Structure and function of the middle ear

1. Anatomy

1.1 Overall anatomy of the human ear

• external ear (air)
• middle ear (air)
• inner ear (liquid)

1.2 Anatomy of the human middle ear

• Eardrum (tympanic membrane)
• Three small bones (ossicles)

• Two muscles

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

• Joints - synovial

• Ligaments
  • joint ligaments
  • suspensory ligaments
    • posterior incudal
    • others – somewhat variable
• Mucosal folds - variable
• Smooth-muscle fibres

• Facial nerve (both motor & sensory)*
• Chorda tympani nerve (taste)

* Major branches: temporal, zygomatic, buccal, mandibular, cervical

(Image from Daren Nicholson's 3D Ear site.)

1.3 Differences among species

1.3.1 Air cavities

Although the configurations are different, in all three 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.

ant=antrum; ac=air cells; ect=ectotympanic; ent=entotympanic; m=mastoid; p=petrosal; epi=epitympanic.

1.3.2 Eardrum configuration

Varying size of pars flaccida.

Varying orientation of manubrium, and varying degrees of asymmetry.

(Decraemer & Funnell, 2008)

After Gates et al., 1974

• Always conical, and convex towards inside of cone. Even in the platypus.
• Cone inward in mammals, outward in birds.

1.3.3 Ossicles

(not to scale)

1.3.4 Ligaments

(After Funnell, 1972; based on descriptions by Kobayashi, 1955a,b,c)

Different configurations of posterior incudal ligament in different species.

(After Fumagalli, 1949, as presented by Funnell, 1975)

Complex fibre arrangements within ligaments.

1.3.5 Muscles

(After Kobayashi, 1956, as presented by Funnell, 1972)

Stapedius muscle in various species

1.4 Changes with age

(After Ballachanda, 1995, as presented by Qi, 2008)

• Canal walls
• Eardrum orientation, thickness
• Etc.

1.5 Human ear in macroscopic sections

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

1.6.1 Overview

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

30-micron histological sections stained with hæmatoxylin and eosin.

Same section, magnified. Note the eardrum, with the manubrium embedded in it.

Note stapes and cochlea.

1.6.2 Stapes

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.

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.

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

Note the fibrous structure of the annular ligament.

1.6.3 Lenticular process of incus

In a different slice, note the broad, tight joint between the incus (left) and the stapes (right)

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.

Note the blood vessel running down into the lenticular plate.

Note the different layers:

• uncalcified cartilage
• calcified cartilage
• bone

Closer.

Closer.

2. Eardrum structure and properties

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

2.1 Microstructure

Three layers:

• Epidermis on the outside
• Lamina propria
• Mucosal layer on the inside

Layers of lamina propria:

• Subepidermal connective tissue
• Outer layer of radial fibres
• Inner layer of circular fibres
• Submucosal connective tissue

Note the approximately orthogonal fibre organization, like plywood.

2.2 Material properties

Uniaxial measurements on relatively large samples:

• Békésy (1949)
• Kirikae (1960)
• Decraemer (1980)
• Cheng, Dai & Gan (2007)

2.2.1 Békésy (1949)

Transverse. Békésy's arrangement for transverse measurement of the Young's modulus of the eardrum. A calibrated hair was used to produce a known bending force on a flap cut from the eardrum.

Békésy's arrangement for measuring Young's modulus for a calf eardrum. Led to a very low value.

2.2.2 Kirikae (1960)

Longitudinal. Kirikae measured the stiffness of a strip of human eardrum, 10 mm by 1.5 mm. He used a vibrator consisting of a cantilever beam with a natural frequency of 890 Hz. When the strip of eardrum was attached to the beam and stretched by a mass, the natural frequency changed.

2.2.3 Decraemer (1980)

Longitudinal. Measured properties as a function of frequency.

2.2.4 Cheng, Dai & Gan (2007)

Longitudinal. Off-the-shelf instrument.

• Uniaxial tensile test
• Stress-relaxation test
• Strength test

3. Middle ear as transformer

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

• surface area
• lever arm
• curvature

Simple model with fixed axis.

3.1 Surface area

Ratio of eardrum area to footplate area.

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

Example?

Differences in ratios among different families

How to measure the surface areas?

3.2 Lever arm

Length of manubrium
vs.
length of long process of incus

Example?

Lever arm depends on ...

3.3 Curvature

Simplified model

One side only, with distributed load

Further simplification.

Relationship between input x_i and output x_o?

What assumptions?

Example?

3.4 Combination

• surface area
• lever arm
• curvature

Relative magnitudes?

Mechanisms can’t really be separated.

3.5 Muscles

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 dynamic tuning?

4. Middle-ear vibration patterns

4.1 Eardrum at low frequencies

4.1.1 Békésy (1941)

Low-frequency measurement with capacitive probe.

4.1.2 Khanna (1970)

Laser holography.

Simple vibration pattern at low frequencies.

4.1.3 Other

Literature review shows agreement with Khanna even in older data.

For example, Owada (1959), cat and rabbit

Kirikae (1960), human

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

4.2 Eardrum at high frequencies

4.2.1 Khanna (1970)

Laser holography.

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

Great variability among individuals.

Actual holographic images.

4.2.2 Khanna & Decraemer (1997)

Point-by-point measurements.

4.3 Ossicular vibrations

Point-by-point laser heterodyne interferometry combined with optical sectioning microscopy and computer-driven positioning.

Note two experimenters, three computers, racks of instruments and sound-proof room in the rear.

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

Note nested horizontal and vertical goniometers on the left.

Combined laser interferometer and optical sectioning microscope.

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.

4.4 Off-the-shelf laser vibrometer

Laser Doppler vibrometer mounts on standard operating microscope.

PC with data-acquisition board and bundled software.

4.5 Variability

4.5.1 Voss et al. (2000)

Great variability between ears.

4.5.2 Ellaham et al. (2007)

One problem is drying.

4.5.3 Todd (2005)

Anatomical variability.

For example, orientation of manubrium in human.

5. Challenges

5.1 Sizes

• up to mm’s
• down to µm’s
• down to nm’s

5.2 Displacements

• down to nm’s
• up to mm’s

5.3 Time scales

• down to µseconds
• up to tens of seconds
• up to Mseconds

5.4 Tissue types

• bone
• fibrous connective tissue
  • collagen, elastin
  • ground substance
• muscle – striated, smooth
• cartilage – calcified, uncalcified
• synovial fluid

6. References

Ballachanda BB (1995): The human ear canal: Theoretical implications and clinical considerations including cerumen management. Singular, San Diego

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

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)

Fumagalli Z (1949): Sound-conducting apparatus: a study of morphology. Arch Ital Otol Rinol e Laringol 60 Suppl. 1: ix+323 pp. (in Italian; title, figure captions and Chapter 7 in English also)

Funnell, WRJ (1972): The acoustical impedance of the guinea-pig middle ear and the effects of the middle-ear muscles. M.Eng. thesis, McGill University, vi+93 pp.

Funnell, WRJ (1975): A theoretical study of eardrum vibrations using the finite-element method. Ph.D. thesis, McGill University, x+200 pp.

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

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

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

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

Qi L (2008): Non-linear finite-element modelling of newborn ear canal and middle ear. Ph.D. thesis, McGill University, x+133 pp.

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

BMDE-501 Modelling middle-ear mechanics