How Hair Cells in the Ear Actively Respond to Sound
Tiny hair cells located in the inner ear help us hear and maintain balance. On top of each hair cell is a hair bundle, a sensory organelle that converts mechanical input from sound or movement into electrical output, which is then passed on to the brain. Previous research has shown that hair bundles aren’t simply passive entities. They actively oscillate to amplify weak audio signals or to tune into specific frequencies. Biologists have also observed bundles oscillating in the absence of stimuli. Models have tried to capture this bundle behavior, but the connection between active oscillation and the audio response has not been made clear. A new thermodynamic model of energy flow within hair bundles suggests that they work like tiny machines [1]. Depending on the stimulus, the bundles either extract power from incoming sound waves or inject power into them—corresponding, respectively, to sensing or amplifying a stimulus.
In the inner ear, an active process called cochlear amplification helps humans (and other mammals) hear the faintest of sounds. When a faint whisper enters the ear, for example, the outer rows of hair cells respond to the weak signal by moving in a way that amplifies the sound waves for the inner hair cells, which are the ones that send a message to the brain. Molecular motors propel the movement or twisting of hair bundles required for these functions.
Previous work has explored how much energy a hair cell consumes to drive bundle oscillations, but the resulting models have typically assumed that bundles are moving spontaneously—that is, in the absence of external stimuli. Roman Belousov from the European Molecular Biology Laboratory in Germany and his colleagues have developed a stochastic thermodynamic model that includes an energy input from sound waves. “Instead of just looking at how a hair bundle moves on its own, we wanted to add what happens when it interacts with sound,” Belousov says.
To make their model realistic, the researchers first performed experimental observations of a bullfrog’s sacculus—an inner-ear organ like the cochlea but one that is simpler to study. They placed the dissected sacculus between two chambers, each filled with a distinct inner-ear fluid, mimicking the physiological environment of the bullfrog’s ear. They captured spontaneous oscillations of the hair bundles with a microscope and used the data to determine relevant hair-bundle parameters, such as stiffness and friction coefficients, which were then incorporated into the team’s model.
The model featured three energy channels: an external environment acting like a heat reservoir, an external signal representing the sound stimulus, and an internal energy source driving active processes. The researchers found that the simulated hair bundle operated in one of four different thermodynamic regimes, depending on the amplitude and the frequency of the signal.
For two of the thermodynamic regimes, the hair bundles acted as work-to-work machines, converting mechanical work from one source into another, with minimal heat loss. In the first regime, mechanical energy from the signal flowed through the hair bundle into the cell. Conversely, in the second regime, energy flowed outward from the hair cell into the signal channel. Although the two work-to-work regimes are simplified, the team believes that they correspond to the hair cell’s main functions of sensing and amplification. The switching between regimes depends on the strength of the incoming signal, with the active cell motion (amplification) only turning on when the signal is weak, Belousov says.
The other two regimes, likely not biologically relevant, were thermodynamic peculiarities. In one, the moving hair bundle actively dissipated heat. Surprisingly, in the remaining regime, the hair bundle could “work as a tiny refrigerator cooling down the surrounding environment around the cell,” Belousov says. This heat transfer illustrates how inspiration from biology continues to provide new insights into thermodynamics. “Because these organelles are nanoscale, it gives us possible principles on how thermodynamic machines can work at such small scales,” Belousov says.
Dáibhid Ó Maoiléidigh, a Stanford University biophysicist, is impressed by the study and its defining of amplification and detection in terms of energy flows. “That’s a useful way to look at the system,” he says. He wonders if the amplification and detection regimes in the model might explain the selective functioning of hair bundles in the mammalian cochlea. “Maybe the difference between the bundles of inner hair cells and outer hair cells is that they just operate in different regimes,” Ó Maoiléidigh says. He adds that the study has implications for medicine, since loss of hair cells is the leading cause of permanent hearing damage. “Understanding how hair cells work is very important for treating hearing loss.”
–Sachin Rawat
Sachin Rawat is a freelance science writer based in Bangalore, India.
References
- Y. Thipmaungprom et al., “Thermodynamic signatures of sensing and amplification by periodically driven hair-cell bundles,” PRX Life 4, 013039 (2026).