Essential Cell Biology 5th edition

Ion Channels and the Membrane Potential
409
entry of
positively
charged ions
supporting cells
auditory
hair cells
tectorial membrane
CHANNEL
CLOSED
linking
filament
CHANNEL
OPEN
stereocilia
(A)
basilar membrane
auditory nerve fibers
STEREOCILIA NOT
TILTED
(B)
STEREOCILIA
TILTED
Figure 12–28 Mechanically-gated ion channels allow us to hear. (A) A section through the organ of Corti,
which runs the length of the cochlea, the auditory portion of the inner ear. Each auditory hair cell has a tuft of
spiky extensions called stereocilia projecting from its upper surface. The hair cells are embedded in an epithelial
sheet of supporting cells, which is sandwiched between the basilar membrane below and the tectorial membrane
above. (These are not lipid bilayer membranes but sheets of extracellular matrix.) (B) Sound vibrations cause the
basilar membrane to vibrate up ECB5 and down, E12.27/12.28
causing the stereocilia to tilt. Each stereocilium in the staggered array
of stereocilia on a hair cell is attached to the next, shorter stereocilium by a fine filament. The tilting stretches
the filaments, which pull open mechanically-gated ion channels in the stereocilium plasma membrane, allowing
positively charged ions to enter from the surrounding fluid (Movie 12.7). The influx of ions activates the hair cells,
which stimulate underlying nerve endings of the auditory nerve fibers that relay the auditory signal to the brain.
The hair-cell mechanism is astonishingly sensitive: the faintest sounds we can hear have been estimated to stretch
the filaments by an average of about 0.04 nm, which is less than the diameter of a hydrogen ion (Movie 12.8).
Voltage-gated Ion Channels Respond to the Membrane
Potential
Voltage-gated ion channels play a major role in propagating electrical
signals along all nerve cell extensions, such as those that relay signals
from our brain to our toe muscles. But voltage-gated ion channels are
present in many other cell types, too, including muscle cells, egg cells,
protozoans, and even plant cells, where they enable electrical signals
to travel from one part of the plant to another, as in the leaf-closing
response of a Mimosa pudica plant (Figure 12–29).
Voltage-gated ion channels have domains called voltage sensors that
are extremely sensitive to changes in the membrane potential: changes
(A) time 0 sec (B) 1 sec (C) 3 sec (D) 5 sec
Figure 12–29 Both mechanically-gated and voltage-gated ion channels underlie the leaf-closing response in the touch-sensitive
plant Mimosa pudica. (A) Resting leaf. (B–D) Successive leaflet closures in response to touch. A few seconds after the leaf on the left is
touched, its leaflets snap shut. The response involves the opening of mechanically-gated ion channels in touch-sensitive sensory cells,
which then pass a signal to cells containing voltage-gated ion channels, generating an electric impulse. When the impulse reaches
specialized hinge cells at the base of each leaflet, a ECB5 rapid loss E12.28/12.29
of water by these cells occurs, causing the leaflets to fold into a closed
conformation suddenly and progressively down the leaf stalk (Movie 12.9).