ICL 9.1: Peripheral & Central Auditory Systems Flashcards
what are sound waves?
patterns of alternating compressions and rarefaction
what are the 3 properties of sound waves?
- amplitude
- frequency
- pure tone
what is amplitude?
the magnitude of alternation
it is perceived as loudness
it’s measured as sound pressure level (SPL) and it’s rated on logarithmic scale (decibels dB = 20 log10(P/Po) )
what is frequency?
the temporal spacing of alternation
it’s perceived as pitch!
it’s measured in cycles per second (Hertz)
the adult human hearing range ~ 100 – 160,000 Hz but the most useful range is 4,000-6,000 –> animals can hear higher and lower frequencies
what is pure tone?
the sound of a single frequency or a range of frequencies
vs. complex sound which contains mixed frequencies and amplitudes
what are the components of the peripheral auditory system?
- outer ear
- middle ear
- inner ear (cochlea and organ of Corti)
- auditory (spiral) ganglion and cochlear nerve
what are the 5 auditory centers?
- cochlear nuclei (the primary center!)
- superior olivary nuclei
- nuclei of lateral leminiscus
- inferior colliculus
- auditory cortex
what are the auditory reflexes and pathways?
- acoustic reflex
2. sound localization reflex
what are the components of the outer ear?
the outer ear and the external acoustic meatus funnel sounds to vibrate the tympanic membrane
- external ear (auricle) = pinna can filter sound and assists vertical localization
- external acoustic meatus
channels sound to tympanic membrane
it also has hair follicles and gland (cerumen) which warm the ear and prevent things from entering
- tympanic membrane
what are the components of the middle ear?
- ossicles = malleus, incus and stapes
- stabilizing ligaments
- muscles: tensor tympani m. (CN V) and stapedius m. (CN VII)
- eustachian tube
what are the eustachian tubes?
it’s the part of the middle ear that connects middle ear with nasopharynx
this is what helps balance the pressure in the ear
so clinically if this tube is blocked, the pressure will build up
how do the oss
the tympanic membrane receives pressure from the external auditory canal and the the ossicles amplify the sound!
the tympanic membrane surface area is much larger than the oval window which is what helps amplify the sound
the oval window is at the end of the middle ear and the beginning of the inner ear and it connects the tiny bones of the middle ear to the scala vestibuli, which is the upper part of the cochlea
where is the inner ear located?
inside the petrous portion of the temporal bone!!
what are the components of the inner ear?
- vestibule
- semicircular canals
- cochlea
what is the structure of the cochlea?
part of the inner ear that is a snail-shaped tunnel about 2 ¾ turns
it’s a bony tube with 3 parallel membranous ducts inside; the cochlear duct is in the middle and it’s surrounded by the scala tympani and scala vestibuli ducts as they all curl around into a snail shape
what are the 3 parallel membraneous ducts that make up the cochlea?
- scala vestibuli
- scala tympani
- scala media (cochlear duct)
the scala vestibuli and tympani are connected at the apex of the cochlea
the scala vestibuli connects the oval window of the middle ear to the helicotrema so it’s like the top layer of the coil in the cochlea while the scala tympani connects the helicotrema to the round window –> they both contain perilymph which has high Na+ and low K+
the cochlear duct contains endolymph which has low Na+ and high K+ and it also contains the tectorial membrane and organ of corti
what is the important structure inside the cochlear duct?
the organ of corti which generates electrical signals from the mechnical stimulation
it’s composed of:
1. outer hair cells (3 rows)
- inner hair cells (1 row)
- supporting cells
- inner tunnel
there is also a basilar membrane which is an array of hair cell supporting cells at the bottom of the organ of corti –> so think of it like a sandwich; the hair cells are in the middle while the tectorial membrane is on the top and basilar membrane is on the bottom (the basilar membrane is NOT part of the organ of corti)
what are the characteristics of the hair cells of the organ of corti?
the organ of corti is the part of the inner ear which generates electrical signals from the mechanical stimulation
they have 3 rows of cilia arranged by height which are embedded in tectorial membrane!!!
the hair cells are polar so there is an apical and basolateral surface –> the apical surface has cilia which exposed to endolymph in the cochlear duct while the basolateral surface is exposed to the perilymph in the scala tympani duct
what is the function of the outer vs. inner hair cells of the organ of corti?
