| The olfactory (smelling) sense of mammals detects
certain airborne chemicals. The olfactory receptor cells are neurons that
line the upper portion of the nasal cavity and send impulses along their
axons directly to the olfactory bulb of the brain. The receptive ends of
the cells have cilia that extend into the layer of mucus coating the nasal
cavity. When an odorous substance diffuses into this region, it binds to
specific receptor molecules on the plasma membrane of the olfactory cilia.
The binding triggers a signal-transduction pathway involving a G protein
relay, an effector enzyme (adenylyl cyclase), and a second messenger
(cyclic AMP). Na+ channels are opened by the second messenger in
the olfactory receptor cell membrane, depolarizing it and producing action
potentials that go to the brain.
Although the receptors and brain pathways for taste and olfaction
are independent, the two senses do interact. Much of what we call taste
is really smell. If the olfactory system is blocked, the perception of
taste is significantly inhibited. |
