Structure and function of the middle ear

Human ear, frontal view

1. Anatomy

1.1 Overall anatomy of the human ear

After Cull (1989): The Sourcebook of medical illustration, Parthenon, Carnforth, xxiii+481 pp.

1.2 Anatomy of the human middle ear

Human middle ear, frontal view

After Cull (1989): The Sourcebook of medical illustration, Parthenon, Carnforth, xxiii+481 pp.

Human middle ear, medial view

After Deaver JB (1900): Surgical anatomy, Vol. 2, Blakiston, Philadelphia, 709 pp.
Inner and middle ear

* : temporal, zygomatic, buccal, mandibular, cervical

Daren Nicholson's 3D Ear site

1.3 Differences among species

Schematic of air cavities

1.3.1 Air cavities

Although the configurations are different, in many species there is a second cavity which communicates, through a relatively narrow opening, with the main middle-ear cavity.

This configuration leads to an acoustic resonance, like a Helmholtz resonator.

1.3.2 Eardrum configuration

Eardrum configurations 1 Varying size of pars flaccida.


Sheep after Lim, Acta Otolaryngol. 66: 515–532 (1968);
human after Filogamo, Acta Anat. 7: 248–272 (1949)

Eardrum configurations 2 Varying orientation of manubrium, and varying degrees of asymmetry.

(Decraemer & Funnell, 2008)

Platypus eardrum

After Gates GR, Saunders JC, Bock GR, Aitkin LM & Elliott MA (1974): Peripheral auditory function in the platypus, Ornithorhynchus anatinus. J Acoust Soc Am 56: 152-156


1.3.3 Ossicles

human, cat, g.p. ossicles

(not to scale)

1.3.4 Ligaments

posterior incudal ligaments

Based on descriptions by Kobayashi, 1955a,b,c

Different configurations of posterior incudal ligament in different species.


Kobayashi M (1955a): On the ligaments and articulations of the auditory ossicles of cow, swine and goat. Hiroshima J Med Sci 3: 331-342
Kobayashi M (1955b): On the ligaments and articulations of the auditory ossicles of the rat and the guinea pig. Hiroshima J Med Sci 3: 343-351
Kobayashi M (1955c): The articulations of the auditory ossicles and their ligaments of various species of mammalian animals. Hiroshima J Med Sci 4: 319-349

Ligament fibre arrangements

After Fumagalli (1949): Sound-conducting apparatus: a study of morphology. Arch Ital Otol Rinol e Laringol 60 Suppl. 1: ix+323 pp.

Complex fibre arrangements within ligaments.

1.3.5 Muscles

Stapedius

After Kobayashi M (1956): The comparative anatomical study of the stapedial muscles of the various kinds of mammalian animals. Hiroshima J Med Sci 5: 63-84

Stapedius muscle in various species

1.4 Changes with age

Comparison of newborn and adult ears

After Fowler EP Jr. (1947): Medicine of the ear, 2nd ed., T. Nelson, New York)

Eardrum becomes more vertical with age.

Comparison of newborn and adult ears

After Fowler EP Jr. (1947): Medicine of the ear, 2nd ed., T. Nelson, New York)

Or does it?

1.5 Human ear in macroscopic sections

Human coronal

Coronal (frontal) section, reconstructed from horizontal sections from NLM's Visible Human Project.

Click on the image to view a set of images cropped from the original horizontal sections, in the vicinity of the ear. These are from the Visible Human female data. The pixel size and slice thickness are both 0.33 mm.

1.6 Cat ear in histological sections

Cat histology: ears

1.6.1 Overview

Section through cat skull, showing middle-ear cavities on both sides.

30-micron histological sections stained with hæmatoxylin and eosin.

Cat histology: middle ear Same section, magnified. Note the eardrum, with the manubrium embedded in it.

Note stapes and cochlea.

1.6.2 Stapes

Cat histology: ossicles Note the stapes in the oval window, opening into the basal turn of the cochlea. Note also the second and third turns of the cochlea, and the auditory nerve coming out of the middle of the turns.
Cat histology: stapes

Note the tip of the long process of the incus, just above the head of the stapes, and the body of the stapedius muscle. The position of the annular ligament, between the footplate of the stapes and the oval window, is just visible.
Cat histology: stapes

Note the annular ligament between the footplate of the stapes (above) and the rim of the oval window (below).
Cat histology: stapes

Note the fibrous structure of the annular ligament.

