Physiology of the Auditory and Vestibular Systems (Pierce) Flashcards
Displacement along what results in our ability to hear sound
Basilar membrane
Basilar membrane
Narrow and stiff where?
What frequency of sounds does this region receive?
Near the oval/round windows
High frequency (bird singing or whistling)
Basilar membrane
Wider and more flexible where?
What frequency of sounds does this region receive?
Near the helicotrema/apex
Low frequency sound (Thunder or Tuba)
What kind of receptors are ear hair cells
They are mechanoreceptors. They convert mechanical signal to electrical signal
They receive afferent and efferent input, but are themselves not neuronal
What are the characteristics of the ear hair cells
- Polarized epithelial cells
- Have a basal and apical end
- Stereocilia exist on apical surface
- Stiff, graded in size and rich in actin
- Neural synapses on basal side
Endolymph
- Produced by Stria Vascularis
- Found in Scala Media
- Potassium Rich fluid that bathes the apical end of the hair cells
Perilymph
- Found in Scala vestibuli and scala tympani
- Potassium Poor fluid that bathes the Basal end of the cochlear hair cells
How do the hair cells convert mechanical signal to an electrical signal
- Sound causes deflection of the stereocilia that opens mechanically gated K+ channels, which allows K+ to enter the cell, depolarizing it.
- Change in membrane voltage opens TRPA1 channels, allowing Ca2+ to flow in, further depolarizing the cell and causing release of NTs glutamate and generation of an AP in CN VIII
NOT: K+ enters cells because stereocilia are bathed in endolymph
Deflection of streocilia towards the long end result in what
Depolarization
Deflection of streocilia towards the short end result in what
Hyperpolarization
Because K+ channels close so K+can’t enter cell
What is the Stria Vascularis
Highly vascularised area on the lateral wall of the Scala media (cochlear duct) that maintains the electrochemical properties of the endolymph by producing high levels of K+ into the endolymph
Blood labyrinth barrier (BLB)
formed by the stria vasculais. One of the main sites of drug entry to access the inner hair cells, sometimes to its detriment.
What is a common source of otoxic drug secretion into the cochlea
Stria vascularis (corsses BLB)
What can diminish the endocochlear potential
Any substance (medication, drugs, carbon monoxide, etc.) that disrupts the function of the stria vascularis. This will impact hearing.
What is the primary source of auditory information
Inner hair cells
What are the primary cells that amplify sound waves and how do they do this?
Outer hair cells
They are contractile cells. Their contractile properties boosts the mechanical vibrations of the basilar membrane
What is the arrangement of inner and outer hair cells
Inner: single layer
Outer: three rows
What do inner and outer hair cells synapse with
Inner: peripheral terminal of primary afferent neurons & efferent neurons
Outer: peripheral terminals of afferent neuroms from the spiral ganglion & efferent neurons
Impulses produced by the ear itself originate where and what are the sounds called
Superior olivary complex
olivocochlear efferents
Medial olivary complex neurons innervate ____ hair cells
Lateral olivary complex neurons innervate ____ hair cells
- Outer
- Inner
How are the sounds produced by the ears measured
Otoacoustic emissions (OAE)
What are the two types of Otoacoustic emissions (OAE)
- Spontaneous
- Occurs in 1/3 of people
- usually pure tones or clicks
- probably evoked by motile outer hair cells
- Evoked
* Used to test for hearing loss
What are the clinical importance of Otoacoustic Emissions
Newborn hearing screen
Tinnitus testing
Ototoxicity testing
Where does the central portion of the cochlear nerve (CN VIII) branch to?
Cochlear nuclear complex in the medulla
What is the cochlear nuclear complex comprised of
Dorsal (posterior) cochlear nuclei
Ventral (anterior) cochlear nuclei
Function of the dorsal (posterior) cochlear nuclei
Integrates the acoustic information with somatosensory information
Function of the ventral (anterior) cochlear nuclei
Begins processing the features of the sound (nature of sound - high,low)
What is the first site in the brainstem where information from both ears converges.
Superior olivary complex
This binaural processing is essential to accurately localize sound.
Function of the medial superior olivary nucleus (MSO)
- Generates a map of the interaural TIME difference which helps localize the location of the sound
- Receives excitatory input (glutamate &/or asparate)
This is important because sound is received by one 1 faster than the other
Function of the lateral superior olivary nucleus (LSO)
Generates a map of the interaural INTENSITY differences, which helps localize the source of the sound.
Information from the medial and lateral superior olivary nucleus converge where
Inferior Colliculus
Function of inferior colliculus
Suppresses information related to echoes, which would interfere with localization of sound
Together with echo suppression helps create a precise origin of sound location along the horizon.
What integrates precise information regarding intensity, frequency and binuaral properties of sound and relays them onward
Medial geniculate nucleus (MGN) of the thalamus
What structure is essential for the conscious perception of sound and higher order processing of sound (loudness, modulations in volume, rate of frequency modulation)
Primary auditory cortex (A1)
What is the tonotopic arrangement of the primary auditory cortex
More rostral areas are activated by low frequencies and more caudal areas respond to higher frequencies
Function of the auditory (secondary) association cortex
Responds to more comples sounds (music), identifying (naming) a sound and speech. Composed of multiple areas (Broca’s, Wenicke’s, etc.)
What are the efferent inputs to the auditory system
Olivocochlear Efferent
Middle ear muscle motoneurons
Autonomic Innervation of the inner ear
Where do Olicochlear Efferents originate and what is their function
Originate in the Superior Olivary Complex
They decrease basilar membrane motion which reduces responses of inner hair cells and auditory nerve fibers. This reduces the response to noise and may protect hair cells from damage due to intense sounds
Function of the middle ear efferents
- Includes motor innervation to the tensor tympani to the malleus and tympanic membrane and the stapedius to the stapes. Attenuates sound.
- Bilateral response to high sound levels.
- Contractions decrease transmission of sound.
- Act at low frequencies.
• May prevent damage.
- May prevent low frequency masking (improving speech discrimination).
- May prevent hair cells from damage due to intense sounds.
- Implicated in tinnitus.
What is the action of the autonomic efferents in the intrinsic protection of hair cells from intense sounds
- Can arise from CN VIII
- Caroticotympanic nerve off superior cervical ganglion: innervates the mucous glands of the tympanum and the blood vessels of the ear drum and all its contents.
- Acoustic nerve supplies blood vessel sympathetic regulates vascular tone in blood supply to the cochlea
Comprised of sympathetic adrenergic fibers
What is the purpose of cochlear prosthesis
Stimulate surving nerve fibers by threading multiple electrodes through the coclea
What causes sensorineural hearing loss
Damage to hair cells or nerve fibers or both through noise damage, ototoxic drugs, age related or loss of unknown etiology
Which end of the cochlea is more susceptible to damage:
Base or apical?
Base (high frequency)
Which is more susceptible to injury:
inner or outer hair cells
Outer hair cells
Injury to outer hair cells causes what?
Injury to inner hair cells causes what?
Outer: decreased sensitivity and broader tuning
Inner: Cuts off auditory inputs to the CNS
What force activates the Anterior Semicircular canal
Rotation in the vertical plane forwards maximally activates it.
falling forward
What force activates the Horizontal Semicircular canal
Rotation in the horizontal plane maximally activates it.
spinning
What force activates the Posterior Semicircular canal
Rotation in the vertical plane backwards maximally activates it.
falling backwards
What force activates the Urticle
Linear acceleration foward and backwards
Running
What force activates the Saccule
Detects linear acceleration up and down
jumping up and down
