The basilar membrane is a stiff structural element within the

cochlea The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, the modiolus. A core component of the cochlea is the Organ of Corti, the sensory org ...

of the

inner ear The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in t ...

which separates two liquid-filled tubes that run along the coil of the cochlea, the

scala media Scala or SCALA may refer to: Automobiles * Renault Scala, multiple automobile models * Škoda Scala, a Czech compact hatchback Music * Scala (band), an English electronic music group * Escala (group), an electronic string quartet formerly k ...

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

The basilar membrane is a pseudo-resonant structure 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. 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). High-frequency sounds localize near the base of the cochlea, while low-frequency sounds localize near the apex.

Function

Endolymph/perilymph separation

Along with the

vestibular membrane The vestibular membrane, vestibular wall or Reissner's membrane, is a membrane inside the cochlea of the inner ear. It separates the cochlear duct from the vestibular duct. It helps to transmit vibrations from fluid in the vestibular duct to the ...

, several tissues held by the basilar membrane segregate the fluids of the

endolymph Endolymph is the fluid contained in the membranous labyrinth of the inner ear. The major cation in endolymph is potassium, with the values of sodium and potassium concentration in the endolymph being 0.91 mM and 154 mM, respectively. ...

and

perilymph Perilymph is an extracellular fluid located within the inner ear. It is found within the scala tympani and scala vestibuli of the cochlea. The ionic composition of perilymph is comparable to that of plasma and cerebrospinal fluid. The major ca ...

, such as the inner and outer sulcus cells (shown in yellow) and the reticular lamina 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 lamina, the endolymph side of the

organ of Corti The organ of Corti, or spiral organ, is the receptor organ for hearing and is located in the mammalian cochlea. This highly varied strip of epithelial cells allows for transduction of auditory signals into nerve impulses' action potential. Transd ...

A ''base'' for the sensory cells

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

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: 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. 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. The outer hair cells feed back energy to amplify the traveling wave, by up to 65 dB at some locations. 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

The movement of the basilar membrane causes

hair cell Hair cells are the sensory receptors of both the auditory system and the vestibular system in the ears of all vertebrates, and in the lateral line organ of fishes. Through mechanotransduction, hair cells detect movement in their environment ...

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

. 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

, 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

File:Gray928.png, Diagrammatic longitudinal section of the cochlea. File:Gray929.png, Floor of cochlear duct. File:Gray930.png, Spiral limbus and basilar membrane. File:Gray931.png, Section through the spiral organ of Corti (magnified) File:Gray932.png, The reticular membrane and subjacent structures.

References

External links

Auditory Neuroscience , The Ear several animations showing basilar membrane motion under various stimulus conditions

* ttp://sites.google.com/site/nowwehaveyourattention/ Basilar Membrane Simulator Video and Scripts to Simulate the Basilar Membrane
The role of the basilar membrane in sound reception: good explanation and diagrams
{{DEFAULTSORT:Basilar Membrane Auditory system Ear