Overview
This lecture covers the basics of the special senses of taste and smell, emphasizing their dependence on chemoreceptors and their interrelated roles in flavor perception.
Chemoreceptors & Sensory Systems
- Taste and smell both use chemoreceptors, which detect specific dissolved molecules.
- Vision uses photoreceptors (detect light); hearing and equilibrium use mechanoreceptors (respond to physical force).
Smell (Olfaction)
- Smell begins with air molecules dissolving in nasal mucus and contacting the olfactory epithelium.
- Olfactory receptor cells in the epithelium each have a specific receptor protein (over 1,000 types).
- Humans have about five million olfactory receptors; dogs have over 220 million.
- Odors are identified by unique combinations of receptor activation, not single molecules.
- The olfactory nerve (tract) relays information to the brain.
- Anosmia is the loss of smell, usually from nerve or epithelium damage; nerve damage is often permanent but epithelium can regenerate.
- Smell greatly influences taste perception; loss of smell can reduce appetite.
Taste (Gustation)
- Taste molecules must be dissolved in saliva to bind to taste cell receptors.
- Dry mouth can impair taste by reducing molecule dissolution.
- Gustatory (taste) cells have microvilli (gustatory hairs) to increase surface area for detection.
- Taste buds are clusters of taste cells located mainly in papillae on the tongue.
- Taste buds contain taste pores for molecule entry and are connected to sensory nerve endings.
- There are five basic tastes: sweet (sugars), sour (hydrogen ions/acids), salty (sodium ions), bitter (various often toxic compounds), and umami (glutamate/proteins).
- Artificial sweeteners activate sweet receptors similarly to natural sugars.
- Bitter tastes often signal toxins; humans are sensitive to bitter for protection.
- Umami taste detects glutamate, common in protein-rich foods and flavor enhancers like MSG.
- Tastes are identified by patterns of receptor activation, with learning associating certain combinations to specific foods.
Key Terms & Definitions
- Chemoreceptor — Sensory cell that responds to specific chemicals in solution.
- Olfactory epithelium — Nasal tissue containing smell receptor cells.
- Gustatory (taste) cell — Sensory cell responsible for detecting taste molecules.
- Papillae — Bumps on the tongue where taste buds are primarily located.
- Anosmia — Loss of the sense of smell.
- Umami — Taste sensation triggered by glutamate, associated with protein-rich foods.
Action Items / Next Steps
- Review the mechanisms of chemoreceptors in taste and smell.
- Be able to describe the general anatomy and function of olfactory and gustatory systems.
- Prepare to identify and explain the five basic taste sensations.