<p>The wide, slack composition of the basilar membrane (BM) in the apex and the narrow, stiff characteristics at the base are among the factors that underlie the tuning of the BM. Slack, wide structures vibrate at low harmonic frequencies, stiff and narrow structures at high frequencies, just like the strings of a guitar. The apex of the cochlea transduces low frequencies, the base high. <a href="http://www.cochlea.eu/en/cochlea/function" rel="nofollow noreferrer">Cochlea.eu</a> has an accessible explanation this. What also adds to this is that the hair cells in the apex are longer, and in the base they are shorter. Basically, the BM allows a standing wave to form, where each place on the BM resonates at a typical harmonic frequency that increases from apex to base. The harmonic frequency is governed by the physical properties and is sharpened by the outer hair cells <a href="https://www.ncbi.nlm.nih.gov/books/NBK10946/" rel="nofollow noreferrer">(Purves <em>et al</em>., 2001)</a>.</p>
<p><a href="https://i.sstatic.net/WhdWa.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/WhdWa.png" alt="cochlea" /></a><br />
<sup>Fig. 1. Cochlear structures with a focus on the frequency tuning of the basilar membrane. source: <a href="https://www.ncbi.nlm.nih.gov/books/NBK10946/" rel="nofollow noreferrer">Purves <em>et al</em>. (2001)</a></sup></p>
<p><sub><strong>Reference</strong><br />
<strong>-</strong> <a href="https://www.ncbi.nlm.nih.gov/books/NBK10946/" rel="nofollow noreferrer">Purves <em>et al</em>., eds. <em>Neuroscience</em>, 2<sup>nd</sup> ed. Sunderland (MA): Sinauer Associates; 2001</a></sub></p>

CC BY-SA 4.0
2021-10-13T06:30:44.347Z
thank you!!!!!!
Math_Man1
CC BY-SA 4.0
2021-10-10T18:51:36.257Z
2021-10-11T07:49:48.470Z
2021-10-11T07:49:48.470Z
AliceD
<p>The basilar membrane becomes thicker and heavier from the basal end to the apical upper end - this is why high frequencies are perceived in the lower range and low frequencies in the upper range.</p>
<p>But why? Why does the traveling wave of the membrane reach its maximum amplitude at high frequencies at the bottom and at low frequencies at the top? How is this related to the width, stiffness and mass of the basilar membrane?</p>
<p>This question is related to:
<a href="https://psychology.stackexchange.com/questions/27699/sound-decomposition-of-the-brain">Sound decomposition of the brain</a></p>

<neuroscience><neurophysiology><neuroanatomy><hearing>
Deflection of the basilar membrane
Math_Man1
# Deflection of the basilar membrane

(Asked by: username_0 on 2021-10-10)

The basilar membrane becomes thicker and heavier from the basal end to the apical upper end - this is why high frequencies are perceived in the lower range and low frequencies in the upper range.

But why? Why does the traveling wave of the membrane reach its maximum amplitude at high frequencies at the bottom and at low frequencies at the top? How is this related to the width, stiffness and mass of the basilar membrane?

This question is related to:
[Sound decomposition of the brain](https://psychology.stackexchange.com/questions/27699/sound-decomposition-of-the-brain)

**Tags:** neuroscience, neurophysiology, neuroanatomy, hearing


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## 1 answers

### Answer by username_1 on 2021-10-10. Score: 2 | Accepted answer

The wide, slack composition of the basilar membrane (BM) in the apex and the narrow, stiff characteristics at the base are among the factors that underlie the tuning of the BM. Slack, wide structures vibrate at low harmonic frequencies, stiff and narrow structures at high frequencies, just like the strings of a guitar. The apex of the cochlea transduces low frequencies, the base high. [Cochlea.eu](http://www.cochlea.eu/en/cochlea/function) has an accessible explanation this. What also adds to this is that the hair cells in the apex are longer, and in the base they are shorter. Basically, the BM allows a standing wave to form, where each place on the BM resonates at a typical harmonic frequency that increases from apex to base. The harmonic frequency is governed by the physical properties and is sharpened by the outer hair cells [(Purves *et al*., 2001)](https://www.ncbi.nlm.nih.gov/books/NBK10946/).

[![cochlea](https://i.sstatic.net/WhdWa.png)](https://i.sstatic.net/WhdWa.png)  
Fig. 1. Cochlear structures with a focus on the frequency tuning of the basilar membrane. source: [Purves *et al*. (2001)](https://www.ncbi.nlm.nih.gov/books/NBK10946/)

**Reference**  
**-** [Purves *et al*., eds. *Neuroscience*, 2nd ed. Sunderland (MA): Sinauer Associates; 2001](https://www.ncbi.nlm.nih.gov/books/NBK10946/)

> **Comments:**
> **username_0**: thank you!!!!!!  

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