Second Cause of Hidden Hearing Loss Identified

Gabriel Corfas and his research team at University of Michigan have discovered that disruption in the Schwann cells that make myelin, which insulates the neuronal axons in the ear, leads to hidden hearing loss.

They contrast this against loss of synapses which they have previously found to cause hidden hearing loss. Hence the title "second cause of hidden hearing loss identified".

News article:
https://www.eurekalert.org/pub_releases/2017-02/mmu-sco021717.php

Here is the research paper:
http://www.nature.com/articles/ncomms14487

 

The article on Eurekalert was not clear about what that previous research was that identified the first cause of hidden hearing loss (HHL). I thought they were referring to a previous work by Corfas at Michigan university. I now realize that they were referring to the work by Liberman at Massachusetts Eye and Ear. I thought it seemed familiar...

You can see the location of IHC synapses in the image above.

In other words the Schwann cell loss and axonal demyelination is not permanent (hence "transient"). But the natural restoration that follows is not perfect. Myelin is a protective sheath that surrounds nerve fibers.

In other words there is no nerve fibre loss. There is a disruption in the Heminode which sits at the Habenula perforata. Heminode is a node of Ranvier situated at the junction of myelinated (protected) axons and bare (unprotected) axons. In the cochlea this happens at a place called Habenula perforata.

In other words myelin defects are bad, there are two types or causes for HHL, and they can add up.

To help you navigate around and better understand this I have include two images below.

Location of Habenula perforata:



Schematic view of cochlear nerve fibres:



Explanation:

Schematic illustration of the peripheral glial cells in the adult human cochlea. Satellite glial cells (green) envelop all spiral ganglion neuron (SGN) cell bodies. Non-myelinating Schwann cells (light blue) ensheath both the central and peripheral processes of the type II SGNs (yellow) that innervate the outer hair cells (OHC). Myelinating Schwann cells (dark blue) ensheath and myelinate both processes of the type I SGNs (red) that innervate the inner hair cells (IHC). Beyond the habenula perforata, in the organ of Corti, neither Schwann cell types ensheath the most distal part of the peripheral processes of type I and type II SGNs.