Structure and Function of the Auditory System

Question 1

anatomy

Answer 1

Anatomy: the science of structure of living things, including their systems, organs, and tissues
The body’s architecture

Question 2

Physiology

Answer 2

Physiology: the science of function in living systems
What it does
In this case, we’re referring to normal function

Question 3

CONDUCTIVE:

Answer 3

Outer ear
Middle ear

Question 4

SENSORINEURAL

Answer 4

Inner ear (sensory)
Auditory nervous system (neural)
CN VIII
Central auditory nervous system

Question 5

Three Parts of the Human Ear

Answer 5

Outer ear Acoustic energy

Middle ear Mechanical energy

Inner ear Hydraulic energy

CNVIII/Brain Electrical energy

Question 6

Auricle: aka pinna

Answer 6

Visible portion of the ear
Serves mainly to collect sound waves and funnel them into external auditory meatus

Question 7

External auditory meatus: aka ear canal

Answer 7

About 2.5 cm in length
Cerumen glands located in cartilaginous portion
Directs sound to the tympanic membrane
Measures about 2–3 cm
First 2/3rds is skin-covered cartilage
Inner 1/3rd is skin-covered bone
Junction between cartilaginous and bony portions is called the osseocartilaginous junction
Has a downward bend toward the middle ear

Question 8

Cerumen

Answer 8

glands located in lateral 1/3rd of cartilaginous portion of EAM

Question 9

Outer Ear Physiology

Answer 9

Collects and resonates sound
Assists in sound localization
Provides primary cue for determination of sound source elevation
Assists in distinguishing sounds from front vs. behind
Functions as protective mechanism for middle and inner ears
Protects the more delicate middle and inner ears from foreign bodies
Boosts (amplifies) high-frequency sounds

Question 10

Acoustic amplification of the outer ear

Answer 10

Acoustic amplification of the outer ear
Influence of the pinna (p) and of the auditory meatus (m) on the amplitude of the signal (incidence angle: 45 in the horizontal plane)
At 3000 Hz, the total amplification (t) is 20 dB

Question 11

middle Ear Anatomy

Answer 11

Tympanic membrane
Ossicles (malleus/incus/stapes)
Held in place by ligaments
Tensor tympani muscle
Stapedius muscle
Tympanum (air-filled space)
“Four walls”
Eustachian tube

Question 12

Middle Ear Anatomy

Answer 12

Middle ear cavity: filled with air
Ossicles: ear bones; the smallest bones in the human body

Malleus: (hammer)
Manubrium attached to TM
Incus: (anvil)
Short crus fitted into recess in wall of tympanic membrane; long crus attached to head of stapes
Stapes (stirrup)
Crura attach to footplate, which fits into oval window of cochlear wall

Question 13

Middle Ear Anatomy

Answer 13

Oval and round windows
Medial wall of the middle ear cavity
Middle ear cavity
Two muscles
Stapedius
Tensor tympani

Specially designed for efficiency
Very tense (stop vibrating instantly, limiting the chance for distortion)
Very elastic (dampens vibrations)
Very small (anatomical efficiency)

Question 14

Middle Ear Primary function

Answer 14

Primary function: serves as an impedance-matching transformer for about 30 dB lost
Impedance: measure of the opposition of energy flow
Matches the transfer of energy between air in outer and middle ears (lower energy) to fluid in inner ear (higher energy)
Area ratio difference between tympanic membrane and oval window
Lever action at incudomalleolar joint
Conical shape of TM
The mechanical link formed by the ossicles directs vibrations at the eardrum directly to the oval window
Oval window in? Round window out!

