HIS Anatomy of the Auditory System 1 — Questions and Answers
Question 1: What are the three ossicles of the middle ear and what is their function?
- Cochlea, vestibule, semicircular canals—they convert electrical signals to sound
- Malleus, incus, stapes—they form a mechanical lever system that transmits and amplifies vibrations from the tympanic membrane to the oval window of the cochlea (Correct answer)
- Eustachian tube, round window, oval window—they equalize pressure in the middle ear
- Helicotrema, basilar membrane, Reissner's membrane—they separate cochlear compartments
Correct answer: Malleus, incus, stapes—they form a mechanical lever system that transmits and amplifies vibrations from the tympanic membrane to the oval window of the cochlea
The malleus attaches to the tympanic membrane, connects to the incus, which connects to the stapes whose footplate covers the oval window—they transmit and amplify sound vibrations across the middle ear by approximately 25-30 dB through mechanical advantage.
Sound vibrations cause the tympanic membrane to vibrate, moving the malleus (attached to the drum). The malleus connects to the incus, which connects to the stapes. The stapes footplate presses on the oval window membrane, transmitting vibrations to the cochlear fluids. The ossicular chain provides impedance matching (converting air vibrations to fluid vibrations): the lever action of the malleus-incus system provides about 1.3:1 mechanical advantage, and the area ratio of the tympanic membrane to the oval window (~17:1) provides most of the ~25-30 dB gain needed to overcome the impedance mismatch between air and cochlear fluid.
Question 2: What is the function of the Eustachian tube?
- To transmit sound from the outer ear to the middle ear
- To equalize air pressure between the middle ear cavity and the nasopharynx (atmosphere), maintaining the pressure differential needed for optimal tympanic membrane vibration (Correct answer)
- To drain endolymph from the cochlea
- To connect the cochlea to the vestibular system
Correct answer: To equalize air pressure between the middle ear cavity and the nasopharynx (atmosphere), maintaining the pressure differential needed for optimal tympanic membrane vibration
The Eustachian tube connects the middle ear to the nasopharynx and opens briefly during swallowing and yawning to equalize middle ear pressure with atmospheric pressure, essential for normal tympanic membrane function.
The Eustachian tube (auditory tube) runs from the middle ear cavity to the nasopharynx and is normally closed, opening briefly with swallowing, yawning, or sneezing through contraction of the tensor veli palatini muscle. Its primary function is to equalize middle ear pressure with ambient atmospheric pressure; failure causes negative middle ear pressure and, chronically, otitis media with effusion (glue ear). The tube also drains secretions from the middle ear. Children's Eustachian tubes are shorter and more horizontal, predisposing them to otitis media.
Question 3: What is the basilar membrane and how does it contribute to frequency analysis in the cochlea?
- The outer membrane of the tympanic cavity that vibrates to sound
- A membrane running the length of the cochlea that is narrow and stiff at the base (responding to high frequencies) and wider and more flexible at the apex (responding to low frequencies), creating a tonotopic frequency map (Correct answer)
- The membrane separating scala vestibuli from scala media
- A stiff membrane that reflects high-frequency sounds away from the cochlea
Correct answer: A membrane running the length of the cochlea that is narrow and stiff at the base (responding to high frequencies) and wider and more flexible at the apex (responding to low frequencies), creating a tonotopic frequency map
The basilar membrane has graded mechanical properties creating a traveling wave that peaks at different locations for different frequencies—high frequencies peak near the base, low frequencies near the apex—forming the cochlea's tonotopic frequency map.
Georg von Bekesy discovered the traveling wave on the basilar membrane (Nobel Prize 1961). The basilar membrane changes gradually from narrow and stiff at the basal (oval window) end to wide and flexible at the apical (helicotrema) end. High-frequency sounds cause peak displacement near the stiff basal end; low-frequency sounds travel further along the membrane and peak near the flexible apex. This mechanical frequency analysis is the physical basis of pitch coding. Outer hair cells enhance frequency selectivity through active amplification (electromotility), sharpening tuning curves beyond what passive mechanics can achieve.
Question 4: What is the role of the outer hair cells (OHCs) in the cochlea?
