What is the significance of the basilar membrane in hearing

As a consequence, the increase in pressure in the cochlear fluid caused by the inward movement of the stapes also displaces fluid in the direction of the cochlear partition, which is deflected downwards.

This downward deflection in turn causes the elastic basilar membrane to move down and also increases the pressure within the scala tympani. The enhanced pressure in the scala tympani displaces a fluid mass that contributes to outward bowing of the round window. When the stapes pulls back, the process is reversed and the basilar membrane moves up and the round window bows inwards.

In other words, each cycle of a sound stimulus evokes a complete cycle of up-and-down movement of the basilar membrane and provides the first step in converting the vibration of the fluid within the cochlea into a neural code. The mechanical properties of the basilar membrane are the key to the cochlea's operation.

One critical feature of the basilar membrane is that it is not uniform. Instead, its mechanical properties vary continuously along its length in two ways. First, the membrane is wider at its apex compared to the base by a factor of about 5, and second, it decreases in stiffness from base to apex, the base being times stiffer.

So, the base is narrow and stiff compared to the apex Figure 8. This means that stimulation by a pure tone results in a complex movement of the membrane. If it were uniform, then the fluctuating pressure difference between the scala vestibuli and the scala tympani caused by the sound would move the entire membrane up and down with similar excursions at all points.

However, because of the variation in width and stiffness along its length, various parts of the membrane do not oscillate in phase. Over a complete cycle of sound each segment of the membrane undergoes a single cycle of vibration but at any point in time some parts of the membrane are moving upwards and some parts are moving downwards.

The overall pattern of movement of the membrane is described as a travelling wave. To visualise the motion of a travelling wave, think of a wave that travels along a piece of ribbon if you hold one end in your hand and give it a flick. Figure 9 a is a representation of what you might expect by flicking a ribbon.

Figure 9 b represents a more realistic representation of the wave on the basilar membrane because the basilar membrane is attached at its edges and is displaced in response to sound in a transverse crosswise direction as well as a longitudinal direction.

As it travels, the wave reaches a peak amplitude that then rapidly falls. The amplitude of the wave is therefore greatest at a particular location on the membrane.

A travelling wave then, is a unique moving waveform whose point of maximal displacement traces out a specific set of locations. The shape described by the set of these locations along the basilar membrane is called the envelope of the travelling wave Figure The point along the basilar membrane where the wave, and hence the envelope traced by the travelling wave, reaches a peak differs for each frequency.

In other words, each point along the basilar membrane that is set in motion vibrates at the same frequency as the sound impinging on the ear, but different frequency sounds cause a peak in the wave at different positions on the basilar membrane Figure 11 a. Look at Figure 11 b. For the lowest frequency 60 Hz the maximum displacement is near the apical end, for the highest frequency Hz the maximum displacement is near the base, while the intermediate frequency has maximal displacement between the two.

Therefore, high-frequency sounds cause a small region of the basilar membrane near the stapes to move, while low frequencies cause almost the entire membrane to move. However, the peak displacement of the membrane is located near the apex. This shows that the travelling wave always travels from base to apex, and how far towards the apex it travels depends on the frequency of stimulation; lower frequencies travel further. What would the response of the membrane be if the sound impinging on the ear was a complex sound consisting of frequencies of Hz and Hz?

Each frequency would create a maximum displacement at a different point along the basilar membrane as shown in Figure 11 c. The separation of a complex signal into two different points of maximal displacement along the membrane, corresponding to the sinusoidal waves of which the complex signal is composed, means that the basilar membrane is performing a type of spectral Fourier analysis. Fourier analysis is the process of decomposing a waveform into its sinusoidal components.

The basilar membrane displacement therefore provides useful information about the frequency of the sound impinging on the ear by acting like a series of band-pass filters. Each section of the membrane passes, and therefore responds to, all sinusoidal waves with frequencies between two particular values. It does not respond to frequencies that are present in the sound but fall outside the range of frequencies of that section.

The filter characteristics of the basilar membrane can be studied using the technique of laser interferometry. Figure 12 shows the results of such a study. The data were collected by presenting different frequency sounds to the inner ear of a chinchilla and then measuring the level of each tone that is required to displace the basilar membrane by a fixed amount.