outer hair cells
are targets of efferent modulation (olivocochlear reflex) and they improve frequency selectivity
inner hair cells are a major source of auditory afferent fibers and input to the auditory nerve
what’s the difference between the composition of endolymph vs. periplymph?
perilymph has high Na+ and low K+
endolymph has low Na+ and high K+
what is mechanosensitive transduction?
when the oval window is compressed by the ossicles, alternating compressions and rarefaction causes endolymph fluid waves and vertical displacement of basilar membrane
the vibration of the basal membrane will cause a shearing of the tectorial membrane and the embedded cilia
displacement of cilia bundles towards the kinocilia (the tallest sterocilia) stretches open the mechanosensitive K+ channels –> the endolymph is high in K+ which will move into the hair cell and depolarizes it; the fact that the perilymph has low K+ also helps the K+ move into the hair cells
once K+ has depolarized the cell, Ca+2 gated channels will open and Ca+2 will also enter the hair cells –> this causes NTs to be released from the basolateral membrane which go to the auditory nerve and then to the brain (usually the NT is glutamate)
what are the 2 types of afferent nerve fibers in the auditory nerve?
so these nerve fibers are receiving NTs released from the hair cells after they’ve been depolarized
- type 1 afferent:
myelinated, each fiber synapses with a few inner hair cells, and they contribute to >90% of auditory input
- type 2 afferent fibers are unmyelinated, each fiber synapses with only like 20 outer hair cells, and their function is unknown – they might modulate frequency selection
auditory nerve projects to neurons in cochlear nucleus (primary auditory center)
what is the function of the efferent fibers of the auditory nerve?
they are inhibitory fibers from superior olivary nucleus
they protects hair cells from loud sound damage
where are high vs. low frequency sounds registered on the basilar membrane?
high frequency stuff is closer to the oval window and in the narrow part of the basilar membrane
low frequency stuff is towards the apex of the cochlea and in the wide part of the basilar membrane
where does the auditory nerve send the auditory information?
auditory nerve sends information to the cochlear nucleus primarily
each subdivision of the cochlear nucleus receives different preferential input from the auditory nerve –> the anterior cochlear nucleus encodes sound intensity and timing, the posterior encodes sound onset, and the dorsal integrates multimodal information
auditory inputs are both bilaterally and preferentially projected to the brain
where is the auditory cortex in the brain?
broadmann area 41 and 42, or Heschel’s gyrus
the primary and secondary auditory cortex are responsible for complex analysis and perception of sound features
how do we localize sound?
- cochlear nucleus
- horizontal sound localization
- vertical sound localization
- inferior colliculus
- medial geniculate body
what is the acoustic reflex?
aka the stapedius reflex and its function is to protect the cochlea from mechanical damage when sounds are too loud
it’s involuntary and driven by high intensity sound stimulation and vocalization
pathway = auditory inputs -> superior olivary nucleus -> motor nuclei of trigeminal and facial nerves -> contraction of tensor tympani and stapedius muscles -> damping the vibration of the tympanic membrane and stapes because the stapedius muscle tightens the stapes bone so that there isn’t as much vibration!
so when you hear a gun shot over and over, it’s really not that bad because you’re getting used to it
what is the sound localization reflex?
it’s an involuntary vigilant reaction toward the source of sound
when there’s a sound coming you turn your head towards the sound and it’s mediated by the inferior collicular nucleus with the superior colliculus
pathway = auditory inputs -> inferior colliculus -> superior colliculus -> tecospinal and tectobulbar tracts -> motor neurons or cervical spinal cord and cranial nerves III, IV, and VI -> head and eyes turning toward source of loud sound
if you are in an orchestra concert, what part of the cochlea is more involved when you are trying to listen to the sound from the cello?
the hair cells at the apex of the cochlea!
which part of the cochlea is more involved when you are distracted by a high-pitched babbling 3-year old sitting next to you?
the hair cells by the oval window
a sudden loud sound from a drum makes you initially feel uncomfortable but then you become well adapted. can you explain the mechanism?
acoustic reflex!
what is the overall function of the vestibular system?
this system cares about the positions and motion status of head, eyes, and body –> it detects, modulates, and stabilizes the spatial orientations and the motion statuses of our head, eyes, and body
by doing so, it helps to establish, calibrate, and optimize the positions and orientations of the subject’s head, eyes, and body. This ensures that the intended motor tasks are carried out precisely in both stationary and dynamic conditions!!
it is a fast system that executes its actions even prior to the beginning of the intended motor activities
what are the main anatomical components of the vestibular system?
the peripheral vestibular system is composed of:
- labyrinthine (end organ; sensors)
- vestibular (scarpa’s) ganglion and vestibular nerve
what are the 3 vestibular centers?
