Auditory + Vestibular System Flashcards
Components of external ear
Auricle
EAM
What separates the outer ear from middle ear?
TM
What are the three main chambers of the middle ear?
Tympanic cavity
Mastoid antrum
Eustachian tube
What are the three ossciles and what do they abut
TM-> malleus-> incus-> stapes-> oval window (ossicle)
Contents of the middle ear
Nerves:
Facial nerve
Chorda tympani
Lesser petrosal nerve
Tympanic branch of the glossopharyngeal nerve (Jacobsen’s nerve)
A branch from internal carotid plexus
Ossicles
Tensor tmpani muscle
Roof of middle ear
Tegmen tympani
Posterior wall of middle ear
Mastoid
Anterior wall of middle ear
Carotid wall
Lateral wall of middle ear
Membranous (tympanic)
Floor of middle ear
Jugular wall
Superior relation of middle ear
Tegmen tympani separates it from the middle cranial fossa
Inferior relation of middle ear
Jugular bulb, there can be congenital dehiscnece of floor with jugular bulb in middle ear placing it at risk of injury
Medial relation of middle ear
Labyrinth and lateral semiciruclar canal lies posterosuperior to facial nerve
Posterior relation of middle ear
Sigmoid venous sinus
Anterior relation of middle ear
Petrous part of ICA in carotid canal
Posterior relation of middle ear
Posteromedial to mastoid air cells- cerebellum in posterior cranial fossa
Compartments of the middle ear
Epitympanic recess/attic
Tympanic cavity
Hypotympanum-> Eustachian tube opening
Components of inner ear
Bony labyrinth- formed by openings in petrous portion of temporal bone
Membranous labyrinth- formed by simple epithelial membrane
Relation between membranous and bony labyrinth
Membranous labyrinth lines the contours of the bony labyrinth and is filled with endolymph
The bony and membranous labyrinths are separated by perilymph
What are the components of the membranous labyrinth
Cochlea (auditory labyrinth)
Utricle, saccule and semicircular canals (vestibular labyrinth)
Components of the cochlea
Bony core- mediolus which contains cochlear part of CN VIII
Cochelar canal which winds two and a half times around the mediolus
Divisions of the cavity of the cochlear canal
Scala vestibuli- above
Scala media- middle
Scala tympani- below
Endolymph is found in which scala?
Scala media
Perilymph is found in which scala
Scala vestibuli and scala tympani
How are the scala vestibuli and scala tympani in communication
At the apex of the cochlea called the helicotrema
The scala media ends in a blind-ended sac
What separates the scala vestibuli from scala Media?
Reisnner’s membrane
What separates the scala media from the scala tympani
The basilar membrane
How are Reisner’s and basilar membranes attached to the bony wall of the cochlear canal?
Spiral ligament
What is the organ of Corti?
Composed of sensory hair cells on the scala media side of the basilar membrane
Contains stereocilia that project into the overlying gelatinous structure- tectorial membrane
Describe how audotiry signals are transduced
Inward movement of the oval window by stapes sets up a fluid wave along scala vestibuli and scala tympani across the scala media to cause upward displacement of the basilar membrane
The basilar membrane is forced up against the fixed tectorial membrane with shearing forces exerted on stereocilia causing excitation of sensory hair cells.
Point of maximum displacement is determined by sound frequency displacement- low frequency= maximum near the base, high frequency- maximum near the apex.
The excitability of cochlear hair cells is frequency-dependent.
What happens after fluid waves have crossed the scala tympani
Cause the round window membrane to move in and out
Which two features of the middle ear enhance the efficacy of energy transfer from air to fluid
The oval window is substantially smaller than tympanic membrane- vibratory force is greatly magnified at the fluid interface of the oval window
Use of TM and ossicles reduces energy loss from higher acoustic impedance of transition from air to fluid
Brainstem organisation of afferent auditory fibres
Cochlear nerve fibres terminate on both dorsal and ventral cochlear nuclei which maintain tonogrpahic organisation of cochlea
Dorsal cochlear nuclear fibres project axons across midline in dorsal acoustic striae-> lateral lemnicsuc-> inferior colliculus
Ventral cochlear nuclear fibres-> ipsilateral and contralateral superior olivary nucleus via trapezoid body. Then ascend in contralateral lateral lemniscus-> inferior colliculus
Inferior colliclus-> MGN via inferior brachium
Function of superior olivary nucleus w.r.t auditory tract
Concerned with sound localisation
Compares differences in the timing of sounds received and intensity
Projects to the nucleus of lateral lemniscus and the inferior colliculus
What is the only part of the ascending auditory pathway that does not have commissural connections?
