The Chemical Senses

1. The Chemical Senses

2. Chemoreceptors • Chemically sensitive cells located throughout the body to monitor: • Irritating chemicals on skin or in mucus • Ingested substances in digestive organs • Levels of carbon dioxide and oxygen in blood • Acidity in muscles indicating oxygen debt following exertion • Gustation (taste) and olfaction (smell) – processed in parallel and merged in cortex.

3. Basic Tastes • Sweet – things that are good for us tend to taste sweet. • Bitter – things that are toxic (poisonous) tend to taste bitter. • Salt • Sour • Savory (umami) – associated with proteins and found in meat (MSG – monosodium glutamate).

4. How Taste Works • Taste buds (taste receptor neurons) line papillae found in different areas of the tongue. • Each papillae responds to one taste (sweet, sour) when the stimulus is weak but multiple tastes when the stimulus is strong. • Identification of tastes occurs in the brain. • Population coding – responses of a large number of broadly tuned neurons specifies the taste.

5. Transduction • Transduction – the process by which an environmental stimulus causes an electrical response in a sensory receptor cell. • Tastants (tastes) use multiple mechanisms: • Pass through ion channel directly (salt). • Bind to and block ion channels (sour & bitter). • Bind to and open ion channels (amino acids). • Activate second messengers in complex ways (sweet, bitter, umami).

6. Taste Pathways • Taste buds send information to primary gustatory axons. • Axons go into the brain stem to the ipsilateral thalamus (VPM) and then to the primary gustatory cortex. • Conscious taste is mediated by the cortex. • Control of feeding (swallowing, saltivation, vomiting, digestion) is controlled by medulla. • Motivation to eat is controlled by hypothalamus.

7. Supertasters • Supertasters tend to dislike things other people like to eat: brussels sprouts, brocoli, spinach. • Excess sensory receptors for bitter flavors in the taste buds. • Non-tasters have fewer sensory cells. • In danger when children because they will eat or drink anything.

8. Smell (Olfaction) • Combines with taste to help us identify food and increases enjoyment of flavors. • Warns of potentially harmful substances or places. • Only 20% of smells are pleasant. • Pheromones released by the body are signals for reproductive behaviors, identify individuals, mark territory and dominance.

9. Olfactory Epithelium • We smell with a thin sheet of cells located high in the nasal cavity. • Three cell types: • Olfactory receptors – tranduction of smell to neural activity. • Supporting cells – produce mucus, like glia. • Basal cells – source of new olfactory receptors. • Receptors die and are replaced every 4-8 wks.

10. How Smell Works • Sniffing brings air through the nasal passages to the olfactory epithelium. • Odorants (chemical stimuli in the air) dissolve in the mucus layer before reaching receptors. • Odorants then bind with cilia of the receptor cells causing G-protein activation resulting in an action potential.

11. Olfactory Pathways • Axons from the olfactory receptors form the olfactory nerve. • The axons penetrate a thin layer of bone called the cribiform plate, then enter the olfactory bulb. • Axons map onto glomeruli in the bulb. • Anosmia – inability to smell due to severing the olfactory axons at the cribiform plate.

12. How Smells are Identified • Initial processing at the glomeruli separate smells into broad categories. • Information passes from the bulbs into olfactory tracts (bundles of axons) projecting to primitive regions of cortex, then to the thalamus, and finally to the cortex. • Parallel pathways process smell in many areas of the cortex.

13. How is Smell Coded? • Three ways of telling smells apart: • Population coding – combinations of responses form patterns related to specific smells. • Sensory map – activation of different areas of the glomeruli correspond to specific odors. The form of a map for each odor may be distinct. • Temporal coding – the timing of action potentials along the axons may differentiate smells. Number, temporal pattern, synchronicity, rhythm.