A7 Auditory System Flashcards
Describe the parts of the Aurice (outer ear)
Helix (top curvature) Antihelix (next curvature) Tragus -> Antitragus is opposite Lobule Concha (beneath Antihelix) External Auditory Meatus -> conducts soundwaves to the tympanic membrane
Describe the major components of the tympanic membrane (as viewed by an otoscope)
Tympanic membrane = translucent membrane
Umbo = central depression, attached to the Handle (manubrium) of Malleus
Anterior and Posterior Mallear Folds project from handle of malleus
Cone of light in AnteroInferior Quadrant
Innervation of Tympanic Membrane
Internal Surface: Trigeminal (V3) -> Mandibular Division
External Surface: Glossopharyngeal (IX)
What are the limits of the outer, middle and inner ear?
Outer = Auricle-inner surface tympanic membrane
Middle= Inner surface tympanic membrane-Oval Wintow (tympanic cavity)
Inner=Oval window inwards (includes nerves)
Structures of & connections with the middle ear
Structures:
All within tympanic cavity = air-filled compartment
Tympanic membrane (outer border)
Ossicles:
- Malleus against tympanic membrane surface
- Incus
- Stapes’ footplate against Oval Window
- Muscles: Tensor Tympani and Stapedius
- Nerves present: Chorda Tympani Nerve (arising from facial), Typanic branch of Glossopharyngeal/Lesser Petrosal -> forming Tympanic Plexus
Connects with:
- Auditory/Eustachian Tube
- Mastoid Air Cells
Describe locations, innervations and functions of the 2 muscles of the middle ear
Tensor Tympani:
- Attached to Handle of Malleus (after running alongside auditory tube)
- CNV3 (mandibular division)
Stapedius:
- Attached to neck of Stapes (v v small)
- CNVII
Thought to moderate the articulations between malleus, incus and stapes (the ossicles). Protective function to damped v loud sounds -> Decrease vibrations through ossicles
Which nerves are present in middle ear, and what is their function?
Where abouts do they run?
Facial Nerve: Runs along medial, then posterior wall. Gives off Chorda Tympani Nerve, and innervates Stapedius
Chorda Tympani: Passes through from posterior-anterior, not functional in middle ear
Tympanic Branch of Glossopharyngeal: Enters through inferior wall. Gives rise to Tympanic Plexus
Tympanic Plexus: On Medial Wall
Lesser Petrosal Nerve: From Tympanic Plexus, leaves middle ear, anteriorly to innervate parotid gland (carries PSNS fibres)
Describe the course of Chorda Tympani in the Middle Ear
Arises as branch from Facial Nerve as it runs through the Facial Canal
Runs close to tympanic membrane
Loops between malleus and incus
Exits at base of the skull to enter Inferotemporal Fossa ->
Composition of Chorda Tympani
Arising from Facial Nerve
Taste (SS) fibres to Anterior 2/3 of tongue
Preganglionic PSNS fibres to Submandibular Ganglion
Describe the Structures of the Inner Ear
Bony and Membranous Labrynth (membranous labrynth follows the contours of bony labrynth)
Extends from the Oval Window, which opens into the Vestibule.
Medially/Anteriorly/Inferior to the vestibule, branches the Cochlear. Superiorly & Medically to the Vestibule, branches the Semicircular Canal.
Between the Semicircular Canals (of which there are anterior, posterior and lateral ducts) and the Cochlear, are the Utricle and the Saccule, respectively
The Endolymphatic Duct arises from the Saccule, and ends as the Endolymphatic Sac.
Describe the composition & contents of the Bony Labyrinth
Bony Labyrinth formed from Petrous Temporal Bone
Contains Perilymph: similar to ECF: High Na, Low K
Describe contents of Membranous Labyrinth & its function
Contains Endolymph Similar to ICF: High K, Low Na The specific composition of Endolymph is necessary for: its functions in Auditory Transduction Survival of Auditory Hair Cells
Describe the Parts of the Cochlea:
Entire length ~33mm
Spiral formation (~2.5rotations): larger basal to smaller apical spiral
The Bony Portion of the Cochlea, as well as it’ bone core - the Modiolus - are formed from Spongy Petrosal Temporal Bone.