Question 15

middle ear Secondary function

Answer 15

Equalizes pressure between tympanum and nasopharynx
“Garbage chute”

Question 16

Eustachian Tube

Answer 16

Auditory tube”
Equalizes air pressure between middle ear (ME) cavity and nasopharynx (1º function)

Helps to drain fluids which might accumulate in ME into nasopharynx (2º function)

Mucosa needs equal pressure and oxygen to function

Question 17

middle ear Tertiary function

Answer 17

Tertiary function: serves to protect the inner ear from intense sounds
Acoustic reflex occurs bilaterally when stapedius muscle contracts, which stiffens the ossicles
Contraction of the middle ear muscles results in attenuation (decrease) of sound pressure reaching the inner ear―15 to 20 dB depending on frequency
Reflex is consensual―when either ear is appropriately stimulated, the muscles in both ears contract
We use this clinically when we perform the electroacoustic measurement called the acoustic reflex

Question 18

Inner Ear Anatomy

Answer 18

The inner ear is composed of two labyrinths
Petrous portion of each temporal bone
Osseous labyrinth:
Membranous labyrinth:
Auditory labyrinth:
Vestibular labyrinth:
Vestibule:

Question 19

Osseous labyrinth

Answer 19

Osseous labyrinth: channel in the bone

Question 20

Membranous labyrinth:

Answer 20

soft-tissue fluid-filled channels within osseous labyrinth containing end-organ structures of hearing and vestibular systems

Question 21

Auditory labyrinth

Answer 21

: contains the organ of hearing (cochlea)
Nerve endings encode a sound’s frequency, intensity, and timing information

Question 22

Vestibular labyrinth

Answer 22

contains the organ of balance and posture (semicircular canals), of which there are three (superior, lateral, and posterior, for pitch/roll/yaw)

Question 23

Vestibule:

Answer 23

found between the two inner ear labyrinths
Bounded on its lateral side by the oval window
Contains the utricle and saccule
Part of the vestibular system
Nerve endings encode linear (horizontal/vertical) head movement

Question 24

Inner Ear Anatomy

Answer 24

Auditory labyrinth
Scala media
Basilar membrane: base of the scala media
Right-hand wall of the scala media is the spiral lamina
Left-hand wall (sort of the hypotenuse) is called Reissner’s membrane
No matter where you slice the cochlea, you will see these three divisions (except at the helicotrema, where the scalae vestibuli and tympani are common because the scala media does not extend into the very tip)

Question 25

Auditory labyrinth
-Scala media
—Basilar membrane

Answer 25

Basilar membrane: base of the scala media
Right-hand wall of the scala media is the spiral lamina
Left-hand wall (sort of the hypotenuse) is called Reissner’s membrane

Question 26

scala media

Answer 26

(contains endolymph, which is high K+/low Na+)

Question 27

scala vestibuli

Answer 27

(contains perilymph, which is high Na+/low K+)

Question 28

scala tympani

Answer 28

contains perilymph, which is high Na+/low K+)

Question 29

Inner Ear Anatomy
Labyrinths

Answer 29

Auditory labyrinth
Organ of Corti:
Inner hair cells:
Outer hair cells:

Question 30

Organ of Corti:

Answer 30

in the basilar membrane; foundation on which rests a series of specialized hair cells
These hair cells arranged in rows which extend the length of the basilar membrane

Question 31

Inner hair cells

Answer 31

: one row, shaped like pears

Question 32

Outer hair cells

Answer 32

form three to four other rows, test tube shape; “hairs” shaped like Ws or Vs

Question 33

Basilar membrane

Answer 33

Stapes footplate rocks back and forth in oval window, which creates a wave within the cochlear fluids that displaces the scala media

Inward displacement of cochlear fluids at oval window matched by outward displacement via round window, which communicates directly with scala tympani
When the oval window is pushed inward by the stapes, the round window is pushed outward by the increased pressure in the inner ear fluid

Question 34

The stimulation from the stapes

Answer 34

The stimulation from the stapes creates a travelling wave in the cochlea, which moves from base to apex
Position of the peak in the vibration pattern on the basilar membrane depends on the frequency of the sound
High frequencies produce maximal displacement near the base
Low frequencies produce maximal displacement at the apex
Each point on the basilar membrane is sharply tuned

Question 35

Inner Ear Retrocochlear Pathway

Answer 35

Auditory reception area
Located in the temporal lobe of each of the two hemispheres of the brain
Perception of loudness and pitch controlled at level of brainstem
Why do you think loudness and pitch are controlled at this level?
Higher-level auditory behaviors (e.g., speech understanding) at the level of the auditory cortex