1.6.3 Lenticular process of incus

Cat histology: incudostapedial joint In a different slice, note the broad, tight joint between the incus (left) and the stapes (right)
Cat histology: lenticular process

In another ear, from another angle, and with thinner sections (1-micron, toluidine blue stain), note the very thin bony connection between the long process of the incus and the lenticular plate.
Cat histology: lenticular process

Note the blood vessel running down into the lenticular plate.
Cat histology: lenticular process

Note the different layers:


Cat histology: lenticular process

Closer.
Cat histology: lenticular process

Closer.

1.7 Eardrum microstructure

Cat histology: middle ear

The eardrum is ~10 mm in diameter, but only 10's of microns thick.

Pars tensa cross-section

After Fig. 1 in Lim DJ (1968): Tympanic membrane: Electron Microscopic Observation. Part I: Pars Tensa. Acta Oto-Laryngol. 66: 181–198

Three layers:

Layers of lamina propria:


eardrum fibre organization Note the approximately orthogonal fibre organization, .

2. Middle-ear function

2.1 Middle ear as transformer

Animation with fixed axis

Matching low acoustical impedance of air to high acoustical impedance of liquid in cochlea. Mechanisms:

Simple model with fixed axis.

2.1.1 Surface area

Human middle ear, frontal view

Ratio of eardrum area to footplate area.

Force balance:
ftm = ffp
ptmAtm = pfpAfp
pfp = ptm(Atm/Afp)

After Cull (1989): The Sourcebook of medical illustration, Parthenon, Carnforth, xxiii+481 pp.

Differences in among different families

How to measure the surface areas?

Based on data of Kirikae (1960)

2.1.2 Lever arm

Length of manubrium
vs.
length of long process of incus

Lever arm depends on ...

2.1.3 Curvature

Simplified model

One side only, with distributed load

Further simplification.

Relationship between input xi and output xo?

What assumptions?


2.1.4 Combination

Schematic of middle-ear transformer

Relative magnitudes?

Mechanisms can’t really be separated.

Funnell (1996): On the low-frequency coupling between eardrum and manubrium in a finite-element model. J. Acoust. Soc. Am. 99(5): 3036-3043

2.2 Middle-ear muscles

Zebras grazing

Functions:

  • middle-ear reflex is too slow for protection against sudden noises
  • muscles attenuate low-frequency sounds
  • muscles reduce masking of high-frequency sounds by low-frequency sounds
  • frequency-selection filter
  • dynamic tuning?
leopard stalking

3. Experimental measurements

3.1 Eardrum at low frequencies

3.1.1 Békésy (1941)

Bekesy low frequency Low-frequency measurement with capacitive probe.
Topographic map

3.1.2 Khanna (1970)

low-frequency vibration pattern Laser holography.

Simple vibration pattern at low frequencies.

3.1.3 Other

Owada (1959) Literature review shows agreement with Khanna even in older data.

For example, Owada (1959), cat and rabbit

Kirikae (1960) Kirikae (1960), human

Kirikae I (1960): The structure and function of the middle ear. University of Tokyo Press, Tokyo

Bekesy low frequency, interpreted Even Békésy's own results can be interpreted as agreeing in part with Khanna's observations.

3.2 Eardrum at high frequencies

3.2.1 Khanna (1970)

Variability of high-frequency patterns Laser holography.

Vibration pattern breaks up, becomes more complex at high frequencies.

Great variability among individuals.
Variability of high-frequency patterns

Actual holographic images.

3.2.2 Khanna & Decraemer (1997)

Point-by-point measurements.

Combination of

Measurements require


General view of vibration-isolation table inside sound-proof room.

Note on the left.

Combined .
Point-by-point eardrum measurements

Point-by-point measurements. Cat.

After Fay, Puria, Decraemer & Steele (2005) Fig. 2
Animated point-by-point eardrum measurements

Animated point-by-point measurements.

Courtesy W.F. Decraemer

3.2.3 Maftoon et al. (2013)

Laser Doppler vibrometer

Zinan He (2012), M.Eng. thesis, Mcgill University

Point-by-point measurements. Gerbil.