MGN
How do the lateral lemnisci communicate?
Via commissure of Probst
Electrical stimulation of the primary auditory cortex
Produces sensation of simple sounds such as buzzing or ringing
Electrical stimulation of the auditory association areas
Complex sounds e.g. dog barking or familiar voice
Organisation of auditory cortex
Tonotopicaklly
Low frequency sounds anteriorly
High frequency posteirorly
Summation columns associated with binaural input and suppression columns monaural input
Clinical features of conducitve hearing loss
Reduction in hearing
Maintain good ability to hear loud noises
Hear better in setting of noisy backgrounds
Clinical features of sensorineural hearing loss
Selective difficulty hearing high pitched sounds and vowels
Loss of speech discrimination out of proportion to pure tone deafness
Difficulty hearing speech that is mixed with background noise
Low pitch tinnitus frequently associated with
Conductive hearing loss
High frequency tinnitus assocaited with
Sensorinueral hearing loss
In which condition is sensorineural hearing loss associated with low frequency tinnitus?
Meniere’s
Subjective tinnitus
Only experienced by patient
Objective tinnitus
Experienced by patient and examiner
Causes of objective tinnitus
Eustachian tube dysfunction
Ossicles
Palate
Cerebral vascular malformations or aneurysms
Weber’s lateralises to right
AC>BC on right
R SNHL
Weber’s lateralises to right
BC>AC on right
R Conductive hearing loss
Cochlear lesion causing high frequecny hearing loss
Base of cochlea (due to tonotopic arrangement)
Cochlear lesion causing low frequency hearing loss
Apex of cochlea
Unilateral auditory cortex lesion
Results in difficulty localising signs
May make it difficult for a listener to ignore background noise or competing measures
Draw the ascending auditory pathway
Disequilibrium
Represents malfunction of one of the three communicating systems- visual, proprioceptive, vestibular
Basic function of vestibular system
Coordination of motor control
Posture
Equiibrium
Eye movements
What structures constitute the vestibular portion of the labyrinth
Utricle
Saccule
Semicircular canal
Function of utricle and saccule
Detect linear acceleration and the position of the head in space
Structure of utricle and saccule
Dilatation of membranous labyrinth, surrounded by perilymph
Filled with endolymph and lined by simple cuboidal epithilium except at receptor rich areas called maculae
Components of the utricle and saccular maculae
Supporting cells- columnar epithelial cells continuous with the simple cuboidal epithelial cells that line the utricle and saccule
Hair cells which are specialised receptors cells intercalated between the supporting cells
Otolithic membrane (gelatinous mass embedded with calcium carbonate otoliths) that covers hair cells
Dendrites of cells in the vestibular ganglion
Arrangement of macular hair cells
Hair of maculae are microvilli each containing 40-80 with a single kinocilium that arises from a centriole.
One kinocilium is located in the periphery of each hair cell, thus polarising the hair cell
Polarisation of utricular and saccular maculae
Kinocilia of the maculae are arranged to polarise the maculae in relation to an imaginary curved line called the striola
Utricular maculae are polarised towards the striola
Saccular maculae are polarised away from the striola
Functional significance of the otolithic membrane
Exerts gravitational pull on hair cells.
The orientation of the hair cells relative to this gravitational force determines the direction of their displacement
Displacement of hair cells along the axis of polarisation depolarises the cells and initiates excitatory impulses
Displacement of hair cells in the direction opposite hyperpolarises the cells and initiates an inhibitory impulse
Plane of saccule
Vertical
Plane of utricle
Horizontal
The function of semicircular ducts
Provide information bout the angular acceleration fo the head in any direction but do not detect its ability in a static position
Functional significance of the semicircular canal
Each duct enlarges at point of attachment to the utricle- ampullae which are the functional counterparts of the maculae
Contained thickened sensory epithelium called ampullary crest which contains kinocilia containing hair cells
These hair cells are embedded in a gelatinous mass- the cupula and fill space between crest and roof of the ampulla
Angular acceleration of the head causes displacement of endolymphatic fluid and movement of cupula causing stimulation of hair cells.
