Hearing/audition & vestibular system Flashcards
external ear structures
pinna and external auditory meatus
external ear function
collect sound waves and serves as a resonator
middle ear structures
ossicles: malleus, incus, stapes
structures: stapedius (stapes), tensor tympani (malleus)
eustachian tube: opens into middle ear, joints it to pharynx
middle ear functions
- efficient transmission of sound travelling in air to the fluid filled inner ear
- reflexively control the ossicular movements during loud noises
- adjust for pressure variations
inner ear structures
bony labyrinth
membranous labyrinth: cochlea, scala, media, tympani, vestiboli, oval and round windows, organ of corti
inner ear functions
Transform sound vibrations in fluid filled canals to neural impulses which travel in CNVIII towards CNS
function of stapedius and tensor tympani
Contract in response to a loud noise to decrease the movement of the ossicles to dampen the loud noise
clinical effects if stapedius and tensor tympani are weak
Damage = certain loud noises that are louder than normal; may also report having difficulty understanding speech when these muscles are paralyzed
pathway for sound from the receptor to the primary auditory cortex
RECEPTOR/1st ORDER
Receptor: hair cells in scala media
1st Order:
Bipolar cells → hair cells → vestibulocochlear nerve → brainstem → 2nd order neurons in cochlear nuclei
pathway for sound from the receptor to the primary auditory cortex
CENTRAL PATHWAYS/2nd ORDER
2 routes:
- Directly to inferior colliculus (midbrain)
- turn head in response to sound; reflexive or autonomic - Goes to superior olivary nucleus (pons)
- perception of sound
- goes up bilaterally and up lateral lemniscus
- terminates in medial geniculate nucleus (thalamus)
pathway for sound from the receptor to the primary auditory cortex
REFLEX PATHWAYS
Inferior colliculi (midbrain) and superior olivary nucleus send info to: superior collicluli, medial longidituinal fasciclus, vestibular nuclei, and SC to coordinate head/eye movements toward sound and to CN VII to blink
pathway for sound from the receptor to the primary auditory cortex
DESCENDING PATHWAYS
Provide feedback circuits to sharpen auditory signals and enhance sound localization by suppressing competing signals
pathway for sound from the receptor to the primary auditory cortex
BILATERAL REPRESENTATION
receives info from both ears
More connections from opposite ear than from ipsilateral ear
lesion in cortex will not result in deafness in contralateral ear because of this
function of superior olivary nucleus
- Receives binaural convergence into single neurons and this allows for the timing of input from one ear to be compared to the other.
- Destruction of this nucleus produces difficulties with localization of sound
- Coordinate reflexive movements of head and eyes towards sound!
- CN VII-causes you to blink in response to sound!
lesions of outer and middle ear
mild hearing loss in ipsilateral ear (conductive loss)
lesions of inner ear
CNVIII, cochlear nuclei: significant hearing loss on ipsilateral side (sensorineural hearing loss)
lesions of superior olivary nucleus
after SON: may not notice because bilateral
before SON: may have unilateral deficits
bilateral lesions in the cortex or brainstem affecting the nuclei or tracts
can cause bilateral hearing impairments that can be severe
Otitis Media
(middle ear infection) = most common cause of conductive hearing loss
Common problem in children; if chronic and not treated, may affected speech and language development
Most common cause of sensorineural hearing loss is
exposure to loud noise that damages the hair cells
Other causes of sensorineural hearing loss include
Meniere’s disease and Acoustic neuroma
Meniere’s disease
progressive deafness, tinnitus, and dizziness
motion sickness
Acoustic Neuroma
tumor affecting CN VII and CN VIII at cerebellopontine angle
blood supply to inner ear and lesion
Labyrinthine arteries: interruption of blood supply will affect structures in the inner ear (sensorineural hearing loss)
what does the peripheral sensory apparatus consist of and where does it lie
Consists of bony labyrinth and membranous labyrinth
Lies in inner ear
components of membranous labyrinth
lies w/in bony labyrinth; contains cochlea (hearing) and vestibular apparatus (vestibular functions)
