Life10e Ch46 2 Lecture Notes Sensory Systems

Lecture Notes by Donna Francis to accompany
Life: The Science of Biology, Tenth Edition
Sadava • Hillis • Heller • Berenbaum

Chapter 46: Sensory Systems

46.2 How Do Sensory Systems Detect Chemical Stimuli?

Chemoreceptors: receptor proteins that bind to specific molecules, their ligands; responsible for taste and smell.
Also monitor internal environment, such as CO2 levels in blood.

Olfaction is the sense of smell.
Olfactory sensors are embedded in epithelial tissue at top of nasal cavity (in vertebrates).
Axons extend to the olfactory bulb in the brain, dendrites end in olfactory hairs on the nasal epithelium.

FIGURE 46.3 Olfactory Receptors Communicate Directly with the Brain

Odorant—a molecule that binds to an olfactory receptor protein on the cilia of olfactory receptor neurons (ORNs).
Olfactory receptor proteins are specific for particular odorants.

When an odorant binds to a receptor, it activates a G protein, which activates a second messenger (cAMP). cAMP binds to cation channels in ORN’s membrane, causing an influx of Na+ and generating an action potential.

Many more odorants can be discriminated than there are olfactory receptors.
In the olfactory bulb, axons from ORNs with the same receptor types cluster together to form glomeruli.
Complex odorants can activate a unique combination of glomeruli.

An olfactory system with hundreds of different receptors can discriminate an astronomically large number of smells.
Other mammals have much greater sensitivity to smells. A dog has about 1 billion odorant receptors, compared with about 20 million in the average human.

Pheromones—chemical signals used to communicate among individuals of the same species.
Example: female silkworm moths release bombykol; males have receptors for bombykol on the antennae.
One molecule of bombykol is enough to generate action potentials.

FIGURE 46.4 Pheromones Can Communicate over Great Distances

Vomeronasal organ (VNO) is found in amphibians, reptiles, many mammals:
A paired tubular structure embedded in the nasal epithelium, with chemo-receptors in the walls.
Information goes to an accessory olfactory bulb.
VNO is specialized for pheromones.

Snakes “smell” with their tongues: VNO opens into mouth—the forked tongue picks up odorants and brings them to VNO chemoreceptors.
In mice, olfactory neurons behave differently according to the gender and strain of another mouse—the VNO may aid in pheromone perception.

Gustation, the sense of taste, depends on clusters of chemoreceptors called taste buds.
Human taste buds are embedded in the tongue epithelium, on the papillae.
Some fish have taste buds on the skin; duck-billed platypus has taste buds on the bill.

FIGURE 46.5 Taste Buds Are Clusters of Sensory Cells

Tongue epithelium is shed and replaced at a rapid rate. Taste bud cells last about ten days.
Neurons form new synapses with new taste bud cells as they are formed.

Humans taste salty, sour, sweet, bitter, and umami.
Umami is a savory, meaty taste that originates from receptors for amino acids, including monosodium glutamate (MSG).
Full complexity of taste involves both gustatory and olfactory receptors.

Saltiness and sour receptors are ionotropic and respond to Na+ depolarizing the cell.
Sweetness and bitter receptors are metabotropic, and involve families of receptor proteins.
The sensory cells release neurotransmitters onto sensory neurons, which conduct action potentials to the CNS.