Anatomy and Physiology of Hearing
 

Introduction to Hearing Loss
Anatomy and Physiology of Hearing
Audiology
Conductive Hearing Loss
Sensorineural Hearing Loss
Sudden Sensorineural Hearing Loss
Other Types of Hearing Loss
Reference

Table of Contents

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External Ear

Consists of pinna and the external auditory meatus up to the lateral border of the tympanic membrane.

 

 

 

 

Pinna

Composed mostly of cartilage and has no useful muscles.

The center, concha, leads to the EAM.

Resonance : 5kHx

Resonances and antiresonances.

Differentiate sound sources in front of the listener from those behide.

 

External Auditory Meatus

Lateral 1/3 : cartilaginous portion containing cerumen-producing glands and hair follicles

Remaining 2/3 : bony portion, including a tight dermal lining surrounding the TM

Resonance : determined by the length of the tube.

15dB in 3-kHz region

10dB in 1-5kHx region

Behaves like a quarter wave resonator.

2.5 cm à 3.5 kHz

Reasons for NIHL.

 

External Ear

Head and external ear

Localization of sound sources

Attenuator at frequencies at which the width of the head is greater than the wavelength of the sound : 2Hz

The Head Shadow Effect

Lower frequencies

Interaural time differences : 0.6 msec

Higher frequencies

Useful for improving the detection and recognition of low-energy, high-frequency sounds such as voiceless fricatives.

Hearing-aid and evaluations

8kHz resonance in infant

Adult values after about age 2.5 YO

 

Middle Ear

 

 

 

 

The middle ear consists of the tympanic cavity and the osseous eustachian tube.

Transmits acoustic energy from the air-filled EAM to fluid-filled cochlea.

Impedance-matching of air to the high impedance of the fluid-filled cochlea.

Impedance-matching 3 ways

Effective vibratory area : most important

Of TM is17-20x greater than of the stapes footplate.

Lever action of ossicular chain

Arm of the long process of the incus : 1.3x

The length of manubrium and neck of malleus

Shape of TM

Pressure gain : 25-30 dB

Tympanic Membrane

Transfer of power to inner ear

Protection from FB of EAM

Maintaining the air cushion that prevents insufflation of FB from NP through eustachian tube

Transformer action of TM and ossicular chain provides for a relatively efficient transfer of power to the inner ear and the fidelity of sound transmission across the middle ear is outstanding.

Distortion of signal does not occur even >130 dB SPL

Passive mechanical system with both mass and compliant elements and, therefore, resonant properties

Linear system coupled to the cochlear which contributes a large resistance.

Highly damped and linear and has a wide frequency response

Ratio of volume velocity of the stapes to sound pressure at the TM increases in humans to about 800-900 Hz the resonance of middle ear. (50% loss is only 3 dB)

Less than half of the power entering the middle ear reaches the cochlea.

Inefficient at frequencies above 2kHz

The reason humans do not detect and recognize sounds above 20 kHz

The frequency region of greatest energy concentration is 3-5 kHz

2 striated muscles : tensor tympani and stapedium

Protect the cochlea from loud sound

Providing strength the rigidity to the ossicular chain

Reducing physiologic noise: Chewing and vocalization

Improving the signal-to-noise ratio for high-frequency signal

Automatic gain control and increasing the dynamic range of the ear

Smoothing out irregularities in the middle-ear transfer function

Protection function of the muscles

Consensual reflex of stapedius

Increases the stiffness of ossicular change and TM

Attenuating sounds below 2 kHz

Stapedial reflex protects the cochlea

Tensor tympani does not normally respond to intense acoustic stimulation

Acoustic reflex latency is greater than 10 msec

 

Cochlea

A coiled, bony tube about 3.5 mm long.

Divided into the scala vestibuli, scala media and scala tympani

Perilymph : extracellular fluidlike material

Endolymph : intracellular-like fluid

Scala media has a positive DC RP of about 80 mV, decreasing slightly from the base to apex : produced by the heavily vascularized stria vascularis of the lateral wall The Battery of cochlea (Na+/K+-ATPase pumps).

