CH7: AUDITOY SYSTEM

What is the middle ear?

An air-filled cavity between the tympanic membrane (eardrum) and the cochlea of the inner ear

What are the three ossicles (in order)?

Malleus → Incus → Stapes

What is the function of the ossicles?

They serve as a mechanical lever system to amplify sound vibrations

What is the Eustachian tube and what does it do?

A tube running from the middle ear to the nasopharynx; it ventilates the middle ear by equalizing air pressure with atmospheric pressure

What connects to the oval window?

The stapes (last ossicle in the chain)

What is the round window?

A membrane-covered opening that allows pressure release from fluid displacement in the cochlea

What structures form the inner ear?

Fluid-filled saccule, utricle, semicircular ducts, and cochlea

What are the three semicircular ducts?

Horizontal duct, Posterior duct, Anterior duct

What are the two functions of the inner ear?

Audition (hearing) and Equilibrium (balance)

Which structures mediate equilibrium?

Saccule, utricle, and semicircular ducts

What do the saccule and utricle sense?

Saccule = vertical linear acceleration; Utricle = horizontal linear acceleration

What do the semicircular ducts sense?

Angular acceleration

What is the bony labyrinth?

The outer bony wall of the inner ear

What is the membranous labyrinth?

The structure lodged within the bony labyrinth, filled with endolymph

What fluid fills the space between bony and membranous labyrinth?

Perilymph (extracellular fluid)

What fluid fills the membranous labyrinth?

Endolymph (intracellular fluid)

What is the modiolus?

The central bony axis of the cochlea, conical shaped, containing nerve fibers and housing the spiral ganglion

How many turns does the cochlea make?

2.5 turns

What are the three fluid-filled scalae (cavities)?

Scala vestibuli, Scala tympani, and Cochlear duct (scala media)

Which scalae contain perilymph?

Scala vestibuli and Scala tympani

Which scala contains endolymph?

Cochlear duct (scala media)

What is the approximate voltage of endolymph?

+80 mV

What is the approximate voltage of perilymph in scala tympani?

0 mV

What is the helicotrema?

The opening at the apex of the cochlea where scala vestibuli and scala tympani meet

What is the organ of Corti?

The sensory organ containing hair cells, located on the basilar membrane

What are the two types of hair cells?

Inner hair cells and Outer hair cells

Which type is more numerous?

Outer hair cells (more numerous than inner hair cells)

What percentage of cochlear nerves innervate inner hair cells?

95%

What is the function of inner hair cells?

Sensory cells - they detect sound and send signals to the brain

What is the function of outer hair cells?

Amplifiers - they amplify vibrations of the basilar membrane

What are stereocilia?

Hair-like projections on hair cells that bend in response to movement

What are tip links?

Filaments connecting adjacent stereocilia that open ion channels when stretched

What happens when hair bundles are deflected?

Tip links stretch → increased permeability to potassium ions → depolarization

Step 1: What happens when sound waves arrive?

Sound waves strike the tympanic membrane (eardrum), causing it to vibrate

Step 2: How is energy transformed in the middle ear?

TM vibration converts pressure waves into mechanical energy, setting ossicles in motion

Step 3: How is energy transformed in the inner ear?

Mechanical energy becomes hydraulic energy in cochlear fluid

Step 4: What do hydraulic waves stimulate?

The basilar membrane

Step 5: What produces the final mechanical displacement?

Basilar membrane movement displaces cilia of hair cells relative to the tectorial membrane

Step 6: How are neural impulses generated?

Deflection stretches tip links → K+ channels open → hair cells depolarize

Step 7: Where do cochlear hair cells send impulses?

Through the vestibulocochlear nerve (CN VIII) to cochlear nuclei in the brainstem

What is the ultimate transformation in audition?

Sound vibrations → neural impulses

Where do high frequency (high pitch) sounds act?

At the base of the cochlea (short waves)

Where do low frequency (low pitch) sounds act?

At the apex of the cochlea (long waves)

What is tonotopic organization?

The systematic mapping of sound frequencies onto anatomical locations

What are otoacoustic emissions?

Sounds emitted BY the ear (up to ~20 dB), caused by outer hair cell activity

How do vibrations travel backward?

Through perilymph and middle ear ossicles to vibrate the tympanic membrane

Why are otoacoustic emissions clinically important?

They are the basis for important audiological tests (hearing screening)

What is the first central synapse in the auditory pathway?

Cochlear nuclei (dorsal and ventral) in the medulla

What is the first brainstem nucleus receiving bilateral input?

Superior olivary complex

What does the superior olivary complex do?

Compares time of arrival and intensity from both ears for sound localization

What comes after the superior olivary complex?

Nuclei of the lateral lemniscus

What structure is in the midbrain?

Inferior colliculus

What is the thalamic relay nucleus for hearing?

Medial geniculate body (MGB)

Where is the primary auditory cortex located?

Transverse Heschl's gyrus (Brodmann area 41), embedded in the lateral sulcus

What is the tonotopic arrangement in PAC?

Higher frequencies = posteromedial; Lower frequencies = anterolateral

What are three possible roles of descending auditory pathways?

1) Selective attention, 2) Protection from intense sound, 3) Adjust input to maintain equal binaural input

What is the acoustic reflex?

Contraction of middle ear muscles in response to loud sounds

What does the acoustic reflex do?

Protects the inner ear and helps extract meaningful sounds from noisy backgrounds

Where do olivocochlear neurons end?

At the base of hair cells, forming inhibitory synapses

What is the "Where" pathway?

Parietal → lateral prefrontal; processes location of sounds

What is the "What" pathway?

Anterior temporal → inferior frontal; processes identification of sounds

What is Wernicke's area and where is it?

Located in BA 22 (auditory association area); analyzes and interprets auditory signals

What is the McGurk effect?

An auditory illusion where auditory and visual speech signals conflict, creating a fused perception

What is unique about the auditory pathway compared to visual?

Auditory has multiple synaptic stops in brainstem; Visual goes directly from retina to thalamus (LGN)

What is the somatosensory thalamic relay?

Ventroposterolateral nucleus

What is the visual thalamic relay?

Lateral geniculate nucleus

What is the auditory thalamic relay?

Medial geniculate nucleus

BA 41

• Primary auditory cortex

• Tonotopic processing of frequency, loudness, temporal info

BA 42

• Auditory association

• Adjacent to BA 41

BA 22

• Auditory association cortex

• Complex processing; Wernicke's area for language comprehension