Within the tube-structure of the Cochlea’s spirals are 3 compartments: Superiorly, the Scala Vestibuli (perilymph), Inferiorly, the Scala Tympani (perilymph) and the Scala Media between them - formed from membranous labyrinth (contained within bony structure of the Cochlear Duct) and containing endolymph.
The Helicotrama = small opening connecting the Scala Vestibuli and Scala Tympani.
Describe the Boundaries and Components of the Cochlear Duct
Basilar membrane (inferiorly) houses the Organ of Corti
Vestibular (Reissner’s) Membrane (superiorly)
Stria Vascularis on the lateral wall - synthesises and secretes endolymph
Where is endolymph produced/secreted and then reabsorbed?
Synthesised and secreted in Stria Vascularis of Cochlear Duct (part of Scala Media)
Reabsored in Endolymphatic Sac
What is Meniere’s Disease? Symptoms?
Defective circulation and/or absorption of endolymph
Causes swelling of the membranous labyrinth
Symptoms: Transient attacks of vertigo & Ringing in the ears (tinnitus)
Describe the structure of the Organ of Corti
Cosists of inner and outer hair cells and supporting cells
Outer hair cells outnumber inner hair cells 4:1
Stereocilia on apical surface of outer hair cells
The stereocilia are rigid, due to cross-linking of actil filaments
The free ends of the stereocilia are embedded in the tectorial membrane
Inner hair cells are more heavily innervated by CNVIII (vestibulocochlear -> cochlear nerve)
Describe the process of Auditory Transduction
- vibration at oval window is transmitted through the perilymph
- This causes displacement of the vestibular and basilar membranes
- This creates a shearing force against hair cells
- When hair cells are displaced, so are the stereocilia. Tip-links between apical sterocilia are tethered to ion channels on the stereocilia.
- Thus, tension changes on the tip-links can open or close these ion channels (mechanical gate)
- Direction of stereocilia deflection determines whether the hair cell depolarises or hyperpolarises.
- Displacement towards taller stereocilia = High tension on tip-links, and opening of channels -> depolarisation -> increased glutamate release
- Displacement towards shorter stereocilia = Lower tension -> Closing of channels -> Repolarisation -> Less glutamate release
On the cellular level, what happens when direction of stereocilia deflection is towards taller stereocilia?
High tension on tip links
Opening of mechanically gated channels
Influx of K+
Triggers opening of voltage-gated Ca++ channels -> Glutatmate release -> Binds to receptors on afferent fibres
Describe the tonotopic map of the cochlea. What accounts for the differences in sensitivity?
At the base of the cochlea, the basilar membrane (organ of corti) responds to higher frequence of sounds. This is because the basilar membrane is narrower and stiffer at this point.
At the apex of the cochlea, the basilar membrane (organ of corti) responds to low frequencies. This is because the basilar membrane is wider and more flexible at this point.
Additionally, there are more layers of hair cells toward the apex.
What are the physical properties of sound, and what is the human ear most sensitive to?
Intensity = loudness (decibels) Frequency = pitch (measured in Hz)
Human ear most sensitive to sounds at ~3000Hz
What & where is the Spiral Ganglion?
Cell bodies located in bony projections of the modiolus, that radiate out toward the organ of corti.
The ganglion contains cell bodies of bipolar afferent neurons, sending information from the hair cells.
The central processes of these nerves coalesce in the body of the modiolus, forming the Cochlear Nerve (of CNVIII)
Describe the course of information from hair cells to auditory cortex
*Where does CNVIII enter brainstem?
Glutamate released from depolarised hair cells -> AP in bipolar sensory cells (cell bodies in spiral ganglion) -> central processes coalesce within modiolus bone forming the cochlear nerve -> cochlear nerve joins with vesibular nerve forming vestibulocochlear nerve.
