動物生理学 III

Handout No. 11②①
Figure 30-6 Hair cells in thecochlea are stimulated when thebasilar membrane is driven up anddown by differences in the fluidpressure between the scala vestibuli and scala tympani. Because
this motion is accompanied byshearing motion between thetectorial membrane and organ ofCorti, the hair bundles that link thetwo are deflected. This deflection
initiates mechanoelectrical transduction of the stimulus. (Adaptedfrom Miller and Towe 1979.)
A. When the basilar membrane isdriven upward, shear between thehair cells and the tectorial membrane deflects hair bundles in theexcitatory direction, toward theirtall edge.
B. At the midpoint of an oscillation
the hair bundles resume theirresting position.
C. When the basilar membranemoves downward, the hair bundles
are driven in the inhibitory direction.

Figure 30-7 Tuning curves forcochlear hair cells. To construct a curve, the experimenter
presents sound at each frequency at increasing amplitudes until
the cell produces a criterion
response, here 1 mV. The curvethus reflects the threshold of thecell for stimulation at a range offrequencies. Each cell is mostsensitive to a specific frequency,its characteristic (or best) frequency. The threshold rises
briskly (sensitivity falls abruptly)
as the stimulus frequency israised or lowered. (From Pickles
1988. )③
Figure 37-7 Structure of a vertebrate hair cell.
A. The epithelial character of the haircell is evident in this drawing of thesensory epithelium from a frog's
internal ear. The cylindrical hair cell isjoined to the adjacent supporting
cells by a functional complex around
its apical perimeter. From the cell's
apical surface extends the hairbundle, the mechanically sensitiveorganelle. Afferent and efferent
synapses occur upon the basolateralsurface of the plasma membrane.

④Figure 31-2 Mechanical sensitivity of a hair cell.
A. A schematic drawing of a hair cell with a recording electrode inserted into its
cytoplasm.
B. Application of a mechanical force to the hair bundle deflects this elastic structure.
C. When the top of a hair bundle is displaced back and forth by a stimulus probe(lower trace), the opening and closing of mechanically sensitive channels produces
an oscillatory receptor potential (upper trace).
D. The sigmoidal relation between hair-bundle deflection (abscissa) and receptor
potential (ordinate) is a stimulated hair cell.

⑥

Figure 31-4 Adaptation of mechanoelectrical transduction in hair cells.
A. The hair bundle is subjected to a
family of deflections in a series oftests. Test stimuli of various sizes areapplied before (1) and at two times
during a bundle deflection maintainedfor 100 ms (2-3). The family of receptor potentials reveals a rapid depolarization at the outset, followed by gradual decline toward a plateau during
maintained stimulation.
B. The relation between displacement
and the electrical response of the hairbundle before and during the maintained displacement. As adaptation proceeds, the sigmoidal relation between
bundle displacement and receptor
potential shifts along the abscissa
without substantial changes in the
curve's shape or amplitude. This result
implies that, during adaptation to a
protracted stimulus, a hair bundle'srange of mechanical sensitivity migrates toward the position at which thebundle is held.

⑤Figure 31-3
A model for the mechanism of mechano-electricaltransduction by hair cells.
A. Top: The ion channels that participate in mechanoelectrical transduction inhair cells are gated by elastic structures in the hair bundle. The channel isassumed to be a membrane-spanning protein with a cation-selective pore. Ion
permeation through this channel is regulated by a molecular gate, whose
opening and closing is controlled by the tension in an elastic element, the gating
spring, that senses hair-bundle displacement. (Adapted from Howard andHudspeth 1988.) Bottom: When the hair bundle is at rest each transduction
channel clatters between closed and open states, spending most of its time shut(leftl. Displacement of the bundle In the positive direction increases the tension in
the gating spring, here assumed to be a tip link, attached to each channel'smolecular gate (middle). The enhanced tension promotes channel opening andthe influx of canons, thereby producing a depolarizing receptor potential (right).
(Adapted from Hudspeth 1989.)
B. The links that connect each stereociliary tip to the side of the longest adjacentstereocilium are visible in this scanning electron micrograph of a hair bundle's
top surface (left) and transmission electron micrograph (right). Although each tiplink is only 3 nm in diameter, the links appear stouter in the illustration on the left
because of metallic coating during specimen preparation. (From Assad et al.
1991 and Hudspeth and Gillespie 1944.)