- vestibular nuclear complex (primary center)
- vestibulocerebellum
- vestibular cerebrum
what are the 3 vestibular pathways and reflexes?
- Vestibulo-Ocular Reflex (VOR)
- Vestibulo-Spinal Reflex (VSR)
- Vestibulo-Autonomic Reflex (VAR)
what is the labyrinth? where is the labyrinth located?
the bony labyrinth is a cavity in the temporal bone that is divided into three sections: the vestibule, the semicircular canals, and the cochlea
it’s part of the peripheral vestibular system and it’s seated inside the petrous portion of the temporal bone on either side of the face
it’s bilateral so that it can help you with spatial orientation
what is the structure of the labyrinth?
so there’s a bony labyrinth and within it is the membranous labyrinth made of chambers and tunnels
chambers = otolith organs (utricule and saccule) and endolymphatic sac
tunnels = one horizontal and two vertical semicircular canals
what are the components of the membranous labyrinth? what is their function?
- otolith organs = utricule and saccule
the sensory epithelia of the utricle and saccule called maculae are concerned with static head position and dynamic head motion such as linear velocity changes
- semicircular canals (3)
the sensory epithelia of the semicircular canals are called ampullae and crista and they are concerned with dynamic head motion such as the angular velocity changes
what are the two types of lymphatic fluids in the labyrinth?
- perilymph
2. endolymph
where is perilymph vs. endolymph found in the labyrinth?
perilymph fills the space between the bony and membranous labyrinth
the endolymph fills the space inside the membranous labyrinth
what produces endolymph?
stria vascularis
how do the hair cells of the vestibular system work?
the semicircular canals have hair cells that are embedded in the gelatinous layer of the crista of the ampulla
the otolith organs (utricule and saccule) also have hair cells embedded in their gelatinous layer –> on top of the gelatinous layer are crystals called otoconia which help increase inertia and shift the hair cells
when the gelatin layer moves, it will cause the hair cells to move and either send stimulatory or inhibitory signals to the vestibular nerve
so again, when you push the hair cells towards the kinocilium the K+ channels will open which will depolarize the hair cell vs. pushing the hair cells away from the kinocilium which will hyperpolarize the hair cell
there are type 1 and type 2 hair cells but type 2 hair cells are more important!
what is the static vs. dynamic system in relation to the labyrinth?
static system = you’re not moving but you still know where you’re head is –> this is mainly the otolithic organs!
dynamic system = you’re moving and you know where you’re head is –> the maculae in your otolith organs tell you about linear movement while you’re ampullae in your semicircular canals tell you about angular movement
what are the two branches of the vestibular nerve?
the vestibular nerve receives information from its 2 branches:
- superior branch –> receives information from the anterior and horizontal semicircular canals and the utricle
- inferior branch –> receives information from the posterior semicircular canal and saccule
these two branches fuse together to form the vestibular nerve! they then join up with the auditory nerve from the cochlea to form the vestibulocochlear nerve (:
what is the arterial supply to the cochlear and vestibular parts of the ear?
anterior inferior cerebellar artery (AICA)
so you can have a stroke to the inner ear!
what is the primary vestibular center?
the vestibular nuclei in the brainstem
it has 4 subdivisions in the upper medulla and lower pons:
- medial vestibular nucleus
- lateral vestibular nucleus (receives inputs from the otolithic organs and cerebellum)
- superior vestibular nucleus
- inferior vestibular nucleus
what is the blood supply to the vestibular nucleus?
posterior inferior cerebellar artery (PICA)
this is important because you can have a PICA stroke in the brainstem which would cause vestibular problems
what part of the cerebellum dedicated to vestibular information?
the vestibulocerebellum = flocculus + nodules
which cortices of the brain are related to the vestibular system?
