Pathology of hair cells and neurones
Overview / Aminoglycosides / Noise Trauma / Excitotoxicity / Presbycusis / Tinnitus
Pictures : M. Lenoir, R. Pujol

Noise trauma
Exposure to loud noise (above 90 dB SPL) causes noise-induced hearing loss (NIHL) by damaging the organ of Corti. Damage and NIHL depend on both the level of noise and the duration of exposure. Hearing loss (i.e. threshold shift) may be temporary (TTS) if a repair mechanism is able to restore the organ of Corti. However, it becomes permanent (PTS) when hair cells or neurones die.
Structural correlates of noise trauma are are of two types:
- mild damage of synapses and/or hair cell stereocilia, which can be repaired, accounts for TTS and recovery;

- severe damage inducing hair cell and/or neuronal apoptosis (programmed cell death), which is impossible to repair, accounts for PTS.

Acutely, noise trauma causes two types of damage to hair cells and neurones
R. Pujol

R. Pujol
 The same noise exposure (6 kHz, 130 dB SPL, 15 min) causes acutely:
- a synaptic damage (left) with all afferent dendrites being disconnected from the IHC because of an excitotoxic injury;
- and a damage to first row OHCs (right).
Whereas the synaptic damage may recover, the damaged OHC, entering apoptosis, is destined to die.
scale bar: 10µm


In everyday life, exposure to a 130 dB / 15 min noise is extremely uncommon: one would have to stay close to plane engines without ear protection. However, the same cochlear damage can be obtained with longer exposure to lower levels of noise: e.g. a few hours exposure to levels of 100 dB, and this can be easily happen in a disco or by listening to a walkman!

Acute hearing loss after noise trauma   (ref. d25)
JL. Puel

 The same noise exposure (6 kHz, 130 dB, 15 min) causes:

---either (blue curve) a profound hearing loss (up to 80 dB) in the non protected ear,

---or (red curve) a much less severe hearing loss (40 dB) in the ear protected by kynurenate (a wide spectrum glutamate antagonist).

This indicates that about 50% of the hearing loss may be attributed to synaptic damage (excitotoxicity)

Damage to auditory neurones: two phases of excitotoxic lesions

Noise induces an excess release of glutamate. The resulting excitotoxic damage occurs in two phases:

an acute phase

a delayed phase

Acoustic trauma and hair cells
Depending on the level of exposure, damage to hair cells may: 1) only involve stereocilia, and some repair mechanism are possible, or 2) involve the entire hair cell, which undergoes apoptosis and dies.
M. Lenoir
1) Left: mild damage to stereocilia with broken tip links and broken roots.In such a case, electromechanical transduction is altered, or stopped, but slow repair may occur. Right: more severe and irreversible damage with fused and giant stereocilia.

C. Gervais d'Aldin

M. Lenoir

2) Severe and definitive damage, when the hair cell itself is altered and disappears.
On the left, TEM of a dying OHC. The flat preparation on the right shows a traumatised cochlea where 7 to 8 OHC have disappeared, together with a pillar cell (damaged area outlined in blue).
scale bar: 20 µm.
Note that beside hair cells, support cells are also often damaged, as in the above picture, right.

High intensity impulse noise
Machine guns, machines and fireworks may produce impulse noise above 130 or 140 dB SPL. At these levels, in addition to neurones and hair cell damage, an actual hole can be observed in the reticular lamina, allowing merging of endolymph and perilymph and leading to total deafness.


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