The Inner Ear

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Left photo: human right temporal bone, outer view: . . . . . . . . .Right photo: plastic model skull for orientation

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inner view of same human right temporal bone; cochlea contained within pyramidal mass of bone at center

The Cochlea

Apex

Base

Left: longitudinal section through human cochlea . . . . .Right: image of section through guinea pig cochlea

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The Three Scalae

Base . . . . . . . . . . . . . . . . . . . . . . . . . .Apex

section along (imaginarily) "stretched out" chambers of the inner ear

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transverse section through the three chambers of the inner ear

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medical illustrator's rendering of the three chambers and the organ of Corti

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plastic model made from a cast of a human scala tympani, (made for surgeons to practice inserting cochlear electrodes)

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The Organ of Corti

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Variation along length of cochlea

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Differences between inner and outer hair cells

Inner (single row) Outer (three rows)
about 7,000 such cells per ear about 20,000 such cells per ear
about 60 cilia per cell 100-200 cilia per cell
surrounded by other cells touched by other cells only at top and bottom ends
cilia do not touch tectorial membrane cilia ends fit into sockets in tectorial membrane
nerve endings around bottom 1/3 of cell nerve endings concentrated at base of cell

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scanning electron micrograph of the upper surface of the Organ of Corti, tectorial membrane retracted, looking down on reticular lamina and cilia at end of each hair cell

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.the scale bar shown corresponds to a distance of 10 µm (micrometers)

a human hair is about 50 µm in diameter

single row of inner hair cell cilia at higher magnification

[5 µm = 0.005 mm = 1/200 mm]

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two of three rows of outer hair cell cilia at higher magnification

cilia shorter, more uniform, more numerous

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Different innervation of inner and outer hair cells

Afferent nerve fibers: carrying signals from ear toward brain

30,000 exit each cochlea, 30 per opening to spiral ganglion, 20 of those 30 come direct from the nearest inner hair cell, only about 1/10 connect to outer hair cells -- each cell to many fibers and each fiber to many cells

Efferent nerve fibers: carrying signals from CNS to ear

500 enter cochlea, branch to 3,000 near Organ of Corti; 8,000 cross tunnel to outer hair cells, where they make 40,000 connections

Outer Hair Cell Inervation: diffuse afferent and profuse efferent

Inner Hair Cell Inervation: multiple, specific afferent and little if any efferent

The further from the oval window, the relatively more afferent and fewer efferent fibers

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Mechanical Response of the Basilar Membrane

(von Békésy measurements on cadaver cochleas)

Tiny starch crystals were placed at various points along a basilar membrane while pure tones of various frequeincies were played through an earphone, producing vibrations of the eardrum and ossicles and, in turn, travelling waves along the basilar membrane. By reflecting a beam of light off the surface of the crystals, the amplitude of vibration could be measured. It was found that different regions of the membrane responded most strongly to different frequencies. The response peaks, however, were much to broad to explain humans' ability to distinguish musical pitches.

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Basilar Membrane Displacement Model Animations

250 Hz -- 1kHz -- 4 kHz -- Click

[Waveform files for 250 Hz, 1 and 4 kHz]

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The Observed Sharper Tuning of Neural Impulses from the Organ of Corti

These measurements of the response of various individual nerve fibers to frequency ranges of pure tones were done in a living cat, while Von Bekesy's studies above -- indicating much poorer frequency resolution -- were done in cadavers. Minima in these thresholds correspond to peak sensitivities, which are still too broad to fully explain human pitch discrimination abilities.

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Active Tuning

Outer Hair Cell Model Animations

A -- B -- C

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Organ of Corti Model Animations

A -- B -- C

It now is known that outer hair cells can be instructed by the central nervious system to change their lengths in response to basilar membrane vibrations -- either in phase with those vibrations (reducing the amplitude of the vibrations) or out of phase (increasing the amplitude). By sending appropriate instructions to outer hair cells in particular regions, our central nervious system can mechanically tune the basilar membrane's response to achieve much higher frequency resolution. The inner hair cells detect the vibrations that have been tuned by the outer hair cells and produce the sequences of neural pulses ("spikes") we interpret as sounds.

How movement of the stereocilia cause hair cells to initiate electrochemical neural pulses ("spikes")

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tips of adjacent stereocilia connected by ultrafine helical fibers that open ion channels when tugged by relative motion

the 5 µm (micrometer) scale shown corresponds to 5000 nm (nanometers), scales below left to only 100 nm

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Left: tunneling electron micrographs of adjacent chinchilla stereocilia, connected by tip link fibers

Center: noise filtered versions of the same images

Right: high power images of the tip links, showing helical structure, and Fourier transforms of rectangular areas

[scale bars are 100 nm long in left and center, 10 nm in right panels]

To visualize the changes of scale we have traversed in studying the anatomy of the ear, think of the 100 nm scale bars above as being 2 inches in length. At that scale, the streteched out Organ of Corti would be 11 miles long, with an average width of 1.5 miles. A typical human hair at the same scale would be about the diameter of the Duke Chapel tower. Our initial diagram of the whole ear would be over 30 miles across.

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