Several cerebral areas in the upper lateral temporal and the upper parietal lobes
what is the primary vestibular afferent pathway?
fibers from the vestibular system project to the ipsilateral vestibular nuclei or to the ipsilateral vestibulocerebellum
glutamate is the NT that is used to convey these signals from the vestibular system to the brain
what are the secondary afferent connections in the vestibular pathway?
so sometimes there are afferent projections to the vestibular nuclei from things other than the vestibular system!
like sometimes there’s fibers coming from the vestibular nerve, the cerebellum or the contralateral vestibular nuclei to the vestibular nuclei!
what are the secondary efferent connections in the vestibular pathway?
so there are projections FROM the vestibular nuclei to other structures such as:
- vestibulocerebellum
- contralateral vestibular nuclei
- the brainstem motor neurons in the nucleus of CN 3, 4 and 6
- motor neurons in the spinal cord
- brainstem reticular formation
- thalamus and vestibular cortex
- peripheral end organs
what is the vestibulo-ocular reflex?
its function is to stabilize gaze during head motion
its driven by head motion/velocity changes and results in a compensatory eye movement to maintain foveation –> so your head moves in one direction and your eyes move in the opposite direction to compensate and stabilize your gaze!
a linear VOR is mediated by the otolithic organs and compensates for translation
an angular VOR is mediated by the semicircular canals and compensates for rotation
what is the vestibulo-spinal reflex?
its function is to stabilize body posture during head motion
it’s driven by head motion or changes of stance and it results in compensatory phasic changes in the tones and lengths of posture muscles (extensors) in trunk and extremities to offsets the effects on the original head position and status
two the VSR connects with the gamma motor neurons control the muscles in the muscle spindles to help you adjust the tone of the muscle
what is the vestibulo-collic reflex?
its function is to stabilize head position by generating counter movements
if you turn your head, your body will kind of go the other way to balance yourself out
it may be involved in the righting reflex
what is the vestibulo-autonomic reflex?
its function is to induce autonomic reactions to harmful head motion
this is what causes nausea and vomiting to bad vestibular stimulation like after a rollercoaster
when encountering turbulence during flying, the plane may jump, shift or tilt repeatedly and vigorously. why do some passengers feel nauseous or even vomit?
the vestibular system is involved
specifically the vestibulo-autonomic reflex
what part of the labyrinth is affected when the airplane jumps up and down?
saccule
saccule is specifically for linear up/down motion
what part of the labyrinth is affected when the airplane shifts side to side?
utricule
utricle is side to side motion
what part of the labyrinth is affected when the airplane tilts?
semicircular canals –> vertical canal specifically
what are the clinical signs of people with auditory vs. vestibular problems?
auditory = alternations of hearing thresholds, contents, and comprehension
vestibular = dizziness, vertigo, nystagmus, impaired gaze control, imbalance
what side of the head would you see deficits if there was a lesion in the end organs of the ear?
unilateral symptoms
what side of the head would you see deficits if there was a lesion in the nerves and meninges of the ear?
unilateral symptoms
what side of the head would you see deficits if there was a lesion in the cerebellopontine angle?
unilateral symptoms
what side of the ear would you see deficits if there was a lesion in the brainstem?
ipsilateral vestibular symptoms
bilateral auditory symptoms
what ear deficits would you see if there was a lesion in the cortex?
cortical deficits = auditory agnosia, amusia, cortical deafness, motion perception, gaze/gait control, etc.
what are the different types of hearing loss?
- conductive hearing loss (mechanical conduction problem)
. - sensorineural hearing loss (hair cell or auditory nerve problem)
- mixed type hearing loss (both conductive and sensorineural)
- preferential hearing loss at certain frequency bands
what is hyperacusia?
you hear too much!
maybe they have a brain problem so there’s no dampening of noise
what could cause altered hearing quality?