components of vestibular apparatus
3 semicircular canals, the utricle, and the saccule
which 2 vestibular components are referred to as the otoliths
utricle and saccule (in vestibule)
how the otoliths sits
Maculae (hair cells) sit on specialized structures Saccule – sit on vertical plane sacs sit upright = vertical Utricle – sit on horizontal plane U = urn = dead = horizontal
how is the otoliths stimulated
Otolithic membranes contain calcium carbonate crystals called otoconia that overlie maculae
Movements of membranes cause bending of sensory hair cells and subsequent firing of vestibular nerve fibers
function of otoliths
Register forces related to linear acceleration in anterior-posterior, lateral, or occipito-caudal direction
Register static head position with respect to gravity
Regulate static equilibrium
orientation of semicircular canals
Each canal plane is perpendicular to the others
how pairs of canals function together
The 6 individual semicircular canals together become 3 coplanar pairs
Anterior canal on right operates with posterior canal on left
Posterior canal on right functions with anterior canal on left
2 horizontal canals operate together
When angular head motion occurs within shared plane, neural firing increases in vestibular nerve on one side and decreases in other
This push/pull allows CNS to detect direction of rotation
function of otoliths
register forces related to linear acceleration in anterior-posterior, lateral, or occipito-caudal direction.
They also register static head position with respect to gravity (regulate static equilibrium)
function of semicircular canals
Each pair responds selectively to angular/rotational motion in a particular direction
they are also responsible for detecting the velocity of head movements which enables individuals to generate eye movement that matches the velocity of head movements
pathway from receptor cells to the vestibular nuclei
1st order (bipolar) = Scarpa’s ganglion in internal auditory meatus
central processes travel in CN VIII, leaves internal auditory meatus, enter brainstem at pontomedullary junction
go directly to cerebellum through inferior cerebellar peduncle without synapsing
go to vestibular nuclei located in the lateral aspect of the pons and medulla
cerebellum
coordinates balance responses
Spinal cord via lateral vestibulospinal tracts
activate postural response
Spinal cord via medial vestibulospinal tract
for head righting reactions
Spinal cord via reticulospinal tracts
balance reactions
Motor nuclei of extraocular muscles via medial longitudinal fasciculus
coordination of head and eye movements and reflexive eye movements (vestibule-ocular reflex)
cortex
conscious perception of balance
reticular system and ANS
nausea
purpose of Vestibuloocular Reflex (VOR)
Motor response that generates eye movements to enable clear vision while the head is in motion
purpose of Vestibulospinal Reflex (VSR)
Motor response that generates compensatory body movement to maintain head and postural stability and to prevent falls slippery fall
nystagmus
normal reflexive rhythmic movement of eyeballs in response to either head rotation or visual fixation on a moving object
2 phases of nystagmus
Slow Phase: eyes slowly drift away from center field of vision
Fast phase: eyes suddenly return to central field of vision
stimulation of nystagmus
Normally stimulated by vestibular stimulation, visual (optokinetic) stimulation, or extreme lateral gaze: short duration
Professional skaters and ballet dancers can inhibit with visual feedback
pathologic nystagmus
abnormal nystagmus that is present at rest, with minimal stimulation, or that lasts excessively long after stimulation
Abnormal response: may be spontaneous, positional, or gaze evoked
Often a cardinal sign of vestibular dysfunction
lesion to CN VIII or damage to inner ear
Balance dysfunction, abnormal VOR and VSR, abnormal nystagmus, nausea, vertigo
lesion of vestibular nuclei
Balance dysfunction, abnormal VOR and VSR, abnormal nystagmus, nausea, vertigo AND motor issues
Benign paraoxysmal positional vertigo
sudden false sensation that you’re spinning, intense dizziness
triggered by certain head positions or movements; caused by otoconia that are dislodged in fluid-filled semicircular canals (interfere with normal fluid movement, causes inner ear to send false signals to brain)
