How the ear's works work
Citation: A. J. Hudspeth (1989/10/05) How the ear's works work. Nature (RSS)
DOI (original publisher): 10.1038/341397a0
Semantic Scholar (metadata): 10.1038/341397a0
Sci-Hub (fulltext): 10.1038/341397a0
Internet Archive Scholar (search for fulltext): How the ear's works work
Tagged: Biology (RSS) hearing (RSS), acoustics (RSS)
This paper reviews the known physical origins of hearing and equilibrium in vertebrates, focusing on the results of studies in the 1970s and 80s particularly on the role of hair bundles in converting sound into electrical potential in the nervous system. The contemporary understanding of structural details of the ear are summarized, including the structure of hair cells and mechanoreceptive hair bundles, transduction channels, adaptation to a range of frequencies, and the possibilities for direct mechanoelectrical transduction, driven directly by hair motion without secondary messengers.
Particular attention is paid to mechanisms for transduction and frequency tuning, areas of active research and study at the time. Both positive and negative discoveries are covered, noting areas where further research is needed. Some new micrographs and figures from the author's work are included to tie the review together. Over 100 related papers are cited and synthesized into the review, most by other authors.
Overview of contemporary results
- Hair cell structure and use
Hair cells are essential to the proper function of the ear, and similar structures are found in all vertebrates. Hair bundles in the ear can detect tiny motions and respond to vibrations over 100,000 times a second. It is noted that this mechanism is widely used across the animal kingdom and for a variety of sensory purposes even within the ear, with hair bundles in the cupula itself (sensitive to angular acceleration in three directions), in the organ of Corti (sensitive to sound), and in the otolithic organs (sensitive to linear horizontal and vertical acceleration).
The chain of events in hearing is summarized, from displacement of the eardrum to the bones of the middle ear, to piston-like stimulation and angular acceleration of the cochlea, deflecting the cupula and flexing the basilar membrane up and down, which in turn stimulates hair bundles in the organ of Corti. Similar effects driven by acceleration stimulate hair cells in the otolithic membrane.
Variations in the structure of hair bundles and individual hair cells: Each is used as a strain gauge, opening ion channels when stimulated. And they are very precisely located within the ear: the numer of stereocilia in a hair bundle, and their length and diameter and spacing, are completely determined by their place along the basilar membrane. Within a stereocilium, actin filaments are cross-linked by fimbrin, greatly increasing their stiffness.
Unresolved questions about hair growth and structure: how are the dimensions of hair bundles determined? Can they regenerate in humans, as they do in lower vertebrates?
- Mechanoelectrical transduction
The electrical signals that convert sound into neuronal impulses are driven by opening and closing transduction channels. These seem to be opened and closed directly by the mechanical motion of hair cells, though again the specific mechanism is a subject of open research. The mechanism acting through some sort of elastic gating springs. The specific transduction channels have not been isolated, but their responses have been characterized in many ways, in humans and other vertebrates.
A deflection of a hair bundle by only 0.003 degrees - 0.3nm - leads to electrical response. This seems to suggest a lot of noise from Brownian motion, which is expected (and observed via inteferometry) to have an rms amplitude of 2nm.
Most transduction current involves K+passing through transduction channels. Some chemicals known to be toxic to the ear can enter and obstruct the channels. There is O(1) channel per stereocilium, making them relatively difficult to see in dissection. One potential way of locating them is through the attraction of Ca2+ to them.
Individual bundles seem to contain elements that act as [gating] springs, whose tension controls the opening and closing of these channels. These springs seem to absorb 50% of the force applied to bundles. They have been difficult to locate. However there are small strands that connect every stereocilium to the side of the longest attached process, called 'tip links' which could be those springs.
Unresolved questions: the location and shape of transduction channels and the sites triggering transduction are not precisely known. How could tip links be tested for demonstration that they act as springs? Perhaps showing that they are elongated during excitement and shortened on inhibition of channels, or finding a way to remove them while measuring changes in hearing.
- Frequency tuning
Frequency tuning and adaptation happens through both physical resonance - with longer hairs responding to lower frequencies - and through a separate ion channel which enables electrical resonance and damping. Adaptation involves loosening of gating spring tension. Ca2+ may work as a second messenger in adaptation here as it does in photoreceptors.
The physical structure of each bundle tunes it to a specific frequency, around which it may be able to distinguish sound from noise very well. An electrical tuning and resonance is also predicted, via Ca2+-sensitive K+ channels.
There is evidence of active tuning by the ear as well, with cells exerting force on their surrounding and parts of the ear vibrating and emitting acoustic wavelengths to improve tuning. Similar active tuning and emission is observed in birds and amphibians as well as mammals. The hair bundle may collectively play a role in active tuning as well.
Unresolved questions: it is not known how cells get tuned to an electrical resonant frequency, or how hair bundles generate force.
- Need for speed
Good arguments can be made for hearing requiring very fast response to changing stimuli. And hearing is clearly responsive to high-frequency sound. This provides a reason for direct transduction (which is faster than any intermediated mechanism), and is needed for active echo-location such as bats use. This may rquire gating speeds of up to 3 microseconds. These speeds are fast compared to the diffusion timescale for transduction channels. However direct opening of a gate or channel could speed up the process dramatically with its mechanical energy.
Theoretical and Practical Relevance
This review article summarized the state of understanding of the internals of the vertebrate ear in 1989, particularly the role played by hair cells, summarizing some of the clearest and most unchanging data about it. It remains current as of 2012.