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.^[1] 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.^[2] The stereocilia 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.

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.^[1] 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.^[1]

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.^[1]

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.^[1] 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.^[1] Prior to tilting, 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.^[1]

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.^[1]

Although the pathophysiology of otolithic dysfunction is poorly understood, a disorder of otolith function, at a peripheral or central level, may be suspected when a patient describes symptoms of false sensations of linear motion or tilt or shows signs of specific derangements of ocular motor and postural, orienting and balancing responses. When disorientation is severe the patient may describe symptoms which sound bizarre, raising doubts over the organic basis of the disease. It is important to understand otolithic involvement in a wider neurological context through knowledge of the otolith physiology and the characteristics of proven otolithic syndromes.^[8]

Benign paroxysmal positional vertigo (BPPV) is the most common vestibular system disorder and occurs as a result of otoconia detaching from the otolithic membrane in the utricle and collecting in one of the semicircular canals. It is generally associated with natural age-related degeneration of the otolithic membrane. When the head is still, gravity causes the otoconia to clump and settle. When the head moves, the otoconia shift, which stimulates the cupula to send false signals to the brain, producing vertigo and triggering nystagmus. In addition to vertigo, symptoms of BPPV include dizziness, imbalance, difficulty concentrating, and nausea.^[9]

The otolithic membrane can be affected in patients with Ménière's disease. Sudden falls without loss of consciousness (drop attacks) may be experienced by some people in the later stages of the disease, when they are referred to as Tumarkin attacks, or as Tumarkin's otolithic crisis.^[10][11] Those who experience such attacks (probably less than 10% of people with Meniere's disease) may report a sensation of being pushed sharply to the floor from behind.^[12] The phenomenon is thought to be triggered by a sudden mechanical disturbance of the otolithic membrane that activates motoneurons in the vestibulospinal tract.^[12]

Otolithic function can also be compromised after unilateral vestibular neurectomy. The illusion is that during centrifugal stimulation, a small luminous bar, fixed with respect to the observer, appears to be roll-tilited by the same amount that the observer feels to be roll-tilted. This illusion is felt symmetrically in normal patients, but after vestibular neuroectomy, patients perceive a reduce illusion when the force is directed toward their operated ear.^[13]

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.^[14] 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.^[14]