Biomedical Engineering Reference
In-Depth Information
1. INTRODUCTION TO THE INNER EAR
In vertebrates, the senses of hearing, balance, and acceleration are per-
ceived through discrete sensory structures located within the inner ear. Each
of these structures contains an epithelial sheet that comprises mechanosensory
hair cells and nonsensory supporting cells. Mechanosensory hair cells act as the
primary transducers, detecting movements caused by physical motion or
sound pressure waves and generating graded responses in neurotransmitter
release. Associated neuronal innervation by branches of the VIIIth nerve con-
veys those signals to the CNS. The basis for mechanosensitivity in these cells is
a group of modified microvilli, referred to as a stereociliary bundle, that pro-
ject from the lumenal surface. These stereocilia are invariably arranged in a
staircase pattern that reflects an inherent asymmetry within the bundle. Tip
links, molecular connections that span between adjacent stereocilia, are exclu-
sively oriented along the plane of increasing stereocilia height (
Pickles et al.,
1989
). Each tip link is physically connected to a mechanosensitive channel with
an open probability that increases significantly in response to increased tension
(reviewed in
Stauffer & Holt, 2007
). Opening of the channel leads to a rapid
influx of calcium and potassium. Therefore, only deflection of the bundle in
the direction of the tallest stereocilia leads to increased channel opening and
depolarization. Conversely, if tension is decreased along the axis of the staircase,
open probability decreases leading to a hyperpolarization of the cell. All of the
mechanosensory epithelia within the inner ear contain morphological special-
izations that take advantage of the directional nature of the stereociliary bundles
to achieve increased sensitivity. For instance, in the coiled cochlear sensory
epithelium of mammals, all stereociliary bundles are oriented toward the lateral
edge of the coil. In response to sound waves, an overlying nonliving membrane,
the tectorial membrane is deflected closer and laterally relative to the lumenal
surface. This motion applies a lateral force to the stereociliary bundles, leading
to channel opening and cellular depolarization. In contrast, some vestibular
epithelia contain arrays of hair cells with orientations that are separated by
180
. As a result, deflection of an overlying membrane in one direction leads
to depolarization of some cells and hyperpolarization of others. This arrange-
ment is thought to result in heightened sensitivity. Anatomically, this results
in a “reversal zone” in which bundle orientation switches by 180
.
The direct link between the orientation of stereociliary bundles and the
appropriate function of auditory and vestibular mechanosensory epithelia
suggests that PCP must be highly regulated during development of the inner
