Exam 2 Hearing Science

2 components of the auditory system

peripheral auditory system (outer, middle, inner ear)

central auditory system (auditory brainstem and auditory forebrain)

3 components of the peripheral auditory system

Outer ear, middle ear, and inner ear

Anatomy of the outer ear

pinna and external ear canal

Physiology of the outer ear (4 functions)

i. Collect sound

ii. Sound pressure gain (amply middle frequencies 1-5 kHz)

iii. Sound localization

iv. Protection of the tympanic membrane

Sound pressure gain

peaked around 2.5 kHz

primary contribution from the concha and the outer ear

This gain is crucial for hearing sensitivity in the speech frequency range.

What is sound localization?

The process by which the location of sound is determined.

What cues are used for localizing sound?

Intensity and phase (time) difference.

pinna anatomy

visable, cartiligenous part of the ear

pinna physiology

sound localization in the midplane

acts as a funnel to direct sound waves towards the ear canal

external ear canal anatomy

a tube leading to the eardrum

external ear canal physiology

Provides an acoustic resonance effect, leading to sound pressure gain (peaking around 2.5 kHz)

middle ear anatomy

tympanic membrane

tympanic muscles

auditory ossicles

eustachian tube

Middle Ear Physiology

sound transmission and protection of hearing

tympanic membrane anatomy

eardrum

tympanic membrane physiology

Transduces acoustic energy signals (air molecule vibration) into mechanical motion.

auditory ossicles anatomy

malleus, incus, stapes

auditory ossicles physiology

Transduces the mechanical motion of the TM to fluid motion in the cochlea

Amplifies sound intensity across frequencies

At what frequency do the auditory ossicles provide peak amplification?

1000 Hz

How do the auditory ossicles transduce sound?

through transducing the mechanical motion of the ossicles to fluid motion in the cochlea (from the stapes moving in and out of the oval window)

Sound transfer functions of the middle ear

sound intensity amplified across frequencies with peaked amplification at 1000 Hz

Problem of sound transmission - What happens when vibration of sound wave transfers from the tympanic membrane in the air to fluid-filled cochlea?

Most of the acoustic energy will be reflected because the difference of acoustic impedance between the two sound media.

Solutions to the sound transmission problem

impedance mismatch problem - increase pressure/force at the oval window

sound mismatch issue

During the change between the ear drum and the fluid motion of the cochlea, the two mediums cause impedance mismatching. The sounds are mostly reflected (bouncing off an oval window).

Three mechanisms for impedance mismatch problem

area ratio, lever system, buckling of eardrum

Area Ratio (hydraulic affect)

Sound vibrates the large eardrum, but this force is concentrated onto the much smaller stapes footplate, greatly increasing pressure (approx. 18x gain)

lever system

The malleus and incus act as a lever, multiplying the force by about 2.1 times.

Buckling of ear drum

buckling motion increases the force transmitted to the middle ear's ossicles, effectively amplifying the sound pressure

acoustic reflexes

stapedius and tensor tympani (middle ear muscles) contract to lower sound transmission in the middle ear after receiving intense sounds

acoustic reflexes purpose

protects inner ear from intense sounds

tensor tympani physiology

contracts and increases tension on the tympanic membrane with intense sounds

stapedius physiology

contracts and works with the tensor tympani during high-intensity sounds to limit the motion of the ossicles and protect the inner ear.

two tympanic muscles

stapedius and tensor tympani

Eustachian tube anatomy

Connects the middle ear to the nasopharynx.

Eustachian tube physiology

Equalizes air pressure between the middle ear and the atmosphere.

2 inner ear structures

vestibular apparatus and cochlea

Where is the cochlea located?

Embedded in the temporal bone, medial to the middle ear cavity.

What is the shape of the cochlea?

Coiled shape with about 2.5 turns around a bony, hollow core called modiolus.

What are the dimensions of the cochlea?

35 mm long, 1 cm wide at the base, and 5 mm wide at the apex.

modiolus of cochlea

contains the auditory nerve and cell bodies of auditory neurons, which forms spiral ganglion surrounded by a ledge of bone (osseous spiral lamina)

The Three Scalae of the Cochlea

scala vestibuli, scala media (also called cochlear duct), scala tympani

Two membranes to separate scalae in cochlea

Reissner's membrane and basilar membrane

Helicoterma

joint opening at the apex of the cochlea that connects scala vestibuli and scala tympani

Helicotrema function

allow fluid (perilymph) to move between the scala vestibuli and scala tympani, helping relieve pressure at low-frequency sounds.

scala vestibuli anatomy

Filled with perilymph and connected to the oval window

scala vestibuli physiology

Transmitting sound vibrations from the oval window to the rest of the cochlea

scala media (cochlear duct) anatomy

Filled with endolymph and contains the Organ of Corti

scala media (cochlear duct) physiology

converts sound waves into electrical signals to the brain

scala tympani anatomy

filled with perilymph and connected to the round window

scala tympani physiology

role in transmitting sound vibration to the auditory nerve via a fluid called perilymph

perilymph

fluid inside of cochlea

basilar membrane

A structure that runs the length of the cochlea in the inner ear and holds the auditory receptors, called hair cells.