- it’s caused by peripheral or central lesions that cause demyelination, tumors or strokes
- impaired discrimination and perception from cortical lesions
- auditory hallucinations (epilepsy, psychiatric diseases)
what is a bedside auditory examination?
put a tuning fork close to the ear to test conduction; test both ears
you can also put the tuning fork on the mastoid to bypass the outer and middle ear directly to the cochlea; many times this conduction is worse because there’s no amplification through the middle ear but if it’s better it means the ossicles are having amplification problems!
so you can test hearing levels, pure tone, sound discrimination and sound comprehension
what is audiometry?
the faintest audible pure tone level at which a patient can hear > 50% of the time
the lower the threshold the better!
so an audiogram is the graphic recording of hearing thresholds for pure tones of different frequencies
what are the testing signs of sensorineural hearing loss?problems with what part of the ear would cause this?
if there is both poor auditory and bone conduction WITHOUT an AC-BC gap
causes = inner ear, auditory nerve, central auditory pathways problems
what are the testing signs of conductive hearing loss? problems with what part of the ear would cause this?
poor auditory conduction but normal bone conduction WITH an AC-BC gap
causes = ear drum, middle ear, hearing bones problem
what are the testing signs of mixed hearing loss? problems with what part of the ear would cause this?
poor auditory and bone conduction WITH an AC-BC gap
what are the different types of preferential hearing loss and what could cause each?
- high frequency = from being old, exposure to noise, basal cochlea problem
- low frequency = from cholera apex problems or end-lymphatic hydrops
- selective spectrums = from streptomycin, Meniere’s disease, cisplatin, etc.
what are the two types of speech audiometry tests?
- speech recognition threshold
subject to repeat words after hearing them from an earphone; you’re looking for the softest intensity level at which the patient repeats the words > 50% of the time
- word recognition score
a subject has to repeat the given monosyllabic words that are phonetically balanced and are selected when being presented at a most intelligible (suprathreshold) level
like can they tell apart words that sound similar to each other
how do you treat hearing loss?
- correct causes if possible
- devices like hearing aid or cochlear implants
- auditory rehabilitation
what is tinnitus? how can you treat it?
hearing ringing sounds or noises of different frequencies
very common, 32% of adults have this
it’s really hard to treat but you can try to treat it with medications, masking, counseling or biofeedback
biofeedback
what is ototoxicity?
there are some antibiotics that can kill the hair cells in the ear! and after they die they don’t regrow!
what is Meniere’s disease?
it’s an inner ear disorder that causes episodes of vertigo due to increased endolymph and pressure in the inner ear due to decreased absorption/increased production = increased pressure in the labyrinth irritates the hair cells and eventually kills them
clinical features include recurrent episodic attacks of vertigo, altered hearing and tinnitus, ear pressure and fullness, disequilibrium or otolith crisis –> this leads to progressive auditory and vestibular losses and eventually bilateral permanent losses
could be caused by endolymphatic hydrops or tamponade of membraneous labyrinthine by excessive endolymph fluid (similar to glaucoma)
how do you treat Meniere’s disease?
reduce the endolymph pressure in the inner ear through:
- low sodium diet
- diuretics
- steroid
- surgeries (fenestration and lyabytinthectomy)
- hearing aids
- tinnitus masking
- vestibular rehabilitation
what is vestibular neuroma?
aka acoustic neuroma which is a tumor growing from the vestibular nerve –> the tumor more often originates from the schwann cells of the superior division of the vestibular nerve rather than the inferior
you could do surgery, radiation or vestibular rehabilitation but it’s a benign disease that takes years to develop and there’s no treatment for it really….
clinical features include progressive ipsilateral vestibular paresis such as vertigo, nystagmus, imbalance
there is also progressive auditory paresis such as neurosensory hearing loss and tinnitus
what is a cholesteatoma?
cholesteatoma is an abnormal skin growth or skin cyst trapped behind the eardrum, or the bone behind the ear –> without treatment, it will cause recurrent ear infections
cholesteatoma can erode bone, including the three bones of hearing, which may cause infection to spread to the inner ear or brain –> these infections can lead to meningitis, brain abscess, facial paralysis, dizziness (vertigo), and even death
what is bilateral otosclerosis?
when the ossicles become ossified and the joints between them don’t move well to transmit vibrations
otosclerosis has to do with the three small bones in the middle ear, more specifically the stapes –> a part of the bone will grow abnormally and this bone growth will prevent the stapes from vibrating normally in response to sound
so people can have tinnitus and hearing loss but this can be fixed by the surgeon rebuilding the stapes bone
what is vertigo?
