Anatomy and Physiology of the Ear The Temporal

Anatomy and Physiology of the Ear § § § The Temporal Bone Outer Ear Middle Ear Inner Ear Cochlear Physiology

Which Way? § Anterior/Ventral = toward the front § Posterior/Dorsal = toward the back § Lateral = toward the side § Medial = toward midline § Superior = 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 Landmarks § § § Helix Antihelix Concha Tragus Intertragal Notch Antitragus

External Auditory Canal § lateral portion-cartilage § medial portion-osseous § lined with epidermal (skin) tissue § hairs in lateral part § cerumen (ear wax) secreted in lateral part.

Outer Ear Functions 1 § Amplification / Filtering -- increases sounds between 1500 and 7000 Hz by 10 to 15 d. B -- 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 membrane Tympanic Membrane separates it from EAC Eustachian tube connects it to nasopharynx Also 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 Functions § Impedance 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 lever Whenever there is a pivot: Force x Length in = Force x Length out Force 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 man § because of the difference in length on either side of the pivot point

Increase in Pressure § Remember that Press. = Force/Area § force is increased 1. 3 times § area is decreased 18. 6 times § Pressure is increased 24. 2 times (27. 7 d. B)

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 1 k. Hz § Effect can be seen in Minimum Audibility Curve (Figure 10. 2)

Minimum Audibility Curve (Figure 10. 2) § Plot of threshold of detection (in d. B SPL) for tones as a function of frequency. § Shows: best hearing around 1 k. Hz poorer hearing below 500 Hz and above 4000 Hz

Tympanometry § Acoustic measures of middle ear health § Made using an immittance (or impedance) bridge: l l 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 d. Pa § Should see peak at point where pressures are equal

Tympanogram types: § § § A: peak between +100 and -200 d. Pa: normal C: peak beyond -200 d. Pa: neg pressure B: no peak flat tymp: effusion As: peak but shallow: stiff: otosclerosis Ad: peak off scale: floppy: dysarticulation

Tympanogram Types

The Acoustic Reflex § Stapedius contraction measured as change in compliance § Reflex arc: l peripheral ear, l VIIIth n. l Cochlear nucleus l superior olivary complex l 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?

INNER EAR Two Halves: § Vestibular--transduces motion and pull of gravity § Cochlear--transduces sound energy (Both use Hair Cells)

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

Cochlea is Divided into 3 “Scala” § Scala Vestibuli l Reissner’s Membrane § Scala Media l Basilar Membrane § Scala Tympani § Helicotrema - the opening between 2 outer Scala

Fluids filling the Inner Ear § Perilymph- in S. Vestibuli and S. Tympani l l High Sodium / Low Potassium concentrations Low Voltage (0 to +5 m. V) § Endolymph- in S. Media l l High Potassium / Low Sodium concentrations High Positive Voltage (85 m. V)

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 O = Outer Hair Cells P = Pillar 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 acousticalmechanical 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-chemical § Outer 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