<p><strong>Short answer</strong><br>
Hair cells in the cochlea can code sound intensity via the amount of neurotransmitter they release. Higher sound levels result in more neurotransmitter release and in turn to higher firing rates in the spiral ganglion cells of the auditory nerve. </p>

<p><strong>Background</strong><br>
Sound waves are picked up by the <strong><a href="http://www.nature.com/ncomms/journal/v2/n10/full/ncomms1533.html" rel="nofollow noreferrer">mechanoreceptors</a></strong> in the inner ear: the <a href="http://www.ncbi.nlm.nih.gov/books/NBK10867/" rel="nofollow noreferrer"><strong>hair cells</strong></a>. Hair cells release the excitatory neurotransmitter <a href="http://neurotransporter.org/glutamate.html" rel="nofollow noreferrer">glutamate</a>. Dependent on the direction of the deflection of the hairs (<a href="http://www.ncbi.nlm.nih.gov/books/NBK10867/" rel="nofollow noreferrer">stereocilia</a>) on the hair cells, the hair cell releases more neurotransmitter (activation) or less (inhibition) than at its resting state (Fig. 1). The released glutamate activates the primary sensory neurons that connect to the hair cells, namely the <strong><a href="http://www.cochlea.eu/en/cochlea/organ-of-corti/innervation" rel="nofollow noreferrer">spiral ganglion cells</a></strong> (SGCs). The axons of the SGCs combine to form the auditory nerve <a href="http://onlinelibrary.wiley.com/doi/10.1113/jphysiol.2004.082214/abstract" rel="nofollow noreferrer">(Fuchs, 2005)</a>.  </p>

<p><a href="https://i.sstatic.net/Z88n7.gif" rel="nofollow noreferrer"><img src="https://i.sstatic.net/Z88n7.gif" alt="HC"></a><br>
<sup>Fig. 1. Hair cells are activated when their stereocilia are bent in one direction (depolarization) or inhibited when the cilia bend to the other direction (hyperpolarization). source: <a href="http://www.susqneuro.com/publications/dizzy/" rel="nofollow noreferrer">Yanowski</a></sup></p>

<p>The hair cell cilia contain mechanoreceptor proteins, that open or close, dependent on the direction in which the cilia bend (Fig. 2).</p>

<p><a href="https://i.sstatic.net/Y7Hdi.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/Y7Hdi.jpg" alt="mechanoRs"></a><br>
<sup>Fig. 2. Mechanoreceptors in the cochlear hair cell. source: <a href="http://www.nature.com/nrn/journal/v15/n9/full/nrn3786.html?WT.feed_name=subjects_hair-cell" rel="nofollow noreferrer">Hudspeth (2014)</a></sup> </p>

<p>Because sound waves have sinusoidal shapes, hair cells undergo cycles of activation and inhibition, closely following the input waves. Louder stimuli cause the cilia to bend further. This causes larger responses in the hair cell and larger amounts of neurotransmitter to be released in the depolarizing phase of the auditory stimulus. This in turn leads to <strong>higher spike rates</strong> in the spiral ganglion cell axons <a href="http://onlinelibrary.wiley.com/doi/10.1113/jphysiol.2004.082214/abstract" rel="nofollow noreferrer">(Fuchs, 2005)</a>. Spike rate in the spiral ganglion cells are thought to primarily encode sound intensity <a href="http://www.jneurosci.org/content/31/43/15424.full" rel="nofollow noreferrer">(Heil <em>et al.</em>, 2011)</a></p>

<p>A secondary mechanism of loudness coding <em>may</em> be related to the fact that increased sound levels activate a larger area in the cochlea, due to the low-frequency tail (<a href="http://research.meei.harvard.edu/eplhistory/Kiang_papers_CD_contents/1974_Kiang_Moxon.pdf" rel="nofollow noreferrer">Kiang &amp; Moxon, 1974</a>). Hence, more neurons start firing at higher sound levels when the tone frequency is the same. However, although it is a plausible mechanism, it is a secondary intensity code, if any.   </p>

<p><sub><strong>References</strong><br>
<strong>-</strong> <a href="http://onlinelibrary.wiley.com/doi/10.1113/jphysiol.2004.082214/abstract" rel="nofollow noreferrer">Fuchs, <em>J Physiol</em> (2005); <strong>566</strong>(1): 7–12</a><br>
<strong>-</strong> <a href="http://www.jneurosci.org/content/31/43/15424.full" rel="nofollow noreferrer">Heil <em>et al.</em>, <em>J Neurosci</em> 2011; <strong>31</strong>(43): 15424-37</a><br>
<strong>-</strong> <a href="http://www.nature.com/nrn/journal/v15/n9/full/nrn3786.html?WT.feed_name=subjects_hair-cell" rel="nofollow noreferrer">Hudspeth, <em>Nature Rev Neurosci</em> (2014); <strong>15</strong>: 600–14</a><br>
<strong>-</strong> <a href="http://research.meei.harvard.edu/eplhistory/Kiang_papers_CD_contents/1974_Kiang_Moxon.pdf" rel="nofollow noreferrer">Kiang &amp; Moxon, <em>JASA</em> 1954; <strong>55</strong>(3): 620-30</a></sub></p>

CC BY-SA 3.0
2016-06-20T11:20:20.237Z
2016-06-30T18:33:45.543Z
2016-06-30T18:33:45.543Z
AliceD
<p>Different areas of the inner ear (the cochlea) are sensitive to different acoustic frequencies. Hence, the cochlea basically performs a fast Fourier transform on the audio signal. This spectral information is subsequently sent to the auditory cortex. But <strong>how does the cochlea encodes the intensity of an acoustic stimulus?</strong></p>

CC BY-SA 3.0
2016-07-02T04:09:52.000Z
Possible duplicate of [How is tone volume encoded?](http://cogsci.stackexchange.com/questions/8627/how-is-tone-volume-encoded)
StrongBad
CC BY-SA 3.0
2016-07-02T18:52:46.830Z
@StrongBad - I answered both of these questions and I consider them diffferent enough to be existing side-by-side
AliceD
<neuroscience><perception><sensation><neurophysiology><hearing>
How does the inner ear encode sound intensity?
akm
# How does the inner ear encode sound intensity?

