Amazing stuff! There is a lot going on inside of ears!
"... 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. 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.
... [This work] have developed a stochastic thermodynamic model that includes an energy input from sound waves. ...
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,” ..."
From the abstract:
"Hair cells actively drive oscillations of their mechanosensitive organelles—the hair bundles that enable hearing and balance sensing in vertebrates. Why and how some hair cells expend energy by sustaining this oscillatory motion in order to fulfill their function as sensors and amplifiers remains unknown.
Here, we develop a stochastic thermodynamic theory to describe flows of mechanical energy in a periodically driven hair bundle. Our analysis of thermodynamic fluxes associated with hair-bundle motion and external sinusoidal stimulus reveals that these organelles function as thermodynamic work-to-work machines under different operational modes.
One mode allows the cell to harvest energy of the external signal, whereas another channels the power supplied by the cell into the signal. These two regimes might represent thermodynamic signatures of signal sensing and amplification, respectively, which we further connect to the receptor currents through ion channels controlled by the hair bundles.
In addition to energy harvesting and work transduction, our model also substantiates the capability of hair cells to operate as heaters and, at the expense of external driving, as active feedback refrigerators. We quantify the performance and robustness of the mechanical work-to-work conversion by hair bundles, whose thermodynamic efficiency in some conditions exceeds 80% of the applied power."
Physics - How Hair Cells in the Ear Actively Respond to Sound "Our ability to hear relies on tiny “hair bundles” in the inner ear. A new thermodynamical model offers an explanation for the different ways that bundles oscillate."
Left: A hair cell captured with differential interference contrast microscopy. Right: Energy can flow in and out of the cell through three channels: active driving by molecular motors in the cell, heat from the environment, and work from the external sound signal.
Fig. 1 Left: Schematic of a hair cell with its hair bundle on top. Energy is exchanged between the thermal environment, the hair bundle characterized by the position of its tallest cilium—the kinocilium—and two agents
