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Basilar membrane
Basilar membrane
Basilar membrane
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Basilar membrane.
Cochlea cross section showing thin basilar membrane in brown
Section through organ of corti
Details
Identifiers
Latinmembrana basilaris ductus cochlearis
MeSHD001489
Anatomical terminology

The basilar membrane is a stiff structural element, a type of basement membrane within the cochlea of the inner ear[1] which separates two liquid-filled tubes that run along the coil of the cochlea, the scala media and the scala tympani. The basilar membrane moves up and down in response to incoming sound waves, which are converted to traveling waves on the basilar membrane.

Structure

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The basilar membrane is a pseudo-resonant structure[2] that, like the strings on an instrument, varies in width and stiffness. But unlike the parallel strings of a guitar, the basilar membrane is not a discrete set of resonant structures, but a single structure with varying width, stiffness, mass, damping, and duct dimensions along its length. The motion of the basilar membrane is generally described as a traveling wave.[3]

The properties of the membrane at a given point along its length determine its characteristic frequency (CF), the frequency at which it is most sensitive to sound vibrations. The basilar membrane is widest (0.42–0.65 mm) and least stiff at the apex of the cochlea, and narrowest (0.08–0.16 mm) and stiffest at the base (near the round and oval windows).[4] High-frequency sounds localize near the base of the cochlea, while low-frequency sounds localize near the apex.[5][6] This is because the resonant frequency of the membrane depends on its own width and stiffness, not on the diameter of the cochlea at each point.

Function

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Endolymph/perilymph separation

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Along with the vestibular membrane, several tissues held by the basilar membrane segregate the fluids of the endolymph and perilymph, such as the inner and outer sulcus cells (shown in yellow) and the reticular membrane of the organ of Corti (shown in magenta). For the organ of Corti, the basilar membrane is permeable to perilymph. Here the border between endolymph and perilymph occurs at the reticular membrane, the endolymph side of the organ of Corti.[7]

A base for the sensory cells

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The basilar membrane is also the base for the hair cells. This function is present in all land vertebrates. Due to its location, the basilar membrane places the hair cells adjacent to both the endolymph and the perilymph, which is a precondition of hair cell function.

Frequency dispersion

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A third, evolutionarily younger, function of the basilar membrane is strongly developed in the cochlea of most mammalian species and weakly developed in some bird species:[8] the dispersion of incoming sound waves to separate frequencies spatially. In brief, the membrane is tapered and it is stiffer at one end than at the other. Furthermore, sound waves travelling to the "floppier" end of the basilar membrane have to travel through a longer fluid column than sound waves travelling to the nearer, stiffer end. Each part of the basilar membrane, together with the surrounding fluid, can therefore be thought of as a "mass-spring" system with different resonant properties: high stiffness and low mass, hence high resonant frequencies at the near (base) end, and low stiffness and high mass, hence low resonant frequencies, at the far (apex) end.[9] This causes sound input of a certain frequency to vibrate some locations of the membrane more than other locations. The distribution of frequencies to places is called the tonotopic organization of cochlea.

Sound-driven vibrations travel as waves along this membrane, along which, in humans, lie about 3,500 inner hair cells spaced in a single row. Each cell is attached to a tiny triangular frame. The 'hairs' are minute processes on the end of the cell, which are very sensitive to movement. When the vibration of the membrane rocks the triangular frames, the hairs on the cells are repeatedly displaced, and that produces streams of corresponding pulses in the nerve fibers, which are transmitted to the auditory pathway.[10] The outer hair cells feed back energy to amplify the traveling wave, by up to 65 dB at some locations.[11][12] In the membrane of the outer hair cells there are motor proteins associated with the membrane. Those proteins are activated by sound-induced receptor potentials as the basilar membrane moves up and down. These motor proteins can amplify the movement, causing the basilar membrane to move a little bit more, amplifying the traveling wave. Consequently, the inner hair cells get more displacement of their cilia and move a little bit more and get more information than they would in a passive cochlea.

Generating receptor potential

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The movement of the basilar membrane causes hair cell stereocilia movement. The hair cells are attached to the basilar membrane, and with the moving of the basilar membrane, the tectorial membrane and the hair cells are also moving, with the stereocilia bending with the relative motion of the tectorial membrane. This can cause opening and closing of the mechanically gated potassium channels on the cilia of the hair cell. The cilia of the hair cell are in the endolymph. Unlike the normal cellular solution, low concentration of potassium and high of sodium, the endolymph is high concentration of potassium and low of sodium. And it is isolated, which means it does not have a resting potential of −70mV comparing with other normal cells, but rather maintains a potential about +80mV. However, the base of the hair cell is in the perilymph, with a 0 mV potential. This leads to the hair cell have a resting potential of -45 mV. As the basilar membrane moves upward, the cilia move in the direction causing opening of the mechanically gated potassium channel. The influx of potassium ions leads to depolarization. On the contrary, the cilia move the other way as the basilar membrane moves down, closing more mechanically gated potassium channels and leading to hyperpolarization. Depolarization will open the voltage gated calcium channel, releasing neurotransmitter (glutamate) at the nerve ending, acting on the spiral ganglion cell, the primary auditory neurons, making them more likely to spike. Hyperpolarization causes less calcium influx, thus less neurotransmitter release, and a reduced probability of spiral ganglion cell spiking.

