Lectures 20-22: Auditory System Flashcards
Audible range in Hz and dB. What frequency are we most sensitive?
20 - 20,000 Hz (1,000 - 3,000 we are most sensitive) and 1 - 120 dB
External ear: parts and functions
Pinna (reflects low frequencies for vertical sound localization) and external auditory meatus (magnifies 3,000 Hz sound by passive resonance)
Middle ear: parts
Ossicles (malleus, incus, stapes), muscles (tensor tympani, stapedius), eustachian tube
Where does the stapes foot plate insert?
Oval window
Tensor tympani muscle innervation and function. What bone does it attach to?
Innervated by CNV, improves transmission of high frequencies; malleus
Stapedius muscle innervation and function
Innervated by CNVII, stiffens chain of ossicles to dampen sound (stapedius reflex)
Cochlea: parts, fluid, and function. Describe why one of the fluids is special.
Bony cochlea (filled with perilymph) and membranous cochlea (filled with viscous endolymph [high K+, low Na+])
Three cochlear compartments
Scala vestibuli (in), scala media (membranous), scala tympani (out)
How is sound transmitted?
Stapes oscillates, pressure wave transmitted through oval window –> scala vestibuli –> scala tympani (base to apex) –> round window
Where does hearing happen? Two membranes flanking scala media? Third membrane? Where are the hair cells?
Scala media; vestibular and basilar membrane; tectorial membrane; sit on basilar membrane and extend to tectorial membrane
Organ of Corti consists of…
Basilar membrane, inner and outer hair cells, tectorial membrane
Stria vascularis
Highly vascular structure that produces endolymph on lateral wall of Organ of Corti
Inner hair cells
Single row, responsible for hearing, 95% of input to auditory nerve
Outer hair cells
Three rows, critical for audition: input from superior olive, sharpen sound frequency resolution, protect hair cells from loud noise, otoacoustic emissions
Hair cells “hairs”
Sterocilia (arranged like staircase) and kinocilium (taller)
Deflection toward kinocilium
Depolarization of hair cell membrane –> exocytosis –> excites distal process of auditory nerve
Deflection away from kinocilium
Hyperpolarization of hair cell membrane –> less signal –> inhibits
Basilar membrane is wider and more flexible at the _______ for which kind of tones? And the converse? What is this called?
Apex = low tones; narrower and more rigid at base = high tones; tonotopic organization of basilar membrane
What is a special function of the outer hair cells? What are they vulnerable to?
Amplify movement of basilar membrane in response to low intensity sounds; vulnerable to aminoglycoside antibiotics (ototoxic)
Steps of hearing after hair cells
- Inner hair cells activate distal process of auditory nerve; 2. Cell bodies of auditory nerve fibers are located in Spiral Ganglion; 3. Proximal processes of auditory nerve enter brainstem and pontine-medullary junction
Projection pathway of auditory system
Cochlear nuclei (located lateral to inferior cerebellar peduncle) –> superior olive (some crossing here) –> inferior colliculus –> medial geniculate body (thalamus) –> auditory cortex
Superior olive is responsible for…how? Fibers from here ascend in what tract? What other fiber tract projects from here?
Horizontal localization of sound because it receives binaural input from cochlear nuclei; calculate interaural intensity (loudness) and timing differences; lateral lemniscus; also sends a projection back to outer hair cells via olivocochlear bundle
Input from olivocochlear bundle does what? (2)
Selective auditory attention when there is background noise, protection from loud noise
Where do we find the medial geniculate nuclei?
Hanging off the thalamus at the level of the midbrain
Were does the medial geniculate nucleus project?
NOT through internal capsule, but sneak behind posterior limb of IC through the auditory radiation and synapse at Heschl’s gyri
Describe Heschl’s gyri tonotopic organization
Anterior –> low, Posterior –> high frequency (corresponding apex –> base)
What else does the auditory cortex convey?
What and where sound information
Newborn hearing test looks for (2)
Otoacoustic emissions (haircell dance) and auditory brainstem responses
What two important pieces of information are conveyed by the peripheral vestibular system?
- Angular acceleration of the head; 2. Linear acceleration of the head (up and down)
Vestibulo-thalamo-cortical pathway does what? What thalamic nucleus? To where in cortex?
Conscious perception of equilibrium; VP nucleus (contralateral); posterior parietal cortex (primary vestibular cortex)
Vestibulo-ocular reflex (VOR). Afferent and efferent limb?
Steady gaze during head movement; afferent limb = vestibular, efferent limb = oculomotor
Vestibulo-spinal tracts: what two functions and what are their pathways? Ipsi/bilateral?
Postural equilibrium: lateral tract, sends input down spinal cord to innervate motor neurons to extensor musculature (ipsilateral); head stability during body movement: medial tract (vestibulo-colic pathway) maintains head stability during body movement through MLF to cervical spinal cord (bilateral)
Vestibulo-autonomic reflex. Test? Describe.
Effects compensatory changes in BP, respiration and digestion with changes in posture; tilt table test, normal = increased BP when table titled 70 degrees vertically, orthostatic hypotension is a drop in BP when table is tilted
Components of peripheral sensory apparatus (labyrinth)
Semicircular canals, otolith organs
Components of the central pathways of the vestibular system
Four vestibular nuclei; projections to the thalamus, oculomotor neurons, spinal cord, brainstem; reciprocal connections with cerebellum
Semicircular canals: function and names
Detect angular head acceleration; anterior/superior, posterior/inferior, lateral/horizontal
Otolith organs: function and components
Detect linear acceleration of the head; utricle and saccule
Similar to the auditory system, the bony cavities of the vestibular system and filled with _________, while the membranous sacs are filled with __________
Perilymph, endolymph
Swelling at semicircular canals called. What is inside this? What is this made of?
