustation, is one of the special senses responsible for detecting chemical stimuli in the environment. It allows organisms to evaluate the quality and safety of food and plays a role in digestion by stimulating salivation and gastric secretions.

• Taste Buds: Specialized sensory organs primarily located on the tongue within structures called papillae (fungiform, foliate, and circumvallate papillae). Taste buds are also found on the soft palate, pharynx, and epiglottis.

• Gustatory Cells: Each taste bud contains 50–100 gustatory epithelial cells, which are the actual taste receptor cells. These cells have microvilli (gustatory hairs) that project through a taste pore and interact with dissolved chemicals.

• Supporting Cells: Surround and support the gustatory cells within the taste bud.

• Basal Cells: Stem cells that divide and differentiate into new gustatory cells, replacing them every 7–10 days.

• Taste Sensations: There are five primary taste sensations: sweet, sour, salty, bitter, and umami (savory). Each is associated with specific chemicals:

  • Sweet: Sugars, alcohols, some amino acids

  • Sour: Acids (H+ ions)

  • Salty: Metal ions (Na+, K+)

  • Bitter: Alkaloids (e.g., caffeine, quinine)

  • Umami: Amino acids (glutamate, aspartate)

• Neural Pathways: Taste signals are transmitted via cranial nerves VII (Facial), IX (Glossopharyngeal), and X (Vagus) to the gustatory cortex in the insula of the brain.

Example: When you eat a lemon, the high concentration of H+ ions stimulates the sour taste receptors, sending signals to your brain that are interpreted as 'sour.'

Olfaction (Smell)

Olfaction is the sense of smell, allowing detection of airborne chemical stimuli. It is essential for flavor perception, environmental awareness, and triggering memories or emotions.

• Olfactory Epithelium: Located in the roof of the nasal cavity, it contains olfactory sensory neurons, supporting cells, and basal cells.

• Olfactory Sensory Neurons: Bipolar neurons with cilia (olfactory hairs) that extend into the mucus layer and bind odorant molecules.

• Supporting Cells: Provide metabolic and physical support to olfactory neurons.

• Basal Cells: Stem cells that regenerate olfactory neurons every 30–60 days.

• Olfactory Receptors: Humans have about 400 types of olfactory receptors, each binding specific odorant molecules. The combination of activated receptors allows discrimination of thousands of odors.

• Neural Pathways: Olfactory nerve fibers pass through the cribriform plate of the ethmoid bone to synapse in the olfactory bulb. Signals are then relayed to the olfactory cortex, limbic system, and other brain regions.

Example: The smell of baking bread is detected when volatile molecules bind to olfactory receptors, sending signals to the brain that are interpreted as a pleasant aroma.

Comparison of Taste and Olfaction

Additional info: Taste and smell are closely linked; much of what is perceived as taste is actually due to olfactory input. Loss of smell (anosmia) can significantly diminish the sense of taste.