Page 606 - Fundamentals of anatomy physiology

Olfactory reception occurs on the surface membranes of the In general, odorants are small organic molecules. The
olfactory dendrites. Odorants—dissolved chemicals that strongest smells are associated with molecules of either
stimulate olfactory receptors—interact with receptors called high water or high lipid solubilities. As few as four
odorant-binding proteins on the membrane surface. odorant molecules can activate an olfactory receptor.

1 The binding of an odorant to its 2 The cAMP opens sodium ion 3 If sufficient depolarization

receptor protein leads to the activation channels in the plasma occurs, an action potential is
of adenylyl cyclase, the enzyme that membrane, which then begins triggered in the axon, and the
converts ATP to cyclic AMP (cAMP). to depolarize. information is relayed to the CNS.

Odorant MUCOUS Closed ++
molecule LAYER sodium +
channel

Receptor Depolarized
protein membrane

Inactive Active cAMP cAMP
G protein G protein Sodium ++
ions enter +
RECEPTOR adenylate cAMP
CELL ATP cyclase cAMP

Some 90 percent of the gustatory Salt and Sour Channels Sweet, Bitter, and Umami Receptors
receptor cells respond to two or more
different taste stimuli. The different tastes The diffusion of sodium ions from salt Receptors responding to stimuli that produce
involve different receptor mechanisms. solutions or hydrogen ions from acids or sweet, bitter, and umami sensations are linked
A salty stimulus involves the diffusion sour solutions into the receptor cell leads to G proteins called gustducins (GUST-doos-
of Na+ ions through a sodium ion leak to depolarization. inz)—protein complexes that use second
channel common in epithelial cells. messengers to produce their effects.
Stimuli for a sour or acidic taste include Na+ +
H+ ions that diffuse through the same + + Na+ ion Sweet, Membrane
epithelial Na+ channel. The intracellular H+ + + leak channel bitter, or receptor
increase in cations leads to depolarization umami
and neurotransmitter release. Sweet,
bitter, and umami stimuli bind to specific Resting plasma
G protein-coupled receptors. The membrane
resulting multiple chemical pathways
lead to depolarization and Inactive Active
neurotransmitter release. G protein G protein

Depolarized Depolarized
membrane membrane

+ Active
++ G protein

Active Inactive
2nd messenger 2nd messenger

Depolarization of membrane stimulates Activation of second messengers stimulates
release of chemical neurotransmitters. release of chemical neurotransmitters.

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