The Auditory And Vestibular System Flashcards
What are the three parts of the ear?
External (outer)
Middle
Inner
What is a major component that determines the way the ear functions?
Structure
What are the 3 parts of the external ear?
Auricle (pinna)- gathers sound waves, brings them inward
External auditory meatus- amplifies sound waves and directs then medially (inward), forms funnel of sound
Tympanic membrane- vibrates in response to sounds waves
What is the air filled space in the temporal bone?
Middle ear
What are the functions and names of the 3 middle ear bones (ossicles)?
Mechanical displacement of the tympanic membrane, Produced by changes in sound pressure waves, is transmitted to the inner ear
◦ Malleus
◦ Incus
◦ Stapes
What is the final bone of the middle ear that transits sound waves inner?
Stapes
Describe the inner ear (cochlea).
Coiled structure housed within the temporal bone
Contains hair cells
What is the function of hair cells in the inner ear?
Auditory receptors
Sensitive to range of sounds that transit to brainstem
What are the 2 parts of the cochlea, what do they contain?
Bony labyrnth- contains perilymph (stapes movement stimulates perilymph)
Membranous labrynth (inner to bony)- within cochlear duct, contains organ of corti filled with endolymph
From where do the hair cells project in the organ of corti?
Basilar membrane
To where do the hair cells project towards in the organ of corti?
Tectorial membrane
Describe the mechanism that results from movement of fluid and movement of hair cells?
Movement of fluid moves the basilar membrane, causing the hair cells to bend
Movement of the hair cells causes changes in
their membrane potentials causing signals to brain
Describe the pathway of Cochlear reception and transduction of auditory stimuli.
Sound (airborne pressure waves) converted to mechanical energy by vibration of tympanic membrane and sequential movement of ossicles
Movement of stapes pulls or pushes against the oval window of inner ear creating sinusoidal pressure waves in the perilymph
Fluid pressure waves move basilar membrane
Hair cells sense movement through stereocilia
Stereocilia bend
Depolarization or hyperpolarization
Describe how depolarization occurs in the transduction pathway.
Basilar membrane moves upward to scale vestibuli
Hair cells are depolarized (Influx of K+ opens voltage-gated Ca+ channels)
Increases afferent pathway
Describe the hyperpolarization of hair cells in the transduction pathway.
Movement of the basilar membrane downward,
toward scala tympani
Decreases afferent pathway
What are the three transmitting components that are involved in the reception and transduction of auditory stimuli?
Sound
Mechanical
Pressure
Describe the function of the inner ear.
Afferent
Frequency and fine discrimination of hearing
Fewer in number, small receptive fields
Sensory to CNS for ability to hear
Describe the outer hair cells.
Efferent
Receives info from auditory cortex (brain)
Modulates the sensitivity of the organ of corti (block out background noise)
How do we lose our hearing and why?
Hair cell damage since they cannot be mitotically replaced
Decline with aging
exacerbated by ear infection, loud sounds or ototoxic
Cuts off sensory which can lead to neural disease (dementia)
What forms the cochlear nerve? Where are their cell bodies located (step 1)?
The central processes of the inner hair cells form the cochlear nerve and their cell bodies are located in the spiral ganglion
How does the auditory nerve (cochlear nerve) enter into the cranial cavity? Where does it enter in the brainstem?
Pass via the internal acoustic meatus
Enter at pontomedullary junction
Where do the axons of the cochlear nerve travel to and terminate within the brainstem?
Dorsal and ventral cochlear nuclei
Describe the distribution between ipsilateral and contralateral sides? How is this beneficial?
Most auditory info crosses over, each cerebral hemisphere processes from both the ipsilateral and contralateral sides
Preserves hearing during brain damage to one hemisphere
More processing
Localization of sound
Described how the axons of the cochlear nuclei cross between hemispheres and where they go from there.
Most cross over and ascend to contralateral superior olive at pontine level of brainstem
some ascend to ipsilateral superior olive without crossing
Fibers then ascend to inferior colliculus at midbrain level in lateral lemniscus tract
(Olive to inferior colliculus)
From the inferior colliculus where does the fibers of the cochlear nerve ascend to next? Are they ipsilateral or contralateral?
Project to thalamus (medial geniculate nucleus)
Mostly ipsilateral but some cross over
From the thalamus (medial geniculate nucleus) where do the fibers pass?
Pass laterally to join internal capsule
Then project to primary auditory cortex in temporal lobe
Within the primary auditory cortex where is hearing processed?
transverse temporal gyrus (Heschl’s gyrus; area 41/42)
Buried deep within the lateral fissure
To where do auditory cortex send axons back to for modulation and efferent pathway?
Cochlear nuclei
Where do efferent terminate? What is the function of this pathway?
Outer hair cells
Provide auditory feedback that regulates attention and sensitivity to certain sounds (background noise)
Are auditory impulses bilateral or unilateral? What is the benefit of this?
Bilateral enabling localization by direction and distance
Where in the pathway can unilateral sensory hearing occur?
Damage to cochlear nerve only between hair cells and cochlear nuclei (results in vestibular manifestations as well)
What are the effects of damage after the cochlear nuclei?
will not result in significant reduction of hearing on the involved side but will impair localization of sounds
What are the two types of deafness?
Sensorineuronal and conduction
What is sensorineuronal deafness? Is it reversible?