Ampulla; Crista ampullaris = receptor sheet for semicircular canals; hair cells! with the same excitation/inhibition pattern as auditory system
What are hair cells sensitive to? (3)
Trauma, aminoglycoside toxicity, and aging
What is different about the hair cells in the semicircular canals, as compared to the those in the cochlea?
Lodged in the gelatinous cupula (not tectorial membrane)
Clinicians have defined how many planes of movement for the canals? Describe
Three: horizontal, RALP, LARP
Saccule detects…
Linear acceleration/displacement in vertical plane
Utricle detects…
Linear acceleration in horizontal plane
Sensory sheets in otolith organs are called…hairs protrude into…comprised of?
Macula; otolithic membrane; gelatinous structure and otoconia (calcium carbonate crystals)
How is tilt detected in otolithic membrane?
Shifting heaviness of otoconia
Hair cells of the vestibular system synapse on…cell bodies where? Synapses where? How many?
Distal processes of VIII; Scarpa’s Ganglion; vestibular nuclei (4)
Vestibulo-colic reflex does what?
Maintains head stability during body movement (keeps head upright)
Vestibular evoked myogenic potentials (VEMPS) tests what? Describe.
Tests otolith function. Apply sound to patient –> vestibular nuclei –> medial/lateral vestibulospinal tract –> record myogenic potential on neck attributable to sound.
Two types of hearing loss and regions of the ear they affect
Conductive loss (outer –> middle ear) and sensorineural hearing loss (SNHL; inner ear –> CNS)
Weber test assesses which type of hearing loss? How? What if they have SNHL?
Conductive: sound lateralizes to ear with conductive hearing loss; lateralizes AWAY from SNHL
If someone has SNHL, what will happen in the Rinne test? What if they have conductive hearing loss?
SNHL = normal test, air louder than bone; Conductive = positive test, bone louder than air
What does an audiogram assess? (3)
Pure tone threshold for air and bone, and speech reception, speech discrimination
What does tympanometry assess?
Movement of ear drum: assesses middle ear pressure
How do you screen hearing in newborns? What’s the next test?
Otoacoustic emissions; auditory brainstem response
Causes of conductive hearing loss (6)
Cerumen, eustachian tube swelling, tympanic membrane perforation, excess fluid, cholesteatoma, or an ossicular abnormality
Name and describe an ossicular abnormality and treatment
Otosclerosis: fixation of stapes bone; staepectomy
Causes of sensorineural hearing loss (6)
Noise, age, genetics, toxins, virus and acoustic neuroma
T/F: SNHL is usually bilateral
True! (except for Meniere’s or acoustic neuroma)
Hearing loss of age
Presbycusis
What is an acoustic neuroma?
Benign tumor of the schwann cells of the vestsibulochoclear nerve
Who would get a cochlear implant?
Severe bilateral SNHL in an adult or child who has had no benefit from hearing aid
Four-step approach to the dizzy patient
- Understand vestibular physiology; 2. Classify the complaint using history; 3. Peripheral (ENT, labyrinth) or central (neurological); 4. Know the common vestibular syndromes
Classifying dizziness and common causes (3)
- Vertigo = false sense of motion (vestibular); 2. Imbalance/disequilibrium (vestibular, visual, proprioceptive, CNS [cerebellar, etc.]); 3. Lightheadedness (cardiovascular/metabolic)
What test measures eye movement?
VNG Videonystagmography
In a peripheral vestibular lesion, nystagmus is seen away/toward lesion
Away
Five common vestibular syndromes
Acute, unilateral vestibular loss, Meniere, BPPV, bilateral vestibular loss, migraine-associated dizziness
Acute unilateral vestibular loss presents with (4)…prognosis? Common causes (4)
Sudden onset of vertigo, N/V, nystagmus, Romberg; good; infection, neuroma, injury, infarction
Meniere’s syndrome presents with…
Episodic vertigo, N/V, unilateral hearing loss (w/ tinnitus)
What is the pathophysiology of benign positional paroxysmal vertigo (BPPV) and what does this cause? Treatment?
Dislodging of otoconia; acute vertigo brought on by positional change and rotary nystagmus elicited by Dix-Hallpike maneuver; particle repositioning
Four common causes of central vertigo
Cerebellopontine angle tumor, stroke, migraine, MS
T/F: Hearing loss is common with central vertigo
False!
In central vertigo, nystagmus is/is not inhibited by fixation of eyes onto an object
NOT
Bilateral vestibular loss can be caused centrally/peripherally/both
Both
What are some causes of bilateral vestibular loss (5)? Symptoms? Prognosis?
Aminoglycoside or other toxicity, age related factors, infections, trauma, inflammatory/autoimmune conditions; ataxia + oscillopsia (the illusion that the environment is moving when we move our heads) but NO VERTIGO; prognosis = poor
In bilateral vestibular vertigo, what happens with caloric response?
Absent in both ears
Unilateral deafness must be due to damage to what structures. Why?
Ear, auditory nerve, or cochlear nuclei (nothing higher); after the nuclei, all tracts carry bilateral information
All five functional pathways of the vestibular system are under control of this structure.
The cerebellum