Cupula vs otolithic membrane
Cupula lacks calcium carbonate crystals
Central projections of vestibular system
Spinal cord
Cerebellum
Nuclei controlling EOMs
Scarpa’s ganglion
Vestibular ganglion which lies at the base of the IAC
Through which cerebellar peduncle to vestibular fibres pass
Inferior cerebellar peduncle
Longitudinal vestibular spinal tracts
Vestibulospinal tract
MLF
Direct fibres
Medial vestibulospinal tract
Originates in medial vestibular nucleus, projects corssed and uncrossed fibres in the desecending MLF as far as the cervical spinal segements
Lateral vesteibulospinal tract
Arises in lateral vestibular nucleus (Deiter’s nucleus)
Projects uncrossed fibres to all levels of the SC
Deiter’s nucleus
Lateral vestibular nucleus
Sends fibres to lateral vesitbulospinal tract
Where do vestibulospinal tract fibres terminate
Rexed Laminae VII and VIII which facilitate extensor muscles
Provide physiological basis for maintenance of extensor muscle tone required for upright posture
Direction of nystagmus
Named for fast phase
It is actually the direction of slow phase that is pathological with the fast phase representing corrective compensatory eye movements
Visual fixation and nystagmus
Visual fixation inhibits peripheral vestibular nystagmus
Differentatiating factors between peripheral and central nystagmus
Central- multidirectional without inhibition by fixation
Destructive peripheral nystagmus
Fast phase to right
Unopposed tonic firing of right labyrinth-> slow gaze to left with corrective movements to the right (fast phase to right)
Left-sided pathology
Signs and symptoms of peripheral vestibular pathology
Pathology affecting vestibular labyrinth or ganglia
May present with hearing loss, tinnitus, aural fullness, and or pain
Facial weakness and hearing loss if a ganglial lesion
Past-pointing on the affected side
Rotatory nystagmus inhibited by visual fixation
Associated autonomic symptoms may be seen
Central vestibular lesions
CPA lesions:
Hearing loss and tinnitus
Loss of corneal reflex
Facial weakness
Ipsilateral ataxia and intention tremor
Brainstem and cerebellar
Mild vertigo
Other CN palsies- diploploia, dysarthria, perioral numbness
Long tract signs
Hearing loss and tinnitus absent
Location of peripheral vestibular lesions
Labyrinth
Ganglia
Vestibular labyrinth lesions
Episodic vertigo
Hearing loss, tinnitus, aural fullness
Otalgia
On examination patient points towards the affected side
On examination, patient points to the affected side
Spontaneous nystagmus with fast phase away from the affected side, rotatory and inhibited by fixation
Prounced symptoms with nausea and vomiting
Lesions in vestibular ganglia and nerve at IAC
May have facial weakness due to close proximity to facial nerve
Hearing and tinnitus often present but vertigo less prominent and aural fullness/otalgia absent
Central vestibular lesions location
CPA and brainstem
CPA lesions
Progressive hearing loss and tinnitus
Loss of ipsilateral corneal reflex and facial numbness
Facial weakness
Ipsilateral ataxia and intention tremor
Contralateral hemiparesis and hemisensory loss
Vertigo, nystagmus and autonomic symptoms mild
Brainstem and cerebeullm vestibular lesions
Isolated vestibular lesions are rarely the result of lesions in brainstem or cerebellum
Vertigo associated with diploplia, dysarthria, perioral numbness
Long tract signs
Hearing loss and tinnitus typically absent
Localisation of gaze evoked nystagmus
Non-localising
Downbeat nystagmus
DC
Cervicomedullary junction
(posterior fossa)
Upbeat nystgamus
UV
Vermis
Localising nystagmus
Should be in primary position, not gaze evoked
Convergence retraction
CD
Dorsal midbrain
Seesaw nystagmus
ST
Third ventricular/parasellar region
Brun’s nystagmus
BC
CPA
Difference between rebound and periodic alternating nystagmus
PAN alternates side to side with periodicity
Rebound is gaze-evoked
What is the location of the cochlear?
B
