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 to the linear acceleration of the body. The linear acceleration could be in the horizontal direction as in a moving car or vertical acceleration such as that felt when an elevator moves up or down. 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 to the linear acceleration of the body. The linear acceleration could be in the horizontal direction as in a moving car or vertical acceleration such as that felt when an elevator moves up or down. The otolithic membrane is part of the otolith organs in the vestibular system. The otolith organs include the utricle and the saccule. The otolith organs are beds of sensory cells in the inner ear, specifically small patches of hair cells. Overlying the hair cells and their hair bundles is a gelatinous layer and above that layer is the otolithic membrane. The utricle serves to measure horizontal accelerations and the saccule responds to vertical accelerations. The reason for this difference is the orientation of the macula in the two organs. The utricular macula lie horizontal in the utricle, while the saccular macula lies vertical in the saccule. Every hair cell in these sensory beds consist of 40-70 stereocilia and a kinocilium. The sterocilia and kinocilium are embedded in the otolithic membrane and are essential in the function of the otolith organs. The hair cells are deflected by structures called otoconia. Otoconia are crystals of calcium carbonate and make the otolithic membrane heavier than the structures and fluids surrounding it. The otoconia are composite crystallites that overlie the macular sensory epithelium of the gravity receptors of most vertebrates and are required for optimal stimulus input of linear acceleration and gravity. Fishes often have a single large crystal called an otolith, but otoconia from higher vertebrates have numerous crystals, and each apparently single crystal in fact has multiple crystallites that are composed of organic and inorganic components. Ultra-high resolution transmission electron microscopy of rat otoconia shows that the crystallites are 50-100 nm in diameter, have round edges and are highly ordered into laminae. Biomineralization of otoliths and otoconia results mainly from the release of soluble calcium ions, which is in turn precipitated as calcium carbonate crystals. The mechanical coupling of the otoconia to the hair cell sensory sterocilia at the surface of the vestibular sensory epithelium is mediated by two of the extracellular matrix, each on with a specific role in the mechanical transduction process. The first of these layers is the otolithic membrane which uniformly distributes the force of inertia of the non-uniform otoconia mass to all stereocilia bundles. The second layer formed by columnar filaments secures the membrane above the surface of the epithelium. Otolithic membrane structure has been frequently studied in amphibians and reptiles in order to elucidate the differences and to understand how the membrane has evolved in various otolith organs. Otolithic membranes of utricles in reptiles and amphibians represent thin plates of non-uniform structure, while the otolithic membrane in the saccule resembles a large cobble-stone-like conglomerate of otoconia. In fish, amphibians and reptiles there is also a third otolith organ that is not present in humans, and is called the lagena. The otolithic membrane in the lagena of amphibians is poorly differentiated, but well differentiated in reptiles. This difference corresponds to the fact that when vertebrates began to inhabit the earth surface there was a reorganization of the membrane. Over time, there was two changes that occurred in parallel when referring to the evolution of the otolithic membrane. First, otoliths that were present in amphibians and reptiles were replaced by a structurally differentiated otolithic membrane. Second, the spindle-shaped aragonitic otoconia were replaced by calcitic barrel-shaped otoconia. These two changes are referred to as the two directions of evolution of the otolithic membrane. When the head tilts, gravity causes the otolithic membrane to shift relative to the sensory epithelium (macula). The resulting shearing motion between the otolithic membrane and the macula displaces the hair bundles, which are embedded in the lower, gelatinous surface of the membrane. This displacement of the hair bundles generates a receptor potential in the hair cells. In addition to aiding in the sensing of tilting, the otolithic membrane helps the body detect linear accelerations. The greater relative mass of the membrane, due to the presence of the otoconia, causes it to lag behind the macula temporarily, leading to transient displacement of the hair bundle. One consequence of the similar effects exerted on otolithic hair cells by certain head tilts and linear accelerations is that otolith afferents cannot convey information that distinguishes between these two types of stimuli. Consequently, one might expect that these different stimuli would be rendered perceptually equivalent when visual feedback is absent, as occurs in the dark or when the eyes are closed. However, this is not the case because blindfolded subjects can discriminate between these two types of stimuli. The structure of the otolith organs enables them to sense both static displacements, as would be caused by tilting the head relative to the gravitational axis, and transient displacements caused by translational movements of the head. The mass of the otolithic membrane relative to the surrounding endolymph, as well as the membrane's physical uncoupling from the underlying macula, means that hair bundle displacement will occur transiently in response to linear accelerations, and tonically in response to tilting of the head. Prior to tiliting, the axon has a high firing rate, which increases or decreases depending on the direction of tilt. When the head is returned to its original position, the firing level returns to baseline value. In similar fashion, transient increases or decreases in firing rate from spontaneous levels signal the direction of linear accelerations of the head. The range of orientations of hair cells within the utricle and saccule combine to effectively gauge the linear forces acting on the head at any moment, in all three dimensions. Tilts of the head off the horizontal plane and translational movements of the head in any direction stimulate a distinct subset of hair cells in the saccular and utricular maculae, while simultaneously suppressing responses of other hair cells in these organs. Ultimately, variations in hair cell polarity within the otolith organs produce patterns of vestibular nerve fiber activity that, at a population level, unambiguously encode head position and the forces that influence it.