Off-the-shelf vibrometer designed for .

Maftoon et al. (2013), Fig. 1

Maftoon N, Funnell WRJ, Daniel SJ & Decraemer WF (2013): Experimental study of vibrations of gerbil tympanic membrane with closed middle ear cavity. JARO 14(4): 467-481 (doi:10.1007/s10162-013-0389-9)

Ear canal removed, acoustic coupler attached.

Maftoon et al. (2013), Fig. 2

Maftoon N, Funnell WRJ, Daniel SJ & Decraemer WF (2013): Experimental study of vibrations of gerbil tympanic membrane with closed middle ear cavity. JARO 14(4): 467-481 (doi:10.1007/s10162-013-0389-9)

Glass-coated plastic microspheres as laser targets.

Videos of experimental procedure

He Z (2012): Vibration measurements on the widely exposed gerbil eardrum. M.Eng. thesis, McGill University

Maftoon et al. (2013), Fig. 8

Maftoon N, Funnell WRJ, Daniel SJ & Decraemer WF (2013): Experimental study of vibrations of gerbil tympanic membrane with closed middle ear cavity. JARO 14(4): 467-481 (doi:10.1007/s10162-013-0389-9)

Vibrations of points on the pars tensa:

Lower inset shows Bode plot, used to confirm phase unwrapping.

3.3 Ossicular vibrations

Looking into middle ear through hole drilled in bulla. The manubrium is barely visible. Note the moist cotton wool and paper towel.

From a slightly different angle, the eardrum and more of the manubrium are visible.

With sufficient precision, vibrations along 3 axes can be measured.

Close-up. The head of the stapes is barely visible at the back.

Close-up from other side, showing the long process of the incus and the top of the stapes.

Animation showing complex motion of the ossicular chain, as estimated from measurements at multiple points and from multiple directions.

Cf. simple model. Simple model

3.4 Material properties

3.4.1 Uniaxial measurements on relatively large samples

3.4.2 Indentation, etc.

3.4.3 Békésy (1949)

Bekesy stiffness measurement with hair Transverse

A calibrated hair was used to produce a known bending force on a flap cut from the eardrum.

Békésy, Gv (1949): The structure of the middle ear and the hearing of one's own voice by bone conduction. J Acoust Soc Am 21: 217-232

Bekesy stiffness measurement in calf For a calf eardrum. Led to a very low value.

3.4.4 Kirikae (1960)

Kirikae's apparatus Longitudinal

Strip 10 × 1.5 mm.
Vibrator (cantilever beam, natural frequency of 890 Hz).
When the strip of eardrum was attached to the beam and stretched by a mass, the natural frequency changed.

Kirikae I (1960): The structure and function of the middle ear. University of Tokyo Press, Tokyo

3.4.5 Decraemer (1980)

Decraemer's apparatus

Longitudinal

Measured properties as a function of frequency.

3.4.6 Cheng, Dai & Gan (2007)

Gan's apparatus

Longitudinal

Off-the-shelf instrument

Annals of Biomedical Engineering 35(2): 305–314. DOI: 10.1007/s10439-006-9227-0

3.5 Variability

3.5.1 Voss et al. (2000)

Great variability between ears.

Voss SE, Rosowski JJ, Merchant SN & Peake WT (2000): Acoustic responses of the human middle ear. Hearing Research 150(1-2): 43–69

3.5.2 Ellaham et al. (2007)

One problem is drying.

Ellaham NN, Akache F, Funnell WRJ & Daniel SJ (2007): Experimental study of the effects of drying on middle-ear vibrations in the gerbil. Proc 30th Ann Conf Can Med Biol Eng Soc, paper M0173, 4 pp. (CD-ROM)

3.5.3 Todd (2005)

Todd W (2005): Orientation of the manubrium mallei: Inexplicably widely variable. Laryngoscope 115: 1548-1552

Anatomical variability.

For example, orientation of manubrium in human.

Todd NW (2005): Orientation of the manubrium mallei: Inexplicably widely variable. Laryngoscope 115: 1548–1552

4. Challenges

4.1 Sizes

4.2 Displacements

4.3 Time scales

4.4 Tissue types


BMDE-501 Modelling middle-ear mechanics

R. Funnell
Last modified: 2014-07-01 09:19:58