The Chemical Senses

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.

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).

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.

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).

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.

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.

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.

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.

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.

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.

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.

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.