 

 

 

Acoustic energy pathway

Pistonlike action of the stapes footplate in the oval window

Perilymph of the scala vestibuli

Helicotrema

Scala tympani

Organ of Corti : rests on the basilar membrane and osseous spiral lamina

Basilar membrane

0.12mm wide at the base

0.5mm wide at the apex

Organ of Corti

Outer and inner hair cells

Supporting cells : provide structural and metabolic support

Tectorial membrane

Reticular lamina-cuticular plate complex

Outer and inner hair cells of the organ of Corti play a major role in the transduction of mechanical energy (acoustic) into electrical energy (neural)

The spiral ganglion

Cell body of the auditory n

Sends axons to the cochlear nucleus of the brainstem

Dendrite projects through the osseous spiral lamina

Displacement pattern of the basilar membrane is traveling wave

Stiffer at the base than in the apex

Distributed continuously

Progresses from base to apex

Peak of amplitude of displacement varies as a function of stimulus frequency

Extremely sharp tuned response

Remarkable frequency-selective abilities

 

 

 

Cochlear amplifier

Active process

Supported by otoacoustic emission

Energy of the echo can be grater than the energy of the short-duration signal

Motility of outer hair cells and mechanical properties of the stereocilia and tectorial membrane

Stereocilia

Bundles of actin filaments

Inserted into the cuticular plate

Cross-linked between themselves

Of the inner hair cell do not contact to tectorial membrane

Deflection opens and closes nonspecific ion channels at the tips of the stereocilia, resulting in current flow (K+) into the sensory cell

The potassium flux

Arises from

Endocochlear potential of +80 mV

Negative IC potentials of hair cells (45,70)

Results in IC depolarization

Enzyme cascade involving calcium, ultimately releasing chemical transmitter

Subsequent activation of the afferent n

Neurotransmitter

Aff : Glu

Eff : Ach, GABA

Modify the motility changes of outer hair cell

Hyperpolarization of the cell membrane and a doubling of the cells input conductance

 

 

Gross Cochlea Potential

Endolymphatic (endocochlear) potential

DC, 80-10 mV, Scala media

Not generated in response to acoustic stimulation

Cochlear microphonic potential

AC, K+ current flow through outer hair cells

Summating potential

DC, several origin, reflects the DC shifts

Whole nerve action potential

All-or-non discharge of auditory nerve

 

 

Gross Cochlear Physiology

Malfunctioning of the mechanism involved in the production of endolymph and the EP can produce metabolic presbycusis.

When the flow of endolymph is blocked, endolymphatic pressure is increased and hydrops in produced

 

Eighth Nerve Physiology

30,000 neurons that innervate the cochlea

Type I neurons : radial fibers

90-95% synapse directly on inner hair cell

Bipolar, myelinated

Each inner hair cell is innervated by about 15-20 type I neurons

Type II neurons : outer spiral fibers

5-10% innervate the outer hair cells

Monopolar, unmyelinated

Each type II neuron branches to afferent innervation pattern of the cochlea

1,800 efferent fibers originating from SOC projection to the cochlea

Spontaneous rate

High : 18 to 120 spikes per second

Medium : 0.5 to 18 spikes per second

Low : 0 to 0.5 spikes per second

The most sensitive fibers have the most spontaneous activity

The most basic measure of auditory nerve function is the tuning curve of a single fiber.

The tuning curve

Low CF innervate apical region

CF, high and low frequency side, the tail

Normal neural activity, including sensitivity and frequency-resolving power, depends on intact outer hair cells and normal stereocilia

One of the most common features of SNHL is recruitment of loudness.

The tips of the tuning curves are missing and the fibers are not activated until the level of the signal is sufficient to reach the tails of the tuning curves.

Abruptly, many fibers then are activated.

 

Nonlinear Properties of the Ear

The outstanding features of the cochlea and the auditory nerve

Two-tone rate suppression

Otoacoustic emission (OAE)

Spontaneous OAE

Cochlear echo or transiently evoked OAE

Stimulus-frequency OAE

Distortion product OAE

 

Auditory Central Nervous System

 

 

 

Summarized By Thirayost Nimmanon

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