(Asked by: username_0 on 2016-06-20)

Different areas of the inner ear (the cochlea) are sensitive to different acoustic frequencies. Hence, the cochlea basically performs a fast Fourier transform on the audio signal. This spectral information is subsequently sent to the auditory cortex. But **how does the cochlea encodes the intensity of an acoustic stimulus?**

**Tags:** neuroscience, perception, sensation, neurophysiology, hearing

> **Comments:**
> **username_1**: Possible duplicate of [How is tone volume encoded?](http://cogsci.stackexchange.com/questions/8627/how-is-tone-volume-encoded)  
> **username_2**: @username_1 - I answered both of these questions and I consider them diffferent enough to be existing side-by-side  

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## 1 answers

### Answer by username_2 on 2016-06-20. Score: 7

**Short answer**  
Hair cells in the cochlea can code sound intensity via the amount of neurotransmitter they release. Higher sound levels result in more neurotransmitter release and in turn to higher firing rates in the spiral ganglion cells of the auditory nerve.

**Background**  
Sound waves are picked up by the **[mechanoreceptors](http://www.nature.com/ncomms/journal/v2/n10/full/ncomms1533.html)** in the inner ear: the [**hair cells**](http://www.ncbi.nlm.nih.gov/books/NBK10867/). Hair cells release the excitatory neurotransmitter [glutamate](http://neurotransporter.org/glutamate.html). Dependent on the direction of the deflection of the hairs ([stereocilia](http://www.ncbi.nlm.nih.gov/books/NBK10867/)) on the hair cells, the hair cell releases more neurotransmitter (activation) or less (inhibition) than at its resting state (Fig. 1). The released glutamate activates the primary sensory neurons that connect to the hair cells, namely the **[spiral ganglion cells](http://www.cochlea.eu/en/cochlea/organ-of-corti/innervation)** (SGCs). The axons of the SGCs combine to form the auditory nerve [(Fuchs, 2005)](http://onlinelibrary.wiley.com/doi/10.1113/jphysiol.2004.082214/abstract).

[![HC](https://i.sstatic.net/Z88n7.gif)](https://i.sstatic.net/Z88n7.gif)  
Fig. 1. Hair cells are activated when their stereocilia are bent in one direction (depolarization) or inhibited when the cilia bend to the other direction (hyperpolarization). source: [Yanowski](http://www.susqneuro.com/publications/dizzy/)

The hair cell cilia contain mechanoreceptor proteins, that open or close, dependent on the direction in which the cilia bend (Fig. 2).

[![mechanoRs](https://i.sstatic.net/Y7Hdi.jpg)](https://i.sstatic.net/Y7Hdi.jpg)  
Fig. 2. Mechanoreceptors in the cochlear hair cell. source: [Hudspeth (2014)](http://www.nature.com/nrn/journal/v15/n9/full/nrn3786.html?WT.feed_name=subjects_hair-cell)

Because sound waves have sinusoidal shapes, hair cells undergo cycles of activation and inhibition, closely following the input waves. Louder stimuli cause the cilia to bend further. This causes larger responses in the hair cell and larger amounts of neurotransmitter to be released in the depolarizing phase of the auditory stimulus. This in turn leads to **higher spike rates** in the spiral ganglion cell axons [(Fuchs, 2005)](http://onlinelibrary.wiley.com/doi/10.1113/jphysiol.2004.082214/abstract). Spike rate in the spiral ganglion cells are thought to primarily encode sound intensity [(Heil *et al.*, 2011)](http://www.jneurosci.org/content/31/43/15424.full)

A secondary mechanism of loudness coding *may* be related to the fact that increased sound levels activate a larger area in the cochlea, due to the low-frequency tail ([Kiang & Moxon, 1974](http://research.meei.harvard.edu/eplhistory/Kiang_papers_CD_contents/1974_Kiang_Moxon.pdf)). Hence, more neurons start firing at higher sound levels when the tone frequency is the same. However, although it is a plausible mechanism, it is a secondary intensity code, if any.

**References**  
**-** [Fuchs, *J Physiol* (2005); **566**(1): 7–12](http://onlinelibrary.wiley.com/doi/10.1113/jphysiol.2004.082214/abstract)  
**-** [Heil *et al.*, *J Neurosci* 2011; **31**(43): 15424-37](http://www.jneurosci.org/content/31/43/15424.full)  
**-** [Hudspeth, *Nature Rev Neurosci* (2014); **15**: 600–14](http://www.nature.com/nrn/journal/v15/n9/full/nrn3786.html?WT.feed_name=subjects_hair-cell)  
**-** [Kiang & Moxon, *JASA* 1954; **55**(3): 620-30](http://research.meei.harvard.edu/eplhistory/Kiang_papers_CD_contents/1974_Kiang_Moxon.pdf)


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