Journal Club: The role of the tactorial membrane in the sound transduction: mathematical modelling study

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Today's journal article

Deloche F, Thienpont M, Moleti A, Sisto R, Verhulst S. Active control of transverse viscoelastic damping in the tectorial membrane: A second mechanism for traveling-wave amplification? 

Why I picked this article

This study proposes that interactions between the sensory cells in the cochlea and a structure called the tectorial membrane provide damping to sound waves travelling in the cochlea. 

Our peripheral organ for hearing, the cochlea, is an amazing organ which has high sensitivity to be able to detect sounds. To achieve this, the cochlea is thought to be a complex mechanical amplifier of sounds. The "cochlear amplifier" is thought to be the specialised auditory sensory cells and the "basilar membrane" inside the cochlea that moves when sound waves arrive at the cochlea. 

A new study takes a fresh look at how one particular structure, the "tectorial membrane", might help control how energy from sound waves may disperse within the cochlea. Based on recent research using advanced optical imaging and computer simulations, the researchers of this publication built a hypothesis and a simulation model to test this concept. Overall, this publication offers a new perspective on how we achieve our remarkable hearing sensitivity in the cochlea and the importance of considering the tectorial membrane. 

Some of the research findings

There is a series of assumptions made about the computer simulation. The reasoning is explained in the main article, but these are assumptions at this stage and may not be true. Many of the assumptions are provided in the form of mathematical equations, too. Please see the full article for details! 

Assumptions:

  • Tactorial membrane is a nearly incompressible material (Poisson ratio ~0.5)
  • Tectorial membrane has a rectangular shape, parallel to the basilar membrane.
  • Simplified so that structures (reticular lamina, tactorial membrane and basilar membrane) move in phase. 
  • Passive case - this is a scenario where the "cochlear amplifier" is absent. Tactorial membrane is subjected to a normal viscous force on the membrane.
  • Active case - this is when the cochlear amplifier (by outer hair cells) is active. Movement of the reticular lamina and hair cells cancel in a way that the viscous force on the tactorial membrane is attenuated. 
  • Tactorial membrane impedance is dominated by the weight and viscosity.
  • Radial and transverse deformations of the tactorial membrane have equal magnitude

Observations from other literatures considered in this study for the simulation: 

  • Displacement of the tactorial membrane outward is enhanced at low sound levels. 
  • At higher sound levels, the outwards movement of the tactorial membrane and up and down motion of the tactorial movements are at a similar degree (passive case). In an active case with low sound levels, the movement of the tactorial membrane has a hot spot. 

Part of Figure 1. Mathematical model of the tactorial membrane and other structures. Deloche et al. (2025). 
Simulations: 

  • A list of parameters (where possible) was derived from the mouse cochlea. 
  • This includes anatomical constraints like cochlear height, length, surface ratio etc. 
  • The Greenwood function was applied to define frequency mapping within the cochlea. 

Findings:

  • Simulation of travelling wave (sound travelling on the basilar membrane) in response to 10 kHz sounds at different sound levels (= loudness) from 10 - 80 dB SPL equivalent. 
  • The simulation shows that when you vary the damping factor coming from the movement of the tactorial membrane, the viscous load changes in a way that it peaks at a certain location within the basilar membrane. 
  • Different viscous load at different positions leads to sharpening of the movement of the basilar membrane at a hot-spot location. 
  • The damping was dependent on the sound level or the loudness. 
  • Taken together, the simulations suggest that modulating the viscous load affects the travelling wave in the peak region. This could change the movement of the basilar membrane (in terms of the velocity and magnitude) to fine-tune the frequency-specific response.

Haruna's take away

Another tough (for me) simulation article! But again, a number of equations and assumptions clearly described are very informative and I found it very good. It is also great to see how input data from optical imaging and anatomical characterisation, like cochlear size and other structural information, all feed into mathematical modelling. As we obtain a lot of anatomical data, I do think we need to start working more with mathematicians or engineers to make more use of our data. 

I also like about this publication that the focus is on what was less investigated structure - the tactorial membrane. Other than the tectorial membrane, there are still many anatomical features in the cochlea that we have not paid attention to, and maybe that's where some secrets are. 

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This is Haruna's 33/100 of the 100-day challenge to post a science blog article every day! I love inner ear biology & cochlear physiology.