Measurements are taken at a particular point on the basilar membrane. From Figure 12 , determine the frequency of the tone that required the lowest sound level to displace the basilar membrane by a set amount. The image above shows the cochlea unrolled to make the basilar membrane easier to see.

We experience these things every day, but how do our brains create them? Your Brain, Explained is a personal tour around your gray matter. Louis , author, Origins of Neuroscience. The key structure in the vertebrate auditory and vestibular systems is the hair cell. The hair cell first appeared in fish as part of a long, thin array along the side of the body, sensing movements in the water.

In higher vertebrates the internal fluid of the inner ear not external fluid as in fish bathes the hair cells, but these cells still sense movements in the surrounding fluid. Several specializations make human hair cells responsive to various forms of mechanical stimulation. Hair cells in the Organ of Corti in the cochlea of the ear respond to sound. Hair cells in the cristae ampullares in the semicircular ducts respond to angular acceleration rotation of the head.

Hair cells in the maculae of the saccule and the utricle respond to linear acceleration gravity. See the chapter on Vestibular System: Structure and Function. The fluid, termed endolymph , which surrounds the hair cells is rich in potassium. This actively maintained ionic imbalance provides an energy store, which is used to trigger neural action potentials when the hair cells are moved.

Tight junctions between hair cells and the nearby supporting cells form a barrier between endolymph and perilymph that maintains the ionic imbalance. Cilia emerge from the apical surface of hair cells. These cilia increase in length along a consistent axis. There are tiny thread-like connections from the tip of each cilium to a non-specific cation channel on the side of the taller neighboring cilium.

The tip links function like a string connected to a hinged hatch. When the cilia are bent toward the tallest one, the channels are opened, much like a trap door. Opening these channels allows an influx of potassium, which in turns opens calcium channels that initiates the receptor potential. This mechanism transduces mechanical energy into neural impulses. This in turn causes neurotransmitter release at the basal end of the hair cell, eliciting an action potential in the dendrites of the VIIIth cranial nerve.

Press the "play" button to see the mechanical-to-electrical transduction. Bending the cilia toward the tallest one opens the potassium channels and increases afferent activity. Bending the cilia in the opposite direction closes the channels and decreases afferent activity. Bending the cilia to the side has no effect on spontaneous neural activity. The auditory system changes a wide range of weak mechanical signals into a complex series of electrical signals in the central nervous system.

Sound is a series of pressure changes in the air. Sounds often vary in frequency and intensity over time. Humans can detect sounds that cause movements only slightly greater than those of Brownian movement. Obviously, if we heard that ceaseless except at absolute zero motion of air molecules we would have no silence.

The pinna and external auditory meatus collect these waves, change them slightly, and direct them to the tympanic membrane. The resulting movements of the eardrum are transmitted through the three middle-ear ossicles malleus, incus and stapes to the fluid of the inner ear. The footplate of the stapes fits tightly into the oval window of the bony cochlea. The inner ear is filled with fluid. Since fluid is incompressible, as the stapes moves in and out there needs to be a compensatory movement in the opposite direction.

Notice that the round window membrane, located beneath the oval window, moves in the opposite direction. Because the tympanic membrane has a larger area than the stapes footplate there is a hydraulic amplification of the sound pressure. Also because the arm of the malleus to which the tympanic membrane is attached is longer than the arm of the incus to which the stapes is attached, there is a slight amplification of the sound pressure by a lever action. These two impedance matching mechanisms effectively transmit air-born sound into the fluid of the inner ear.

If the middle-ear apparatus ear drum and ossicles were absent, then sound reaching the oval and round windows would be largely reflected. The cochlea is a long coiled tube, with three channels divided by two thin membranes. The top tube is the scala vestibuli, which is connected to the oval window.

The bottom tube is the scala tympani , which is connected to the round window. The middle tube is the scala media, which contains the Organ of Corti. The Organ of Corti sits on the basilar membrane, which forms the division between the scalae media and tympani.

The three scalae vestibuli, media, tympani are cut in several places as they spiral around a central core. The tightly coiled shape gives the cochlea its name, which means snail in Greek as in conch shell. As explained in Tonotopic Organization , high frequency sounds stimulate the base of the cochlea, whereas low frequency sounds stimulate the apex.

This feature is depicted in the animation of Figure The activity in Figure The moving dots are meant to indicate afferent action potentials.