- To transduce sound and send action potentials to the auditory nerve (same as inner hair cells)
- To act as active mechanical amplifiers, changing length in response to electrical signals (electromotility) to boost basilar membrane vibration and sharpen frequency tuning—they power the cochlear amplifier (Correct answer)
- To produce endolymph and maintain the endocochlear potential
- To stabilize the basilar membrane against excessive displacement
Correct answer: To act as active mechanical amplifiers, changing length in response to electrical signals (electromotility) to boost basilar membrane vibration and sharpen frequency tuning—they power the cochlear amplifier
Outer hair cells are unique mechanomotor cells that amplify basilar membrane motion through voltage-driven length changes (electromotility via prestin), enhancing sensitivity by 40-60 dB and sharpening frequency selectivity—damage to OHCs is the most common cause of sensorineural hearing loss.
The mammalian cochlea contains about 3,500 inner hair cells (IHCs) and 12,000 outer hair cells (OHCs). IHCs are the primary sensory transducers with approximately 95% of afferent nerve fibers synapsing on them. OHCs contain the motor protein prestin in their lateral walls and change length rapidly in response to changes in membrane potential, amplifying basilar membrane movement by 40-60 dB for soft sounds. OHCs are selectively vulnerable to noise, aging, ototoxins, and genetic mutations—their loss causes the elevated thresholds and reduced frequency selectivity characteristic of sensorineural hearing loss. Otoacoustic emissions (OAEs) are a byproduct of OHC electromotility.
Question 5: Which cranial nerve carries auditory and vestibular information from the inner ear to the brainstem?
- Cranial nerve V (trigeminal)
- Cranial nerve VII (facial)
- Cranial nerve VIII (vestibulocochlear) (Correct answer)
- Cranial nerve IX (glossopharyngeal)
Correct answer: Cranial nerve VIII (vestibulocochlear)
The vestibulocochlear nerve (CN VIII) has two divisions: the cochlear branch (auditory information from the spiral ganglion and hair cells) and the vestibular branch (balance information from the semicircular canals and otolith organs).
The VIII cranial nerve arises from the internal auditory canal (IAC) and divides into the cochlear nerve (arising from spiral ganglion neurons that synapse on inner hair cells) and the vestibular nerve (superior and inferior branches, arising from Scarpa's ganglion, innervating the three semicircular canal ampullae, utricle, and saccule). CN VIII travels through the internal auditory canal alongside CN VII (facial nerve) and enters the brainstem at the pontomedullary junction. Acoustic neuromas (vestibular schwannomas) are benign tumors of the CN VIII Schwann cells, typically presenting with unilateral sensorineural hearing loss, tinnitus, and vestibular symptoms.
Question 6: What is the endocochlear potential and why is it important for hearing?
- The voltage generated by the tympanic membrane during vibration
- A +80 mV electrical potential in the endolymph of the scala media produced by the stria vascularis, which drives ion flow through hair cell transduction channels and powers sound transduction (Correct answer)
- The resting potential of spiral ganglion neurons
- The voltage required to activate the stapedius muscle reflex
Correct answer: A +80 mV electrical potential in the endolymph of the scala media produced by the stria vascularis, which drives ion flow through hair cell transduction channels and powers sound transduction
The endocochlear potential (EP) is a +80 mV resting potential maintained by the stria vascularis in scala media; combined with the hair cell resting potential of approximately -45 mV (IHC) or -70 mV (OHC), it creates a driving force for mechanotransduction when stereocilia deflect.
The stria vascularis maintains high K+ concentration and a +80 mV positive electrical potential in the endolymph. When stereocilia are deflected by basilar membrane movement, mechanically gated K+ channels open, and K+ flows down its electrochemical gradient (from +80 mV endolymph into the -45 to -70 mV cell interior)—a large electrochemical driving force of 125-150 mV. This K+ influx depolarizes the hair cell, opening Ca2+ channels and triggering neurotransmitter (glutamate) release onto afferent nerve fibers. Genetic mutations affecting K+ recycling (e.g., connexin 26/GJB2) can disrupt the EP and cause profound congenital deafness.
HIS Anatomy of the Auditory System 1
What are the three ossicles of the middle ear and what is their function?