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.