HIS Anatomy of the Auditory System 2 — Questions and Answers
Question 1: What are the three scalae (fluid-filled compartments) of the cochlea and what fluid fills each?
- Scala vestibuli (perilymph), scala media (endolymph), scala tympani (perilymph) (Correct answer)
- Scala anterior (blood), scala media (cerebrospinal fluid), scala posterior (lymph)
- Scala vestibuli (endolymph), scala media (perilymph), scala tympani (cerebrospinal fluid)
- All three compartments are filled with perilymph
Correct answer: Scala vestibuli (perilymph), scala media (endolymph), scala tympani (perilymph)
The cochlea has three fluid-filled compartments: scala vestibuli (above basilar membrane) and scala tympani (below, connected at apex via helicotrema) contain perilymph (high Na+); scala media (cochlear duct) contains endolymph (high K+).
Perilymph fills scala vestibuli and scala tympani; it is similar in ionic composition to extracellular fluid (high Na+, low K+) and is continuous via the helicotrema at the cochlear apex. Endolymph fills scala media (the cochlear duct); it has a unique high K+, low Na+ composition maintained by the stria vascularis. Scala media is separated from scala vestibuli by Reissner's membrane and from scala tympani by the basilar membrane (which supports the organ of Corti). The high K+ endolymph is essential for hair cell transduction.
Question 2: What is the pinna (auricle) and what acoustic function does it serve?
- The inner ear structure containing the cochlea
- The external, cartilaginous visible part of the ear that helps collect sound, and whose asymmetric folds create direction-dependent spectral cues (HRTFs) enabling sound localization, particularly in the vertical plane (Correct answer)
- The membrane at the end of the ear canal
- A structure in the middle ear that connects the malleus to the eardrum
Correct answer: The external, cartilaginous visible part of the ear that helps collect sound, and whose asymmetric folds create direction-dependent spectral cues (HRTFs) enabling sound localization, particularly in the vertical plane
The pinna collects sound and directs it into the ear canal; its complex asymmetric folds create frequency-dependent reflections (head-related transfer functions/HRTFs) that encode sound elevation (vertical localization) cues.
The pinna's irregular cartilaginous folds—helix, antihelix, tragus, concha—create direction-dependent filtering of sound. Sound from above reflects differently than sound from below, creating spectral peaks and notches (HRTFs) that the auditory system uses to determine sound elevation. The pinna also provides modest gain of 5-20 dB in the 2-6 kHz range, boosting speech frequencies. Loss of pinna function (e.g., microphone placement in CIC hearing aids at canal entrance) can impair vertical localization and cause front-back confusion.
Question 3: What is the acoustic reflex (stapedius reflex) and what is its protective function?
- A reflex that opens the Eustachian tube in response to sound
- A bilateral reflex contraction of the stapedius muscle triggered by loud sounds (approximately 70-80 dB above threshold), stiffening the ossicular chain to reduce low-frequency sound transmission and protect the cochlea (Correct answer)
- A reflex that dilates the ear canal in response to loud sounds
- A reflex that reduces tympanic membrane vibration in response to speech
Correct answer: A bilateral reflex contraction of the stapedius muscle triggered by loud sounds (approximately 70-80 dB above threshold), stiffening the ossicular chain to reduce low-frequency sound transmission and protect the cochlea
The stapedius reflex contracts the smallest skeletal muscle (stapedius, innervated by CN VII) in response to loud sounds, stiffening the ossicular chain, reducing transmission of low-frequency sounds by up to 15 dB to protect the cochlea from acoustic trauma.
The acoustic (stapedius) reflex is mediated by an arc: cochlea, CN VIII, cochlear nuclei, superior olivary complex, facial nerve motor neurons, and stapedius muscle (posterior wall of middle ear, attached to stapes neck). When activated, the stapedius contracts, pulling the stapes posteriorly and stiffening the ossicular chain. This attenuates low-frequency sound transmission by about 10-15 dB, providing partial protection against sustained loud sounds. The reflex has a latency of 25-150 ms, making it ineffective against sudden impulse noise such as gunshots.
Question 4: What is the organ of Corti and where is it located?