it’s an illusive sensation of motion (of self or the environment) = an impaired perception of a stationary status
it’s caused by a mismatch between the actual sensory inputs and the pattern of prediction –> when the input and prediction don’t match that’s when you feel motion/vertigo
what is nystagmus?
bouncing of the eyeball
many patterns that often reflect specific neurological conditions
for the jerking nystagmus, e.g., a slow ocular shifting followed by a fast backward rebound, the direction of the fast phase is chosen as the direction of nystagmus
what is caloric irrigation?
you illicit eye movement by stimulating the vestibular system to detect if there’s function in the ear or if the patient is brain dead like in a comatose patient
if the eye responds it means the medulla is still intact and the brainstem is okay which is a good prognosis!
testing vestibulo-occular reflex of horizontal canals by introducing cool (30 C), warm (44 C), or icy water (or air) into the external auditory canals (causes thermodynamic endolymphatic flow)
slow phase shifting (driven by VOR in response to thermal irrigation) followed by a fast rebound (a corrective saccade) in a conscious person, or slow phase shifting only in a comatose patient
cold water will cause the eye to shift towards that side then quickly away if the vestibular nerve is working – warm water will cause the eye to shift away from that side then quickly back if the vestibular nerve is working
what is a head thrust test?
the head thrust test is a test of vestibular function that is performed as part of the bedside examination that tests the vestibulo-ocular reflex (VOR)
the patient sits in front of the examiner and the examiner holds the patient’s head steady in the midline – then the patient is instructed to maintain gaze on the nose of the examiner – the examiner then quickly turns the patient’s head about 10–15 degrees to one side and observes the ability of the patient to keep the eyes locked on the examiner’s nose.
if the patient’s eyes stay locked on the examiner’s nose (i.e., no corrective saccade), then the peripheral vestibular system is assumed to be intact
thus in a patient with acute dizziness, the absence of a corrective saccade suggests a CNS localization
if, however, the patient’s eyes move with the head and then the patient makes a voluntary eye movement back to the examiner’s nose (i.e., corrective saccade), then this suggests a lesion of the peripheral vestibular system and not the CNS
which conditions are considered inner ear diseases?
- benign positional vertigo (canalithiasis/cupulolithiasis)
- labyrinthitis and vestibular neuritis
- endolymphatic hydrops and Meniere’s disease
- perilymphatic fistula
- dehiscence
what is benign paroxysmal positional vertigo?
it’s a technical problem in the ear
the utricule has crystals on it and sometimes a piece of the crystal become detaches and start moving around inside the semicircular canals when you move; especially when you lie down
these crystals irritates the ampulla which make people nauseous or get vertigo
sometimes the crystals become sticky so one side of the semicircular canal is more sensitive than the others
during a PE, usually the eye will move/bounce towards the side of irritation and if you ask the patient to lay down and tilt their head, the crystals will move in that direction and their eyes will bounce really bad and the person will get very nauseous and dizzy = Dix-Hallpike test
you can do Epley Maneuver to get the deposited crystals out of that semicircular canal and back into the utricle and you’re basically just moving their head around till the crystals shake out of the canal that they’re stuck in
what is dehiscence?
a defected of the bony roof of a canal (usually superior canal)
patient will be extremely hypersensitive to sound and it will cause nystagmus or vertigo
it can be congenital or acquired
what is labyrinthitis?
aka vestibular neuritis
when the labyrinth of the inner ear is damage or the vestibular nerve
most often caused by HSV1 or other inflammatory infections; the nerve isn’t damaged from the infection itself but the edema that’s illicit by the infection that compresses and damages the nerve –> that’s why you can treat with steroids or acyclovir
patients will experience rotary vertigo, nausea and vomiting, rotary nystagmus, or disequilibrium
what is a migraine?
it’s a CNS problem that is hypersensitive reaction to the environmental stimulation
what things can cause vertigo?
- migraine
- brainstem tumor
- antiepileptic drugs
- riding a roller coaster
uncomplicated otitis media does NOT cause vertigo because vertigo has nothing to do with the inner ear
