How Hearing Happens
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1In addition to possessing the classical senses of vision, hearing, touch, smell, and taste, humans respond to a variety of other stimuli.
2For example, the vestibular apparatus of the internal ear provides continuous feedback about linear and angular accelerations and allows us to maintain an upright posture. Pressure receptors in our vascular systems and osmoreceptors in our brains help maintain a constant blood volume. Other species employ still more exotic sensory modalities. Many migratory species, especially birds, can orient themselves by reference to the earth’s magnetic field. Pit vipers and boas use thermal imaging to strike warm mammalian prey; bats and cetaceans employ sonar to locate respectively insects and fishes. Despite the wide variety of physical stimuli to which various senses are responsive, there are consistent principles in their operation. A sensory response commences with an antenna, an apparatus for accumulating sensory energy and concentrating it at the receptor cells. In the key step of sensory transduction, the physical stimulus engenders an electrical response that represents the magnitude, duration, and other properties of the input. Most responses are then amplified to assure that their sizes exceed those of any noise sources. Many sensory systems use tuning to enhance responsiveness to stimuli at behaviourally important frequencies and to suppress those at less significant frequencies. In some instances, a sensory receptor bears an axon along which information flows into the central nervous system; in other cases the receptor must forward information across a chemical synapse to excite the nerve fiber. In either event the final step in a peripheral sensory response is the encoding of relevant information in the firing pattern of an afferent nerve fiber.
3How the ear’s works work:
The operation of cochlear hair cells
4Human hearing is extraordinary in its technical specifications. We can perceive frequencies as great as 20 kHz and discriminate between different tones with a precision of 0.2%. At the acoustical threshold, the inner ear responds to vibrations of only ± 0.3 nm, an atomic dimension. Finally, our auditory system can register sound-pressure levels from 0 dB to 120 dB, representing a millionfold range in amplitude and a trillionfold range in power. The defining feature of a hair cell is its mechanoreceptive organelle, the hair bundle. Extending less than 1 μm to more than 100 μm from the flattened apical surface, the bundle comprises from a dozen to over 300 cylindrical protrusions called stereocilia. Each stereocilium consists of a core of parallel actin filaments that are cross-linked into a rigid fascicle. The streocilia are not of equal size, but display a monotonic decrease in length from one edge to another so that the bundle’s top edge is beveled like a hypodermic needle. When the top of a hair bundle is displaced during stimulation, the adjacent stereocilia shear with respect to one another. This movement is communicated to a tip link, a fine filament comprising four cadherin molecules that extends from the tip of each short stereocilium to the side of the longest adjacent one. Each tip link probably contacts a pair of mechanically sensitive ion channels whose opening initiates the hair cell’s electrical response.
5Uniquely among sensory receptors the hair cell is not a passive recipient of stimuli, but instead uses an active process to enhance its inputs. The active process amplifies mechanical stimuli by as much as a thousandfold, thus greatly increasing our sensitivity to weak sounds. When this process fails, we become hard of hearing. Amplification is accompanied by frequency tuning, which restricts each hair cell’s response to a narrow frequency band. If the active process deteriorates, we grow less sensitive to subtle differences in frequency and therefore suffer a diminished ability to discriminate sound sources. Finally, the active process produces a compressive nonlinearity that renders the ear sensitive to sounds over an astonishing trillionfold range in power. By enhancing weak stimuli and suppressing strong ones, this feature allows us to enjoy an instrumental soloist as comfortably as a full orchestra one hundred times as loud.
6Our ability to identify different sound sources – to distinguish predators from prey, for example – rests upon the ear’s ability to decompose complex sounds into their frequency components. Although the cochlear traveling wave initiates this process, individual hair cells are also tuned to specific frequencies both through hair-bundle mechanics and sometimes by electrical resonance. Not only the active process of hair cells, but even their synaptic transmission has been found to be frequency-selective. Hearing deficiency is widespread in industrialized countries, in which about 10% of the population is affected. In all cases, the dominant problem is the loss of hair cells. An important focus of contemporary research is therefore the potential restoration of human hearing through the reprogramming of progenitor cells in the ear.
• The Faculty invited Mr Albert James Hudspeth upon Prof. Christine Petit’s proposal.
• The videos of the lectures are available on www.college-de-france.fr, on Prof. Christine Petit’s page.
Pour citer cet article
Albert James Hudspeth, « How Hearing Happens », La lettre du Collège de France [En ligne], 8 | mars 2014, mis en ligne le 12 août 2015, consulté le 23 avril 2017. URL : http://lettre-cdf.revues.org/2036Haut de page
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