Lecture #17: Anatomy and Physiology of the Auditory System

What is the primary function of the outer ear?

The outer ear captures sound pressure waves from the environment and efficiently channels them toward the tympanic membrane for transmission into the middle ear.

What structures make up the outer ear?

The outer ear consists of the pinna (auricle) and the external auditory canal, which together funnel sound waves toward the tympanic membrane.

What is the role of the tympanic membrane?

The tympanic membrane converts airborne sound waves into mechanical vibrations that are transmitted to the auditory ossicles.

Which bones make up the auditory ossicles?

The auditory ossicles are the malleus, incus, and stapes, the smallest bones in the human body, which transmit vibrations from the tympanic membrane to the oval window.

What is the function of the middle ear?

The middle ear amplifies and converts air-based sound vibrations into fluid-based vibrations that can propagate through the inner ear.

Why is amplification needed in the middle ear?

Air does not efficiently transmit vibrations into fluid, so amplification is required to overcome the impedance mismatch between air and the fluid-filled cochlea.

How does the middle ear increase sound pressure entering the cochlea?

The large surface area of the tympanic membrane relative to the small oval window increases pressure delivered to the cochlear fluid.

What is the role of the Eustachian tube?

The Eustachian tube equalizes air pressure on both sides of the tympanic membrane to allow proper vibration.

What causes conductive hearing loss?

Conductive hearing loss results from impaired transmission of sound through the outer or middle ear, such as fluid, ossicle damage, or tympanic membrane perforation.

Why does tympanic membrane perforation reduce hearing?

Perforation equalizes pressure across the membrane, preventing effective vibration and eliminating ossicular amplification.

What muscles protect the inner ear from loud sounds?

The tensor tympani and stapedius muscles contract reflexively to reduce sound transmission and protect cochlear hair cells.

Which nerve innervates the tensor tympani muscle?

The tensor tympani is innervated by the trigeminal nerve (CN V).

Which nerve innervates the stapedius muscle?

The stapedius muscle is innervated by the facial nerve (CN VII).

Where is the cochlea located?

The cochlea is located in the petrous portion of the temporal bone and contains approximately 2.5 spiral turns.

How does the cochlea encode sound frequency?

The cochlea uses place coding, where different sound frequencies maximally vibrate different locations along the basilar membrane.

Which frequencies are detected at the base of the cochlea?

High-frequency sounds are detected at the base of the cochlea where the basilar membrane is narrow and stiff.

Which frequencies are detected at the apex of the cochlea?

Low-frequency sounds are detected at the apex of the cochlea where the basilar membrane is wider and more flexible.

What are the three main fluid-filled chambers of the cochlea?

The scala vestibuli, scala media (cochlear duct), and scala tympani.

What is the ionic difference between endolymph and perilymph?

Endolymph has a high potassium concentration, while perilymph has a low potassium concentration.

What structure contains the organ of Corti?

The organ of Corti is located within the cochlear duct and is the sensory organ for hearing.

What are the two types of cochlear hair cells?

Inner hair cells and outer hair cells, arranged in one inner row and three outer rows.

Which hair cells are the primary sensory receptors for hearing?

Inner hair cells are the primary sensory receptors and provide the majority of auditory nerve input to the brain.

What is the function of outer hair cells?

Outer hair cells amplify basilar membrane motion and enhance sensitivity and frequency selectivity.

What are stereocilia?

Stereocilia are actin-based projections on hair cells that convert mechanical deflection into electrical signals.

What are tip links and why are they important?

Tip links connect stereocilia and mechanically gate mechanoelectrical transduction channels.

How does stereocilia deflection lead to depolarization?

Deflection opens mechanosensitive channels, allowing potassium influx from endolymph and depolarizing the hair cell.

Why does potassium exit the hair cell after depolarization?

The base of the hair cell is bathed in potassium-poor perilymph, allowing potassium efflux to repolarize the membrane.

Do hair cells generate action potentials?

Hair cells do not generate action potentials; they release glutamate in a graded fashion onto spiral ganglion neurons.

What forms the auditory nerve (CN VIII)?

Axons of spiral ganglion neurons form the auditory portion of cranial nerve VIII.

Where does the auditory pathway first synapse in the brainstem?

Auditory nerve fibers synapse in the ipsilateral cochlear nucleus in the rostral medulla.

Why is sound localization possible in the brainstem?

Bilateral projections to the superior olivary complex allow comparison of timing and intensity differences between ears.

What tract carries auditory information through the brainstem?

Auditory fibers ascend through the lateral lemniscus.

What is the role of the inferior colliculus?

The inferior colliculus integrates auditory information and relays it to the thalamus.

Which thalamic nucleus processes auditory information?

The medial geniculate body serves as the auditory relay nucleus of the thalamus.

Where is the primary auditory cortex located?

The primary auditory cortex is located in the temporal lobe.

What is tonotopy?

Tonotopy is the ordered representation of sound frequency that is preserved from the cochlea through the auditory cortex.

What is the olivocochlear system?

A descending pathway from the superior olivary complex that modulates cochlear hair cell activity.

How do conductive and sensorineural hearing loss differ?

Conductive loss affects sound transmission in the outer or middle ear, while sensorineural loss affects the inner ear, cochlea, or auditory nerve.

Which type of hearing loss is often correctable?

Conductive hearing loss is often correctable with medical or surgical treatment.

What causes sensorineural hearing loss most commonly?

Sensorineural hearing loss is commonly caused by hair cell damage due to noise, aging, drugs, or genetic conditions.

What is presbycusis?

Presbycusis is age-related sensorineural hearing loss that primarily affects high frequencies.

What is tinnitus and what causes it?

Tinnitus is the perception of sound without an external source, often caused by hyperactivity of outer hair cells.

What is a vestibular schwannoma?

A benign, slow-growing tumor of the vestibulocochlear nerve that can cause hearing loss, tinnitus, and vertigo.

What is the most useful diagnostic test for vestibular schwannoma?

MRI of the brain is the most useful diagnostic test.

What tests evaluate conductive vs sensorineural hearing loss clinically?

The Weber and Rinne tuning fork tests help differentiate conductive from sensorineural hearing loss.

How does the Rinne test work?

It compares air conduction to bone conduction; air conduction should be better than bone conduction in normal hearing.

How does the Weber test help localize hearing loss?

Sound localizes to the affected ear in conductive loss and to the unaffected ear in sensorineural loss.

What is the overall cause of hearing loss at the cellular level?

Hearing loss results from dysfunction of mechanotransduction or synaptic transmission along the auditory pathway.