Cochlea and Hearing Loss

The cochlea of the inner ear has the sensory epithelium, an essential tissue made up of three layers, i.e., hair-cell layer, supporting-cell layer, and extracellular matrix layer called the ‘basilar membrane’ (Fig. 1). Acoustic inputs firstly stimulate the basilar membrane and vibrates the sensory epithelium.  This event deflects the stereocilia of the hair cells and further activates ‘mechanoelectrical transduction channels’ at their tips. The channel’s opening enters K⁺ in the endolymph into the hair cells and electrically excites them. In this process, mechanical energy of sounds is transduced to electrical signals. For the details of the process, see the section ‘How Does the Cochlea of the Inner Ear Work?’

Our hearing can enhance small sounds and perceive them with higher sensitivity, whereas it detects loud sounds with lower sensitivity. This property emerges primarily from a unique pattern of the motion in the sensory epithelium. Figure 2 plots the amplitude of epithelium’s vibration as a function of intensity of the acoustic stimuli. The relationship described in this panel clearly represents the aforementioned property. This is referred to as ‘nonlinear response’ or ‘nonlinear amplification’, which disappears when the animal is dead. Note that the amplitude of the vibration in the epithelium is very subtle and ranges from 0.1 nm at minimum to 10 nm at maximum (Fig. 2); the difference is 100-fold. Your hearing can sense a buzzing of a mosquito that flies 3 m away from you as well as accept a jet engine sound emitted form an aircraft nearby. This difference of 1,000,000-fold in sound pressure levels is highly compressed to 100-fold in the motion in the sensory epithelium. The large dynamic range results from the nonlinear response mentioned above and this profile is crucially involved in establishment of the sharp tuning of our hearing.