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 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 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 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 1kHz Effect 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 health Made 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 dPa Should see peak at point where pressures are equal

Tympanogram types: A: peak between +100 and -200 dPa: normal C: peak beyond -200 dPa: 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: 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?

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 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-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