BM base

Narrow and stiff; resonates high frequencies (4-5 kHz)

BM apex

wide and flaccid; resonates low frequencies (below 1 kHz)

tonotopic organization

an arrangement in which neurons that respond to different frequencies and are organized anatomically in order of frequency

Basilar membrane function

Detects and separates different sound frequencies by vibrating at specific locations along its length, allowing the cochlea to encode pitch

Reissner's membrane anatomy

thin membrane inside the cochlea that separates the fluid-filled scala vestibuli from the scala media

Reissner's membrane function

maintain the separation of fluids (perilymph and endolymph), which is essential for proper inner-ear electrical balance and hearing

organ of corti anatomy

basilar membrane on the bottom, structure holding up outer and inner hair cells, outer hair cells with stereocilia, Reisner's/tectorial membrane on top

Outer Hair Cells (OHC)

Enhance the vibration of the basilar membrane, providing sensitivity and frequency selectivity (Cochlear Amplifier)

Inner Hair Cells (IHC)

The actual sensory transducers; convert mechanical motion into electrical signals.

True or False: We have more outer hair cells than inner hair cells

true

hair cells are also called

auditory receptors

supporting cells

- pilor and hensen cells

- structural and metabolic support to outer and inner hair cells

Stereocilia

hairlike extensions on the tips of hair cells in the cochlea that initiate the release of neurotransmitters when they are flexed

The stereocila need to be ______ to open the channel up

flat

Sound waves cause the ________ to vibrate.

basilar membrane

Traveling wave of the basilar membrane

displacement of BM caused by the flow of perilymph in the scala vestibuli and scala tympani

hair cell response to BM displacement

stereocillia bend, Mechanically gated K⁺ channels open, the hair cells are depolarized, which opens up voltage-gated Ca²⁺ channels, allowing for electrical waves to be sent to the brain and interpreted as sound

General sound transmission in cochlea

Flow of the fluid in the cochlea produced by the vibration of the stapes results in the displacement of basilar membrane.

frequency selectivity

The auditory system's ability to respond differentially to different frequencies and bands of frequencies

tuning curve

A graph that relates neural activity to a continuous range of stimulus properties

Characteristic frequency

The frequency of sound that a particular auditory nerve fiber is most sensitive to. It's the frequency that requires the least amount of sound energy for that region of the ear to respond.

Intensity Resolution

the ability to differentiate different sound levels

non-linear intensity resolution

Amplifying or dampening sounds to protect the ear

compression at characteristic frequency

A trick your inner ear uses to turn loud sounds way down, especially the exact sound frequency that the ear listens to, so that loud noises don't hurt you and soft noises are still heard.

Auditory Nerve

a direct synaptic connection between the hair cells of the cochlea and the cochlear nucleus

How many fibers are in each human ear?

30,000

Type I spiral ganglion cells

95% of spiral ganglion cells, many are connected to one IHC (20 fibers to one IHC)

Type II spiral ganglion cells

10% of spiral ganglion cells, connected to OHC, one to many (one fiber to 10 OHC)

spontaneous firing rate

baseline electrical activity (spikes/second) of auditory nerve fibers

High Spontaneous Rate (HSR)

Active even without sound; high sensitivity, low threshold.

low spontaneous rate (LSR)

Requires more intense sound; low sensitivity, high threshold.

Intensity Resolution

Auditory nerve firing rate increases with sound intensity

Intensity resolution threshold

The lowest sound level that causes a nerve fiber to start responding

Intensity resolution saturation

The highest sound level where the neuron's firing rates stops increasing

Intensity Resolution Dynamic Range

Range of sound intensities between threshold and saturation

20-50dB: Fiber can accurately represent sound intensity changes

frequency selectivity

AN fibers are sharply tuned to a specific characteristic frequency (CF).

Phase Locking

AN firing is synchronized to a specific phase of the stimulus waveform (up to 4-5 kHz)

central auditory system contains

auditory brainstem and auditory forebrain

Auditory brainstem contains

Cochlear nuclei (CN), Superior Olivary Complex (SOC), nuclei of Lateral lemniscus (NLL), Inferior Collicus (IC)

Auditory forebrain contains

Medial geniculate body (MGB) and Auditory Cortex

Cochlear Nuclei divisions

Anteroventral (AVCN), Posteroventral (PVCN), and Dorsal (DCN)

Cochlear Nuclei function

first relay center for auditory nerve fibers (complex response patterns)

superior olivary complex

Receives bilateral inputs and localizes sound

nuclei of lateral lemniscus (NLL)

Helps with processing timing and temporal patterns

inferior colliculus (IC)

Combines the analysis of complex sound and the direction in space simultaneously

medial geniculate body (MGB)

The thalamic relay station. Processes and relays specific, detailed auditory information to the auditory cortex.

Auditory cortex divisions

primary auditory cortex and secondary auditory cortex

Auditory cortex tonotopic organization

systematic mapping of sound frequency (pitch) along a physical structure in the auditory system. Frequency of mapping is maintained

AC detection of complex features

Neurons respond to specific patterns, such as frequency-modulation (FM) detectors and temporal-modulation detectors.