3.3 The role of the basilar membrane in sound reception
So far we know that sound-induced increases and decreases in air pressure move the tympanum inwards and outwards. The movement of the tympanum displaces the malleus which is fixed to its inner surface. The motion of the malleus and hence the incus results in the stapes functioning like a piston – alternately pushing into the oval window and then retracting from it. Since the oval window communicates with the scala vestibuli, the action of the stapes pushes and pulls cyclically on the fluid in the scala vestibuli. When the stapes pushes in on the oval window, the liquid in the scala vestibuli is displaced. If the membranes inside the cochlea were rigid, then the increase in fluid pressure at the oval window would displace the fluid up the scala vestibuli, through the helicotrema and down the scala tympani causing the round window to bulge out. This is actually a fairly accurate description of what happens except that the membranes inside the cochlea are not rigid. 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 100 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 9a is a representation of what you might expect by flicking a ribbon. Figure 9b 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.
What do you notice about the change in amplitude of the wave as it travels along the membrane?
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 10). 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 11a).
Look at Figure 11b.
What do you notice about the point of maximum displacement for each frequency?
For the lowest frequency (60 Hz) the maximum displacement is near the apical end, for the highest frequency (2000 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 300 Hz and 2000 Hz?
Each frequency would create a maximum displacement at a different point along the basilar membrane (as shown in Figure 11c).
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.
A little under 10 000 Hz (in fact 8350 Hz or 8.35 kHz).
This frequency is known as the characteristic, critical or central frequency (CF) of that part of the membrane because it is most sensitive to (or tuned to) frequencies in the region of 8 kHz.
For frequencies above and below 8.35 kHz the tone had to be more intense in order to vibrate the membrane to the same extent as that caused by the 8.35 kHz tone. This particular point on the membrane therefore acts as a filter in that it responds maximally to tones of 8.35 kHz, but shows very little response to tones that are higher or lower than this.
In the next section we shall see how the band-pass filtering characteristics of the basilar membrane are preserved in the discharge pattern of nerve fibres that leave the cochlea.
The motion of the basilar membrane also provides information about the temporal pattern of acoustic stimulation: it takes longer for a low-frequency stimulus to reach its point of maximum displacement on the membrane than it does a high-frequency stimulus.
Why is this?
Because high-frequency stimuli cause maximal displacement of the membrane near the base of the cochlea (near the stapes), whereas low frequencies cause maximal displacement at the apical end. If the sound always travels from base to apex, it takes longer for the wave to travel to reach the apex.
Finally, the mechanics of the basilar membrane provide information regarding the level of acoustic stimulation. The greater the stimulus level, the greater the amount of basilar membrane displacement. Therefore, more intense signals cause greater membrane displacement at a particular point than less intense stimuli.
You should now read The mechanics of hearing by Jonathan Ashmore, attached below. There may be some terms and concepts that will not be familiar to you. Do not worry too much at this stage. There is some overlap in the material covered in this course and some of the concepts mentioned in the reading will be more comprehensively covered in later sections of the course.
Click View Document to open The mechanics of hearing by Jonathan Ashmore