Damage to sensory apparatus including cochlear nerve
Irreversible and depends on amount of damage
What is conduction deafness? What is not affected?
Inference with passage of sound waves through external or middle ear (fluid, ear wax, FB, perforated eardrum)
Bone conduction can still occur (internal stimulation)
What tool and tests can we use to evaluate hearing loss?
Tuning fork
Rinne test
Weber test
Describe the rinne test.
Compares hearing via air and bone conduction
Stem of fork placed at mastoid process behind ear until patient cannot hear it
Fork side is then held near ear where it should be heard louder through air
Describe the Weber test. How do we analyze results?
Stem placed on forehead
Unilateral nerve deafness = patient hears it better in
the normal ear
Unilateral conduction deafness = patient hears it better in the impaired ear
What is the vestibular system?
Helps head and body lined up (through vestibulospinal reflexes)
Keeps eyes fixed on target during head movements (through vestibulo-ocular reflex system)
What is the nature of vestibular activity? Is is conscious?
ALL VESTIBULAR ACTIVITY IS REFLEX IN NATURE AND OCCURS SUBCONSCIOUSLY
What can result from disruption or overstimulation of vestibular system? How does this manifest ocularly?
Vertigo-sense of whirling and loss of balance
Can occur with imbalance between right and left sides
Nystagmus
Why can vertigo result in nausea or vomiting?
Vestibular system has strong connection to autonomic center (control vomiting) in brainstem reticular formation
To what area of the brain are there strong connections with the vestibular system?
Cerebellum (vestibulocerebellum)
What is the function of the strong commissural connection between right and left vestibular nuclei?
key role in the compensatory mechanisms that alleviate the vertigo after a unilateral vestibular abnormality
Synchronizes sides
What is equilibrium? From what 3 sources does it receive input?
Sense of awareness of position
Visual, proprioceptive, vestibular
How does disequilibrium manifest through the Romberg sign test?
Patient standing with feet together uses vision to stabilize balance
Eyes closed, visual input stops and patient sway or falls (positive sign)
What are the two components of the vestibular system?
Boney Labyrinth
Membranous Labyrinth
Describe the vestibular parts of the bones labryinth? What are the two parts and their functions?
In the temporal bone, adjacent to the cochlea (auditory part)
Vestibule (head aligned with body)
Semicircular Canals ( eye movements aligned with head movements)
Describe the vestibular parts of the membranous labryinth? What are the two parts and their functions?
Sits within fluid cavity
Utricle and saccule- vestibulospinal system (head/body)
Semicircular ducts- vestibulo-ocular system (head/eye)
Describe the utricle and saccule. What are their components (crystals)?
Macula area in walls of each
Oriented at right angles matching orientation of plane, contains hair cells
Overlaying hair cells is a crystal membrane (otoliths)
What occurs when the crystals (otoliths) shift?
Gravity settle crystals, movement can shift these crystals leading to bending of hair cells which triggers depolarization and excites dendrites of the bipolar vestibular ganglion cells
Following crystal stimulation where does the vestibular nerve travel?
Medially in cranial ver 8 through internal acoustic meatus to brainstem at pontomedullary junction
After pontomedullary junction, where do vestibular nerve fibers project?
Pass dorsally to reach the vestibular nuclear complex
Most will project to spinal motor nuclei via the
lateral and medial vestibulospinal tracts
Some will continue into the cerebellum via the
inferior cerebellar peduncle
What can occur when there are problems in vestibular pathways?
Dizziness
Vertigo
What is positional vertigo?
Otolithic crystals break loose, enter semicircular canal, stimulate hair cells that detect head rotation
Brain receives message that head is spinning, often felt when patient abruptly sits up
What helps reposition the crystals during positional vertigo?
Epley maneuver
How many semicircular canals (ducts) do we have? How are they oriented? What stimulates receptors in functional pairs of the canals?
3
Oriented at right angles
Rotation of head stimulates
What is the vestibulo-Ocular Reflex?
Keeps eyes fixed on target when head rotates
Eyes turn opposite of head rotation
Where are receptors of the semicircular canals?
Ampulla- on each semicircular canal
Projects into cavity as ampulla ray crest containing cupula which are connected to hair cells
Nerve fibers project from ampullary crest
How does transduction to a nerve impulse occur when the head rotates?
Head rotates
Endolymph lags behind and prevents the cupula from moving
Hair cells bend in direction opposite of rotation
When the rotation stops, endolymph
continues to move, causing the cupula to bend
in the same direction of the rotation
results in depolarization (excitation)and
hyperpolarization (inhibition) which alternate
depending on direction.
How are reflexes coordinated?
Receptors in the right and left semicircular canals work in pairs, one side is excited, the other is inhibited
Eyes move synchronously
The hair cells bend in opposite directions
How is the vestibular nerve pathways involved in the VOR?
Vestibular nerve to Semicircular to CN 3, 4, and 6 that controls eye movement.
Enables the vestibular to input into semicircular
What is the oculocephalic (vestibulo-ocular) reflex or dolls eye maneuver?
Patients head is manipulated while focusing on target to test if eye movement in opposite direction of head (normal)
Evaluate brainstem integrity
Can also use warm or cold water in ear to elicit reflex
What occurs when we have damage to the VOR in a responsive patient?
Inability to make coordinated eye movements
Blurry vision with any head movements
Can be disrupted by ototoxic drugs (like hair cells), infection (like hair cells) or inflammation
