Anatomy and Physiology of the Ear

The Temporal BoneOuter Ear Middle EarInner EarCochlear Physiology

Which Way?

Anterior/Ventral = toward the front Posterior/Dorsal = toward the backLateral = toward the sideMedial = toward midlineSuperior = toward upper surface (rostral)Inferior = toward lower surface (caudal)

Gotta Catch a Plane

Sagittal- dividing right from left

Coronal (Frontal) -dividing front from back

Horizontal -dividing up from down

The Temporal Bone - Part of the Skull

Temporal Bone:Lateral/Medial Views

The Temporal Bone houses the “Ear”

The Outer Ear Consists of:

The Pinna - cartilaginous, highly variable in appearance, some landmarks.

External Auditory Canal (or external auditory meatus) - 2.5 cm tube.

Pinna LandmarksHelixAntihelixConchaTragus Intertragal NotchAntitragus

External Auditory Canal

lateral portion-cartilagemedial portion-osseous lined with epidermal (skin)

tissuehairs in lateral partcerumen (ear wax) secreted

in lateral part.

Outer Ear Functions 1

Amplification / Filtering

-- increases sounds between 1500 and 7000 Hz by 10 to 15 dB

-- because of the resonance of

Concha -- 5000 Hz

E.A.Canal -- 2500 Hz

Outer Ear Functions 2

Protection

-- medial displacement of ear drum

-- curvature of canal

-- hairs

-- cerumen

-- skin migration

Outer Ear Functions 3

Localization

-- The ability to identify the location of a sound source

-- (Will be covered more later)

The Middle Ear:A cleft within the temporal bone

Lining is mucous membraneTympanic Membrane separates it from EACEustachian tube connects it to nasopharynxAlso Connected to Mastoid Air Cells

Middle Ear Structures

1- Malleus 2- Incus --Ossicles

3- Stapes 4- Tympanic Membrane

(Eardrum)

5- Round Window

6- Eustachian Tube

Middle Ear Muscles 1. The Stapedius

Attaches to Stapes

Contracts in Response to Loud sounds, chewing, speaking

Innervated by the Facial (VIIth cranial) nerve

Middle Ear FunctionsImpedance Matching -- amplification of

sounds to overcome difference in impedance between the air of EAC and the fluid of the inner ear.

Filtering -- resonant frequency is approximately 1000 Hz, functions as bandpass filter.

Acoustic Reflex -- Contraction of Stapedius muscle in response to loud sounds

Middle Ear Function

Impedance Matching is accomplished through pressure increase produced by the middle ear.

From 2 main effects:

Reduction in AREA

Increase in FORCE

Reduction in AREA

sound striking the (relatively large) tympanic membrane

is delivered to the (much smaller) stapes footplate

Areal Ratio = 18.6 to 1

Increase in FORCE

The malleus and incus act like a leverWhenever there is a pivot:Force x Length in = Force x Length outForce is greater on short side (Think of

wheeled luggage) Malleus manubrium = 1.3 times as long as

Incus long process

Leverage

Small force (baby’s weight) supports manbecause of the difference in length on either side of

the pivot point

Increase in Pressure

Remember that Press. = Force/Areaforce is increased 1.3 timesarea is decreased 18.6 times

Pressure is increased 24.2 times (27.7 dB)

Other Key Middle Ear Function

Oval Window Isolation-- Sound striking the tympanic membrane is delivered through the ossicular chain to the oval window

Without the middle ear, both the oval and round windows would receive sound energy and energy would cancel out.

Middle Ear Filtering:

Band Pass filter Resonant Frequency near 1kHzEffect can be seen in Minimum Audibility

Curve (Figure 10.2)

Minimum Audibility Curve (Figure 10.2)

Plot of threshold of detection (in dB SPL) for tones as a function of frequency.

Shows:

best hearing around 1 kHz

poorer hearing below 500 Hz

and above 4000 Hz

Tympanometry

Acoustic measures of middle ear healthMade using an immittance (or impedance)

bridge: PRESSURE PUMP/MANOMETER MINIATURE SPEAKER MICROPHONE ALL CONNECTED THROUGH A SMALL

PROBE INSERTED IN EAR CANAL

Compliance: opposite of stiffness.

middle ear system is not massive, largely a stiffness-controlled system.

Changes in stiffness/compliance have large effects on functioning of system.

at point where air pressure in canal and middle ear are equal the most sound will be conducted through.

Tympanogram:

A plot of middle ear compliance as a function of ear canal pressure

Pressure is swept from +200 to -200 or -400 dPaShould see peak at point where pressures are

equal

Tympanogram types:

A: peak between +100 and -200 dPa: normal

C: peak beyond -200 dPa: neg pressureB: no peak flat tymp: effusionAs: peak but shallow: stiff: otosclerosis

Ad: peak off scale: floppy: dysarticulation

Tympanogram Types

The Acoustic ReflexStapedius contraction measured as change

in complianceReflex arc:

peripheral ear, VIIIth n. Cochlear nucleus superior olivary complex VIIth n. to the middle ear

Reflex is bilateral.

Clinical Tests using Acoustic Reflexes:

A.R. Threshold: how intense sound must be to elicit the reflex?

A.R. Decay: Is the degree of a contraction maintained throughout a 10 second stimulus?

Two Halves:Vestibular--transduces motion and pull of gravityCochlear--transduces sound energy

(Both use Hair Cells)

INNER EAR

Subdivision into spaces containing endolymph (blue), and spaces containing perilymph (red)

Cochlea is Divided into 3 “Scala”

Scala Vestibuli Reissner’s Membrane

Scala Media Basilar Membrane

Scala Tympani

Helicotrema - the opening between 2 outer Scala

Fluids filling the Inner Ear

Perilymph- in S. Vestibuli and S. Tympani High Sodium / Low Potassium concentrations Low Voltage (0 to +5 mV)

Endolymph- in S. Media High Potassium / Low Sodium concentrations High Positive Voltage (85 mV)

Cross-Section of the Cochlea

Third Turn

Second Turn

First Turn

A Cross Section Shows the 3 Scala

Within S. Media is the Organ of Corti

I = Inner Hair Cells P = Pillar Cells

O = Outer Hair Cells D = Deiter’s Cells

The Stereocilia on IHCs and OHCs

OHCs (at top) V or W shaped ranks

IHC (at bottom) straight line ranks

Cochlear Functions

Transduction- Converting acoustical-mechanical energy into electro-chemical energy.

Frequency Analysis-Breaking sound up into its component frequencies

Transduction-

Inner Hair Cells are the true sensory transducers, converting motion of stereocilia into neurotransmitter release.

Mechanical Electro-chemicalOuter Hair Cells have both forward and

reverse transduction--

Mechanical Electro-chemical

Mechanical Electro-chemical

Frequency Analysis-the Traveling Wave

Bekesy studied cochleae from cadavers, developed the Traveling Wave theory

1. Response always begins at the base

2. Amplitude grows as it travels apically

3. Reaches a peak at a point determined by frequency of the sound

4. Vibration then dies out rapidly