What are the primary structures of the vestibular labyrinth?
The membranous vestibular labyrinth sits in the bony canals and vestibule of the inner ear in the temporal bone.
The labyrinth floats in perilymph, a fluid that is similar to cerebrospinal fluid (CSF) and has a high sodium:potassium ratio. Perilymph communicates with CSF via the cochlear aqueduct.
By contrast, the fluid that fills the labyrinth, the endolymph, has a high potassium:sodium ratio. The labyrinth has three, orthogonal semicircular canals, each of which has two names: superior or anterior , horizontal or lateral , and inferior or posterior .
The superior and posterior semicircular canals are joined at the common crux. The common crux and one end of the lateral canal end in the utricle, which is one of the two sacs. The utricle (approximately in the horizontal plane) is joined to the other sac, the saccule (approximately in the vertical plane).
Each semicircular canal has an enlargement at one end, the crista ampullaris, or ampulla. Within the ampulla is the crista, or tiny hillock that runs across the width of the canal, on three sides of which are cilia that are the apical ends of the hair cells.
The cilia project into a gelatinous cupula, or cup. These mechanical structures transduce rotational motions of the head by bending the cilia when the endolymph moves due to movement of the head.
The base of the chalice-shaped Type 1 hair cell and the columnar-shaped Type II hair cells emit neurotransmitters that are picked up by the vestibular nerve. Each Type I hair cell is encapsulated by a single afferent vestibular nerve ending, but that afferent ending can have multiple efferents.
Type II hair cells communicate with multiple afferent nerve endings and efferents. Hair cells have muscarinic acetylcholine receptors. The afferent vestibular nerve fibers in cranial nerve XIII, however, express glutamate and histamine.
The two sac-like otoliths, the utricle, and saccule each has a slightly curved sensory surface on one wall, known as a macula , in which the hair cells are located.
The macular surfaces are approximately orthogonal to each other, oriented to detect linear motion of the head, including the constant force of gravity, in any plane in space. In the macula the cilia are covered with a layer of calcium carbonate crystals 2.7 times more dense than endolymph, known as otoconia , imbedded in a protein matrix membrane.
When the head moves in a linear direction, for example, forward, the inertia of the otoconia causes the otoconial membrane to lag behind the motion of the head for a fraction of a second, bending the underlying cilia and, if the cell membrane depolarizes, releasing neurotransmitters.
The superior portion of the vestibular nerve innervates the anterior and horizontal semicircular canals, most of the utricle, and a small portion of the saccule. The inferior portion of the vestibular nerve innervates the posterior semicircular canal, most of the saccule, and a small portion of the utricle.
The adequate stimulus to every part of the system is acceleration or deceleration of the head: a change in the rate of speed of the head. When the head is moving at a constant velocity or is at rest (0 velocity) the system does not detect head movement. The mechanical properties of the labyrinth change the signal so that the signal that ascends the vestibular nerve to the vestibular nuclei represents head velocity.