Additional images

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  • Diagrammatic longitudinal section of the cochlea.
    Diagrammatic longitudinal section of the cochlea.
  • Floor of cochlear duct.
    Floor of cochlear duct.
  • Spiral limbus and basilar membrane.
    Spiral limbus and basilar membrane.
  • Section through the spiral organ of Corti (magnified)
    Section through the spiral organ of Corti (magnified)
  • The reticular membrane and subjacent structures.
    The reticular membrane and subjacent structures.

See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Basilar membrane

View on Grokipedia
from Grokipedia
The basilar membrane is a thin, flexible structure within the of the that separates the endolymph-filled scala media from the perilymph-filled scala tympani, and plays a central role in auditory transduction by vibrating in response to sound-induced fluid waves. Located along the length of the uncoiled cochlea, which measures approximately 35 mm, it forms part of the cochlear partition that bisects the cochlear duct. Structurally, the basilar membrane consists of and varies in width and stiffness along its length: it is narrower (about 0.1 mm) and stiffer at the basal end near the oval window, while becoming wider (up to 0.5 mm) and more flexible at the apical end near the . This gradient in mechanical properties enables the membrane to act as a frequency-selective resonator, with high-frequency sounds maximally displacing the basal region and low-frequency sounds the apical region, a phenomenon known as . The membrane supports the , a complex epithelial structure containing sensory hair cells arranged in inner and outer rows. In the hearing process, sound vibrations transmitted through the cause waves in the of the scala vestibuli, which deflect the basilar membrane upward toward the overlying tectorial membrane in the scala media filled with . This displacement generates a shearing force that bends the of cells embedded in or adjacent to the tectorial membrane, opening mechanically gated ion channels and depolarizing the cells to release neurotransmitters onto auditory fibers. Outer cells, equipped with the prestin, actively amplify these vibrations to enhance sensitivity and frequency discrimination, while inner cells primarily serve as the main transducers, conveying 95% of auditory signals to the .

Anatomy

Composition

The basilar membrane is an acellular structure composed primarily of a gelatinous extracellular matrix, consisting of fibers embedded within a rich in glycosaminoglycans such as and proteoglycans that provide viscoelastic properties. This acellular nature ensures that the membrane itself lacks embedded sensory or supporting cells, with the positioned atop its surface to facilitate auditory transduction. The membrane exhibits a layered organization that contributes to its mechanical integrity. The tympanic layer, facing the scala tympani, incorporates fibers extending from the osseous spiral lamina. The middle layer contains a network of radial fibers (primarily types II and XI) oriented from medial to lateral, interspersed with circumferential fibers for added support, all set within the amorphous . The vestibular layer, adjacent to the scala media, includes fibers that attach to the spiral ligament, forming the outer boundary. Collagen fiber density varies along the cochlea's length, with higher packing at the base enhancing stiffness for high-frequency responses and progressively decreasing toward the apex to allow greater flexibility for low frequencies. This gradient in fiber arrangement, without altering the fundamental composition, supports the membrane's role in tonotopic organization.

Morphology

The basilar membrane in humans measures approximately 35 mm in length, spanning the coiled cochlear duct from its base adjacent to the oval window to the apex at the helicotrema. This ribbon-like structure exhibits a pronounced gradient in dimensions along its length, which contributes to its biomechanical properties. At the base, the membrane is narrow, measuring about 0.1 mm in width, and progressively widens to approximately 0.5 mm at the apex. Similarly, its thickness varies, being thicker at the base (approximately 1.1 μm) and thinning toward the apex (approximately 0.2 μm). These gradients in width and thickness are accompanied by variations in , with the basal being stiffer due to a higher of radial fibers, while the apical is more flexible. The decreases by a factor of up to 100 from base to apex, influencing the membrane's overall mass distribution and vibrational characteristics. In transverse sections, the basilar membrane presents a triangular profile, anchored medially to the osseous spiral lamina and laterally to the basilar crest of the spiral ligament. This configuration supports the and facilitates differential responses along the cochlear length.

Location

Position in the Cochlea

The basilar membrane forms the floor of the cochlear duct, also known as the scala media, within the of the . It separates the scala media from the underlying scala tympani, creating a partition that maintains distinct in the cochlear architecture. This positioning allows the membrane to span the width of the cochlear canal, extending from the medial osseous spiral lamina to the lateral spiral ligament. The basilar membrane follows a spiral trajectory along the cochlea, which coils for approximately 2.5 to 2.75 turns around the central modiolus. It begins at the basal turn, near the stapes and oval window, and progresses toward the apex, terminating at the helicotrema where the scala tympani connects to the scala vestibuli. This spiraling path aligns with the overall helical structure of the cochlea, enabling the membrane to support auditory processing across its length. In orientation, the basilar membrane runs longitudinally along the cochlear spiral but extends radially, perpendicular to the modiolus, from the inner bony core to the outer wall. The organ of Corti is situated on its upper surface, facing toward the tectorial membrane in the scala media. This radial alignment facilitates the membrane's role in supporting sensory structures while integrating with the cochlear duct's geometry. The basilar membrane is immersed in from the scala tympani below and from the scala media above, with the latter bathing its upper surface and the . It has no direct exposure to the perilymph of the scala vestibuli, which is separated by the overlying Reissner's membrane. This fluid arrangement underscores the membrane's boundary-forming position without bridging all three scalae directly.

