Structure and Function of the Auditory System Flashcards
anatomy
Anatomy: the science of structure of living things, including their systems, organs, and tissues
The body’s architecture
Physiology
Physiology: the science of function in living systems
What it does
In this case, we’re referring to normal function
CONDUCTIVE:
Outer ear
Middle ear
SENSORINEURAL
Inner ear (sensory)
Auditory nervous system (neural)
CN VIII
Central auditory nervous system
Three Parts of the Human Ear
Outer ear Acoustic energy
Middle ear Mechanical energy
Inner ear Hydraulic energy
CNVIII/Brain Electrical energy
Auricle: aka pinna
Visible portion of the ear
Serves mainly to collect sound waves and funnel them into external auditory meatus
External auditory meatus: aka ear canal
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
Cerumen
glands located in lateral 1/3rd of cartilaginous portion of EAM
Outer Ear Physiology
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
Acoustic amplification of the outer ear
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
middle Ear Anatomy
Tympanic membrane
Ossicles (malleus/incus/stapes)
Held in place by ligaments
Tensor tympani muscle
Stapedius muscle
Tympanum (air-filled space)
“Four walls”
Eustachian tube
Middle Ear Anatomy
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
Middle Ear Anatomy
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)
Middle Ear Primary function
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!
middle ear Secondary function
Equalizes pressure between tympanum and nasopharynx
“Garbage chute”
Eustachian Tube
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
middle ear Tertiary function
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
Inner Ear Anatomy
The inner ear is composed of two labyrinths
Petrous portion of each temporal bone
Osseous labyrinth:
Membranous labyrinth:
Auditory labyrinth:
Vestibular labyrinth:
Vestibule:
Osseous labyrinth
Osseous labyrinth: channel in the bone
Membranous labyrinth:
soft-tissue fluid-filled channels within osseous labyrinth containing end-organ structures of hearing and vestibular systems
Auditory labyrinth
: contains the organ of hearing (cochlea)
Nerve endings encode a sound’s frequency, intensity, and timing information
Vestibular labyrinth
contains the organ of balance and posture (semicircular canals), of which there are three (superior, lateral, and posterior, for pitch/roll/yaw)
Vestibule:
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
Inner Ear Anatomy
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)
Auditory labyrinth
-Scala media
—Basilar membrane
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
scala media
(contains endolymph, which is high K+/low Na+)
scala vestibuli
(contains perilymph, which is high Na+/low K+)
scala tympani
contains perilymph, which is high Na+/low K+)
Inner Ear Anatomy
Labyrinths
Auditory labyrinth
Organ of Corti:
Inner hair cells:
Outer hair cells:
Organ of Corti:
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
Inner hair cells
: one row, shaped like pears
Outer hair cells
form three to four other rows, test tube shape; “hairs” shaped like Ws or Vs
Basilar membrane
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
The stimulation from the stapes
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
Inner Ear Retrocochlear Pathway
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
