The utricle, or utriculus, along with the saccule, is one of the two otolith
organs located in the vertebrate inner ear. The utricle and the saccule are
parts of the balancing apparatus located within the vestibule of the bony
labyrinth. These use small stones and a viscous
fluid to stimulate hair cells to detect motion and orientation. The utricle
detects linear accelerations and head-tilts in the horizontal plane.
Structure The utricle is larger than the saccule
and is of an oblong form, compressed transversely, and occupies the upper and
back part of the vestibule, lying in contact with the recessus ellipticus and
the part below it. The macula utricle is a thickening in the wall of the utricle
where the epithelium contains vestibular hair cells that allows a person to
perceive changes in longitudinal acceleration as well as effects of
gravity. The gelatinous layer and the statoconia
together are referred to as the otolithic membrane, where the tips of
the stereocilia and kinocilium are embedded. When the head is tilted such
that gravity pulls on the statoconia the gelatinous layer is pulled in the same
direction also causing the sensory hairs to bend.
Within the utricle is a small 2 by 3 mm patch of hair cells called the macula
utricle. The macula utricle, which lies horizontally on the floor of the
utricle, contains the hair cells. These hair cells are mechanoreceptors which
consist of 40 to 70 steriocilia and only one true cilium called a kinocilium. The
kinocilium is the only sensory aspect of the hair cell and is what causes hair
cell polarization. The tips of these steriocilia and kinocilium are embedded
in a gelatinous otolithic membrane. This membrane is weighted with calcium
carbonate-protein granules called otoliths. The otolithic membrane adds
weight to the tops of the hair cells and increases their inertia. The addition in
weight and inertia is vital to the utricle's ability to detect linear
acceleration, as described below, and to determine the orientation of the head.
= Histology = The macula consists of three layers. The
bottom layer is made of sensory hair cells which are embedded in bottom of a
gelatinous layer. Each hair cells consists of 40 to 70 steriocilia and a
kinocilium, which lies in the middle of the steriocilia and is the most
important receptor. On top of this layer lie calcium
carbonate crystals called statoconia or otoliths. The otoliths are relatively
heavy, providing weight to the membrane as well as inertia. This allows for a
greater sense of gravity and motion. Labyrinthine activity responsible for
the nystagmus induced by off-vertical axis rotation arises in the otolith
organs and couples to the oculomotor system through the velocity storage
mechanism. That portion which is lodged in the
recess forms a pouch or cul-de-sac, the floor and anterior wall of which are
thickened and form the macula acustica utriculi, which receives the utricular
filaments of the acoustic nerve. The cavity of the utricle communicates
behind with the semicircular ducts by five orifices.
The ductus utriculosaccularis comes off of the anterior wall of the utricle and
opens into the ductus endolymphaticus. Function
The utricle contains mechanoreceptors called hair cells that distinguish
between degrees of tilting of the head, thanks to their apical stereocilia
set-up. These are covered by otolith which, due to gravity, pull on the
stereocilia and tilt them. Depending on whether the tilt is in the direction of
the kinocilium or not, the resulting hair cell polarisation is excitatory or
inhibitory, respectively. Any orientation of the head causes a
combination of stimulation to the utricles and saccules of the two ears.
The brain interprets head orientation by comparing these inputs to each other and
to other input from the eyes and stretch receptors in the neck, thereby detecting
whether only the head is tilted or the entire body is tipping. The inertia of
the otolithic membranes is especially important in detecting linear
acceleration. Suppose you are sitting in a car at a stoplight and then begin to
move. The otolithic membrane of the macula utriculi briefly lags behind the
rest of the tissues, bends the stereocilia backward, and stimulates the
cells. When you stop at the next light, the macula stops but the otolithic
membrane keeps going for a moment, bending the stereocilia forward. The
hair cells convert this pattern of stimulation to nerve signals, and the
brain is thus advised of changes in your linear velocity. This signal to the
vestibular nerve does not adapt with time. The effect of this is that, for
example, an individual lying down to sleep will continue to detect that they
are lying down hours later when they awaken.
See also References
This article incorporates text in the public domain from the 20th edition of
Gray's Anatomy External links
Diagram at ipfw.edu