The utricle and

saccule The saccule (Latin: sacculus) is a bed of sensory cells in the inner ear that detects linear acceleration and head tilting in the vertical plane, and converts these vibrations into electrical impulses to be interpreted by the brain. When the he ...

are the two

otolith An otolith (, ' ear + , ', a stone), also called otoconium, statolith, or statoconium, is a calcium carbonate structure in the saccule or utricle (ear), utricle of the inner ear, specifically in the vestibular system of vertebrates. The saccule ...

organs in the

vertebrate Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...

inner ear The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in the ...

. The word utricle comes . The utricle and saccule are part of the balancing system (

membranous labyrinth The membranous labyrinth is a collection of fluid filled tubes and chambers in the inner ear The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for soun ...

) in the vestibule of the

bony labyrinth The bony labyrinth (also osseous labyrinth or otic capsule) is the rigid, bony outer wall of the inner ear in the temporal bone. It consists of three parts: the vestibule, semicircular canals, and cochlea. These are cavities hollowed out of the ...

(small oval chamber). They 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.

Macula

The macula of utricle (macula acustica utriculi) is a small (2 by 3 mm) thickening lying horizontally on the floor of the utricle where the epithelium contains vestibular hair cells that allow a person to perceive changes in latitudinal acceleration as well as the effects of gravity; it receives the utricular filaments of the acoustic nerve. The hair cells are mechanoreceptors which have 40 to 70 stereocilia and only one true cilium called a

kinocilium A kinocilium is a special type of cilium on the apex of hair cells located in the sensory epithelium of the vertebrate inner ear. Contrasting with stereocilia, which are numerous, there is only one kinocilium on each hair cell. The kinocilium can ...

. The kinocilium is the only sensory aspect of the hair cell and is what causes hair cell polarization. The tips of these stereocilia and kinocilium are embedded in a gelatinous layer, which together with the

statoconia An otolith (, ' ear + , ', a stone), also called otoconium, statolith, or statoconium, is a calcium carbonate structure in the saccule or utricle of the inner ear, specifically in the vestibular system of vertebrates. The saccule and utricle, i ...

form the

otolithic membrane The otolithic membrane is a fibrous structure located in the vestibular system of the inner ear. It plays a critical role in the brain's interpretation of equilibrium. The membrane serves to determine if the body or the head is tilted, in addition ...

. 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. 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. Labyrinthine activity responsible for the

nystagmus Nystagmus is a condition of involuntary (or voluntary, in some cases) Eye movement (sensory), eye movement. People can be born with it but more commonly acquire it in infancy or later in life. In many cases it may result in visual impairment, re ...

induced by off-vertical axis rotation arises in the otolith organs and couples to the oculomotor system through the velocity storage mechanism.

Microanatomy

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 Stereocilia (or stereovilli or villi) are non-motile apical cell modifications. They are distinct from cilia and microvilli, but are closely related to microvilli. They form single "finger-like" projections that may be branched, with normal cel ...

set-up. These are covered by

s which, due to gravity, pull on the stereocilia and tilt them. Depending on whether the tilt is in the direction of the

or not, the resulting hair cell polarisation is excitatory (depolarising) or inhibitory (hyperpolarisation), 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.Saladin, Kenneth S. Anatomy & Physiology: the Unity of Form and Function. Dubuque: McGraw-Hill, 2010. Print. This signal to the vestibular nerve (which takes it to the brainstem) 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. Unbent and at rest hairs in the macula have a base rate of depolarization of 90-100 action potentials a second. The brain suppresses this, and we ignore it and know that our body is stabilized. If the head moves or the body accelerates or decelerates, then bending occurs. Depending on the direction of bending, the hair cells will either be excited or inhibited resulting in either an increase or decrease in firing frequency of the hair cells. The macula is also sensitive to linear acceleration as the inertia possessed by the statoconia can also shift the gelatinous layer during increases and decreases in linear velocity. Blausen 0329 EarAnatomy InternalEar

References

External links