Hearing: The auditory system

Lecture 6- The auditory system

pitch

Frequency (cycles per second, Hertz, Hz)

Humans hear frequency range between 20 Hz to 20 kHz

loudness

Amplitude (log units decibels, dB)

Hair cells

Encode frequency, amplitude, and phase.

→ representation along the cochlea (tonotopy)

→ transmit to auditory fibers

The ear

The external, middle, and inner ear.

The auditory pathway

Cochlea → Cohlear Nucleus → Superior Olive → Inferior Colliculus → Thalamus (MGN) → Auditory cortex

The external ear

• focus sound waves on tympanic membrane (eardrum)

• boosts sound pressure 30 to 100-fold

• selective for frequencies around 3 kHz → related to speech processing, consonants (e.g. ba and pa) have energy in this frequency range (2-5 kHz)

Contains → pinna, concha, and auditory meatus

The middle ear

Transforms airborne sounds into vibrations that can be detected by cells (hair cells in inner ear) that sit in fluid

• Middle ear boosts air pressure 200-fold

→ Large Tympanic membrane funnels sound onto small oval window

→ Lever action of ossicles (3 ear bones – malleus, incus, stapes)

• Conductive hearing loss

• Two small muscles (tensor tympani, stapedius) are activated automatically by loud noises (or self-generated vocalization) and contract to protect inner ear

The inner ear

Includes the cochlea which transforms waveforms from sound pressure into neuronal signals

Normal sounds are complex waveforms (composed of multiple different frequencies)

→ Deconstructs complex waveforms into simple tones

Sensorineural hearing loss

Cochlea

transforms waveforms from sound pressure into neuronal signals

Includes:

• Oval Window

• Round window

• Basilar membrane

• Tectorial membrane

• Fluid Filled Chambers (perilymph and endolymph)

oval window

Where sound waves enter via ossicles (stapes)

round window

vibrates opposite to oval window → allows fluids in cochlea to move

Tonotopy

Topographical mapping of frequencies along the basilar membrane

→ The membrane and auditory nerve fibers are tuned to specific frequencies

→ Basal end and Apical end

Basal end

Is the narrow and stiff part the basilar membrane

→ responds (vibrates) well to high frequency sounds

Apical end

Is the wide and flexible part the basilar membrane

→ responds best to low frequency sounds

The organ of Corti

• Tranforms pressure waves into action potentials.

• The basilar membrane pushes the hair cells against the tectorial membrane as perilymphatic pressure waves pass (Shearing motion).

→ bends stereocilia on the hair cells, causing hyper- or depolarization.

Inner hair cells

(3,500)

Sensory receptors for hearing, constitute 95% of auditory nerve fibers that project to the brain

Outer hair cells

(12,000)

Receive efferent axons from the brain (superior olivary complex)

→ amplify the traveling wave

Tip links

Connect the tips of 2 adjacent stereocilia

→ transform shearing motion into receptor potential

→ movement opens and closes channels

K+ influx

__ __ in apical compartment (through stereocilia) leads to depolarization

Mechanoelectrical transduction by sereocilia in hair cells

At rest, a small fraction of channels are open.

The shear on the hair cells pulls on the tip links to open cation channels, leading to K+ influx and hair cell depolarization (B), or hyperpolarization (when closed, A)

→ biphasic receptor potential

Depolarization opens voltage-gated calcium channels in the cell soma

→ triggers glutamate release

→ induces action potentials in auditory nerve (CN 8)

Bisphasic receptor potential

Because some channels are open at rest, opening and closing of the channels results in a __ __ __

→ Sinusoidal receptor potential in response to sinusoidal sound pressure waves.

→ Enables receptor potential to follow signals up to 3 kHz (but not 20 kHz).

→ Only occurs in direction parallel to symmetry axis (0ᴼ)

“labeled-line” coding of frequencies

Tonotopy of basilar membrane is preserved at higher levels in the auditory pathway

Enables us to hear up to 20 kHz

Tuning curve threshold functions

→ Auditory fibers are tuned to characteristic frequencies

→ Hair cells release NT only when depolarized → auditory nerve fibers fire during the positive phase

Conductive hearing loss

Treated with an external hearing aid

→ amplifies sound

Sensorineural hearing loss

Damage to hair cells in the cochlea can be overcome with a cochlear implant

→ microphone and signal processor convert sounds into electrical stimulation patterns

Major auditory pathways

Auditory projections are organized in parallel

Auditory nerve innervates the cochlear nucleus in the brainstem

→ Tonotopic organization → Low frequencies terminate ventrally, while high frequencies terminate dorsally

From there, there are bilateral projections to the medial and lateral superior olive

→ uses bilateral inputs for sound localization

Medial superior olive

Acts as coincidence detector

Differences in timing of bilateral inputs (determined by length of axon connection) used to locate sound source.

Localize sounds below 3 kHz

Humans can distinguish interaural differences as small as 10 microseconds although transmission between neurons occurs in the millisecond range

lateral superior olive

Differences in intensity are used by the __ __ __ and the medial nucleus of the trapezoid body to locate sound

Localize sounds above 3 kHz

Above 2 kHz, the head acts as an obstacle for short, high-frequency waves, resulting in lower intensity signals in the distant ear.

Inferior colliculus

Located in midbrain

→ topographical representation of space

Neurons have a preferred elevation and a preferred horizontal direction.

Also respond to complex patterns

Auditory thalamus

Medial geniculate nucleus integrates combinations of frequencies

Specific time intervals

Similar to process in lateral superior olive, but for different frequencies and longer time-intervals

Auditory cortex

Located in temporal cortex

→ Maintains topographical map of cochlea

Primary auditory cortex (A1)

Projections from the vental division of the medial geniculate (thalamus) maintains tonotopic map

→ Adjacent belt areas receive projections from the medial & dorsal medial geniculate

• Combination-sensitive neurons

• Species-specific sounds

• Speech (Wernicke’s area)