Journal Club: Wentzel-Kramers-Brillouin framework to model how travelling wave is influenced by contributions from the reticular lamina

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Today's journal article

Sisto R, Moleti A. Contribution of the reticular lamina motion to the traveling wave: a WKB approach. 

Why I picked this article

Sound is vibration in the air. When sound arrives at the eardrum, it vibrates the eardrum. The vibration of the eardrum in turn transfers the vibration to three middle-ear bones and finally to the fluid inside the inner ear organ for hearing, the cochlea. Oscillation of the fluid compartment results in a phenomenon called the "travelling wave", the vibration of the basilar membrane critical for the detection of sounds. 

To understand how hearing detection works in the inner ear, imaging of basilar membrane motion by Optical coherence tomography (OCT) and modelling of the mechanical properties of the cochlear nerve have been used in recent research. While the historic view of the travelling wave focused on the vibration property of the basilar membrane, recent research really points out towards the importance of how the sensory structure on the basilar membrane, the Organ of Corti, interacts with the travelling wave. For example, the upper surface of the organ of Corti is an area called the reticular lamina; it has been suggested that at some sound levels, the reticular lamina can move more than the basilar membrane. 

This research used a biophysical model to investigate how the reticular lamina motion changes the travelling wave.

Some of the research findings

They build in fluid focusing, viscous damping, and a specific outer hair cell (OHC) force model, then track what happens to the local wave properties.
The clinical link is indirect but real: if RL–BM phase and amplitude shape cochlear gain near threshold, this has implications for interpreting otoacoustic and cochlear microphonic measures, and for how drugs or pathology that alter OHC mechanics might shift hearing in noise.

Model:
  • Local wavenumber computed in a Wentzel–Kramers–Brillouin (WKB) framework.
  • Transmission-line cochlear model with two mechanical degrees of freedom per place: the basilar membrane and the reticular lamina.
  • The basilar membrane and the reticular lamina are coupled by the outer hair cell force.
  • The outer hair cell force is modelled as a non-linear instantaneous force proportional to OHC elongation, with low-pass behaviour (which is a new addition in this model). 
  • 2D pressure focusing effect was added. 
  • Hydrodynamics include the damping effect from viscosity, etc. 
  • Cochlear model parameters are summarised in Table 1. 
Finding: 
  • Adding the reticular lamina motion decreases the amplitude of the basilar membrane and the reticular lamina response. (damping effect) 
  • The velocity gain towards the apex (distal side of the cochlea) becomes sharper and nonlinear (enhanced?) when the reticular lamina is included with a phase shift. This was the case for both the basilar membrane and the reticular lamina displacement. 
  • Taken together, the model suggests that the movement of the reticular lamina can have a "damping effect" or an anti-damping effect on the travelling wave, depending on the phase difference between the motion of the basilar membrane and the reticular lamina. 
  • The low-path filter element of the outer hair cells included in this model introduced up to 90-degree phase rotation. 
Figure 3B. The modelling of the reticular membrane gain across cochlear distance under different conditions (reticular lamina contributions). Sisto et al. 2025. 

Haruna's takeaway

I hope I understood the gist of the research right!! While I am fascinated by physics and modelling, my ability to understand this kind of research in this area is still very limited.... and hence the reason for me to read more of them in this journal club challenge, as the opportunity to learn. 

It is fascinating that the travelling wave is still an evolving scientific and physics concept. Compared with the textbook concept that the basilar membrane vibrates differently between the base and apex of the cochlea because of physical properties, and that, added with active processes from the outer hair cells, amplifies and makes the cochlea more sensitive..... the real travelling wave is still mysterious in its behaviour and seems much more complex. Our textbook explanation feels almost too simple and misleading, perhaps, compared with what some of the recent in vivo experimental data and models are suggesting. I would love to watch this space and have my fingers crossed that someone will come along to set up a mathematical simulation model available for teaching and learning of travelling waves. 

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This is Haruna's 53/100 of the 100-day challenge to post a science blog article every day! I love inner ear biology & cochlear physiology.