- A sensory organ in the vestibule that detects gravity
- The sensory epithelium resting on the basilar membrane in the scala media, containing inner and outer hair cells with their supporting cells, where mechanical vibration is converted into neural signals (Correct answer)
- A cartilaginous structure in the external ear canal
- The ganglion in the modiolus containing auditory neuron cell bodies
Correct answer: The sensory epithelium resting on the basilar membrane in the scala media, containing inner and outer hair cells with their supporting cells, where mechanical vibration is converted into neural signals
The organ of Corti sits on the basilar membrane within scala media, housing approximately 3,500 inner hair cells and 12,000 outer hair cells arranged in rows, surrounded by supporting cells, capped by the tectorial membrane.
The organ of Corti is the sensory epithelium of the cochlea. It sits on the basilar membrane and is covered by the tectorial membrane (a gelatinous structure attached to the spiral limbus). Key components: one row of inner hair cells (~3,500, the primary sensory cells), three rows of outer hair cells (~12,000, the cochlear amplifier cells), and supporting cells (pillar cells, Deiters' cells, Hensen's cells, Claudius' cells). Basilar membrane vibration deflects the stereocilia due to shear motion between the basilar membrane and tectorial membrane, opening mechanotransduction channels and initiating the neural response.
Question 5: What are otoacoustic emissions (OAEs) and what do their presence indicate?
- Sounds produced by the tympanic membrane that can be recorded in the ear canal as evidence of normal middle ear function
- Low-level sounds generated by outer hair cell electromotility in the cochlea, recordable in the ear canal, indicating that OHCs are present and functioning normally (Correct answer)
- Electrical potentials recorded from the auditory nerve using surface electrodes
- Sounds emitted from hearing aids that indicate optimal feedback cancellation
Correct answer: Low-level sounds generated by outer hair cell electromotility in the cochlea, recordable in the ear canal, indicating that OHCs are present and functioning normally
OAEs are sounds produced by active OHC electromotility in the cochlea; their presence in the ear canal (recorded with a sensitive microphone) indicates OHCs are functioning, making them a valuable screening tool for outer hair cell integrity.
OHC electromotility not only amplifies basilar membrane motion but also produces back-propagating acoustic energy emitted from the tympanic membrane into the ear canal. This faint sound can be recorded with a sensitive microphone sealed in the ear canal. Types include: Spontaneous OAEs (SOAEs) present without stimulation in ~70% of normal ears; Transient Evoked OAEs (TEOAEs) in response to click stimuli; Distortion Product OAEs (DPOAEs) in response to two simultaneous pure tones. OAEs are absent when OHC function is compromised and are used for universal newborn hearing screening.
Question 6: What is the difference between the utricle and saccule (otolith organs) and the semicircular canals in vestibular function?
- The utricle and saccule detect angular acceleration; semicircular canals detect linear acceleration and gravity
- The utricle and saccule (with otolith crystals) detect linear acceleration and static head position (gravity); the three semicircular canals detect angular (rotational) acceleration in three planes (Correct answer)
- Both structures detect sound vibration; otolith organs detect high frequencies, canals detect low frequencies
- These structures have no vestibular function—they contribute only to bone conduction hearing
Correct answer: The utricle and saccule (with otolith crystals) detect linear acceleration and static head position (gravity); the three semicircular canals detect angular (rotational) acceleration in three planes
The utricle detects horizontal linear acceleration and head tilt; the saccule detects vertical linear acceleration (gravity). The three semicircular canals (horizontal, anterior, posterior) detect rotational head movements in three orthogonal planes.
The vestibular system has five sensory organs in each inner ear: two otolith organs (maculae) and three semicircular canal ampullae. The utricle (horizontal macula) detects horizontal linear acceleration and static head tilt relative to gravity; the saccule (vertical macula) detects vertical linear acceleration (relevant for detecting movement in an elevator). Both contain otoconia (calcium carbonate crystals) overlying sensory hair cells. The three semicircular canals (horizontal, anterior, posterior) are oriented in three orthogonal planes; fluid inertia during head rotation deflects the cupula in the ampulla, activating hair cells to signal angular velocity.
HIS Anatomy of the Auditory System 2
What are the three scalae (fluid-filled compartments) of the cochlea and what fluid fills each?