Structural Relations

The basilar membrane is anchored on its medial side to the osseous spiral lamina, a bony shelf that protrudes from the modiolus, the central axis of the cochlea. This attachment occurs via fibers in the tympanic layer of the basilar membrane, which extend laterally to connect with the tympanic lip at the edge of the osseous spiral lamina, providing structural stability to the membrane's inner boundary. Laterally, the basilar membrane connects to the spiral ligament along the cochlear wall through its vestibular layer, which attaches to the basilar crest, a on the spiral ligament that forms part of the outer boundary of the scala media. This connection via the vestibular layer helps secure the membrane against the lateral cochlear structures, forming a supportive framework that spans the cochlear duct. The basilar membrane serves as a foundational platform for the , supporting the arrangement of inner and outer hair cells along with associated supporting elements such as Deiters' cells and their phalangeal processes. Deiters' cells, also known as phalangeal cells, originate from the basilar membrane and extend upward to form the reticular lamina, enclosing the hair cells within the and contributing to its overall structural integrity. Above the , the basilar membrane maintains an indirect spatial relation to the tectorial membrane through the of the s, which project from the hair cell apices and contact the undersurface of the tectorial membrane. This proximity positions the between the two membranes, facilitating interactions driven by cochlear without direct attachment of the basilar membrane itself to the tectorial structure.

Function

Fluid Separation

The basilar membrane serves as an impermeable barrier that separates the -filled scala media from the in the scala tympani within the . This separation is essential for maintaining distinct fluid compartments, with exhibiting a high concentration (approximately 150 mM) and low sodium (about 1 mM), while has high sodium (around 150 mM) and low (roughly 5 mM). The in the scala media maintains an endocochlear potential of +80 mV relative to the , which has a potential of approximately 0 mV, creating a critical . By preventing the mixing of these fluids, the basilar membrane preserves the ionic gradients necessary for the endocochlear potential, which drives ion influx into s during auditory transduction and enables their . Disruption of this barrier would dissipate the potential, impairing the sensitivity of mechanotransduction and overall cochlear function. The membrane's low permeability to ions stems from its composition of a tight , including fibers and supporting cells of the , which limit diffusion rates—for instance, permeability is on the order of 112 × 10⁻⁶ s⁻¹. Reissner's membrane complements this by forming the upper barrier between and in the scala vestibuli, collectively ensuring compartmentalization. Compromise of the basilar membrane's integrity, as occurs in perilymphatic fistulas, allows fluid leakage and ionic equilibration, potentially resulting in due to the collapse of the endocochlear potential and dysfunction.

Mechanical Vibration

Sound vibrations transmitted to the at the oval window cause displacement of the in the scala vestibuli, initiating a pressure wave that propagates through the cochlear fluids and drives motion of the basilar membrane. This mechanical input results in a traveling wave along the basilar membrane, starting at the basal end of the and moving toward the apex. The traveling wave propagates with velocities that decrease from the base to the apex, typically ranging from approximately 100 m/s near the to less than 1 m/s at apical sites, facilitating between the cochlear fluids and the 's varying mechanical properties. Peak displacement of the occurs at locations determined by the stimulus , where the wave's reaches its maximum due to with the local and characteristics. The envelope of the wave builds gradually before the peak and declines sharply afterward, while the phase exhibits a progressive lag along the 's length, accumulating delays that reflect the dispersive nature of the propagation. Following the point of maximum displacement, energy dissipation is achieved through internal within the basilar membrane's and viscous interactions at its attachments to the surrounding structures, such as the spiral lamina and basilar crest. These damping mechanisms rapidly attenuate the wave's energy, minimizing reflections back toward the base and ensuring unidirectional propagation that supports efficient sound encoding.

Frequency Selectivity

The basilar membrane's frequency selectivity is manifested through its tonotopic mapping, where vibrations peak at specific locations along its length depending on the sound . High-frequency tones up to approximately 20 kHz elicit maximal displacements near the cochlear base, characterized by a narrow and stiff , while low-frequency sounds down to about 20 Hz produce peak responses at the apex, which is wider and more compliant. This spatial organization creates a frequency-place map that enables the to decompose complex sounds into their frequency components. The underlying resonance principles stem from gradients in the basilar membrane's mechanical properties, particularly the , which determines the natural at each position. The ff follows the relation f∝k/mf \propto \sqrt{k/m}
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