The utricle is one of two otolith organs inside the ear canal, the other being the saccule. Otolith organs are able to sense gravitational changes and linear acceleration that comes from movement in a straight line. Also called the utriculus, the utricle occupies the bony labyrinth of the inner ear, nestled between the cochlea and the semicircular canals.
Otolithic organs detect movement depending on their orientation. Animals or humans without otolithic organs are imbalanced. The utricle detects movement on the horizontal plane of the head, and the saccule detects vertical motion.
The utricle is composed of a mass of tiny calcareous stones covered in hair cells embedded in a jelly-like substance containing the calcareous stones. These hair cells are connected to the nervous system by vestibular division fibers of the auditory nerve, with each hair cell connected to one fiber. As the stones accelerate from motion, a force is exerted on the hair cells. When the hair cells detect the force from the moving stones, a signal is sent to the brain through the vestibular nerve that alerts the brain that movement is occurring.
Otolithic organs are subject to gravity, and because gravity always pulls the otoliths toward the ground, a change in orientation alters the direction in which the otoliths pull. This causes a different stimulus to the hair cells, which then send different nerve pulses to the brain. It is through this that the utricle constantly reports the orientation of the head. The information that is sent from the utricle to the brain, when working in conjunction with information from the muscles, also indicates the overall position of the whole body. This is why even a blindfolded person is able to detect that exact position within a space of each part of his or her body.
Certain medical conditions can damage the utricle and cause an imbalance in afflicted persons. These disturbances can come from damage to the ear canal, peripheral or brainstem disturbances, psychiatric disorders and cortical vestibular disturbances. Problems can also arise with certain diseases such as Menieres disease, acoustic neuroma, strokes and seizures that involve the vestibular cortex.
Menieres disease affects both the saccule and the utricle, causing the saccule wall to be much thinner than the utricular wall. When this happens, the saccule grows larger than the utricle, causing a loss of function to the utricle. Acoustic neuroma disconnects the utricle from the brain, which disrupts the nerve pulses sent by the utricle during motion. When a person suffers a stroke involving the vestibular cortex, the world might appear to be turned upside down, and he or she might suffer from several forms of inversion illusions that seem to tilt the visual axis.