Vestibulocochlear (CNVIII) enters the brainstem at the Pontomedullary Junction
Fibres birufcate and branch into the Dorsal and Ventral Cochlear Nuclei
Most fibres from (mainly the dorsal) cochlear nuclei cross the midline and join the Lateral Lemniscus. Some fibres remain on that side to join the Ipsilateral Lateral Lemniscus. (i.e. both lateral lemniscuses carry information from L & R cochleas)
Virtually all fibres from the lateral lemniscuses terminate in the Inferior Colliculus (rather than heading to thalamus)
The Inferior Collilculus then projects bilaterally through the Brachium of the Inferior Colliculus, which travels along the surface of the brainstem to the Medial Geniculate Nucleus
Fibres from Medial Geniculate Nucleus then radiate outwards ‘tonotopically’ to the Primary Auditory Cortex in the Transverse Temporal Gyri.
*Many fbres from ventral cochlear nucleus end up in the Superior Olivary Nucleus (at rostral end of facial motor nucleus) -> sound localization
Unilateral lesion rostral to the cochlear nuclei would cause…
Difficulties localizing sound
Would not cause deafness in either ear
Describe the two major types of hearing loss & the appropriate test for each
Conductive:
Air-borne vibrations are unable to reach organ of Corti: i.e. due to blockage in external ear, middle ear infection, etc.
Hear better by bone conduction
Rinne’s test.
Sensorineural:
Impairment of hair cells, cochlear nerve or central pathways
Imparied hearing persists regardless of whether vibrations delivered by air or bone
Describe Rinne’s Test
Determines the relative sensitivity to air vs. bone conduction
Tuning fork held against mastoid process for bone conduction
Held lateral to tragus process for air conduction
Air conduction should be more sensitive than bone conduction. If bone conduction more sensitive = conductive hearing loss
Describe Weber’s Test. Results?
Tuning fork placed on vertex of skull
Conductive hearing loss = vibration will be percieved as louder on affected side
Sensorineural hearing loss = vibration will be percieved louder on normal side
What are ‘Brainstem Auditory Evoked Responses ‘ (BAER)?
How is it measured? What do results indicate?
Sound clicks presented bilaterally at the same time at various decibels
Electrical activity is recorded from the scalp
Normal result = well defined peaks at normal latencies
Hearing impediment of one ear may be seen as peaks being less pronounced in one ear compared with the other, or by peaks being delayed behind the peaks of the other ear.
Lesions to the Primary Auditory Cortex on one side will cause what defecits?
Which artery supplies this area?
Slight bilateral hearing loss
Slightly worse on contralateral side
(because Lateral Lemniscus pathway carries both L & R ascending fibers from cochlea (as CNVIII))
Also leads to inability to locate the sound
Middle Cerebral Artery
Lesions in the Secondary Auditory Cortex lead to what defecits?
Inability to interpret sound
Bilateral lesions to PAC will cause what defecit?
Which artery involved?
Deafness
MCA
What is the average normal Rinne’s test result?
Air conduction is heard for twice as long as bone conduction
I.e. air conduction hearing is normally greater than bone conduction hearing.
If conduction louder = Conductive hearing loss
Major causes of conductive hearing loss
Blockage of external ear (otosclerosis)
or damage to ossicles (otitis media)
Prevents efficacy of vibrations transmitting from tympanic membrane to oval window
Describe normal test result from Weber’s hearing test.
Tuning fork plaed on vertex of skull
Sound should be equal in both ears
Hearing the tuning fork louder in the LEFT ear, during a Weber’s test, indicates what?
(two options, remember**)
Conductive hearing loss in the LEFT ear
Because: sound perceived louder in affected side because the masking effect of the environmental sound is absent/diminished in the diseased side
OR
Sensorineural Loss of RIGHT ear: In which case there is diminished perception of sound on the affected side.
How do you determine if someone has conductive vs sensorineural hearing loss?
In Weber’s:
CL = defective side louder
SNL = defective side quieter
In Rinne’s:
CL = bone conduction sound louder than air-conduction
SNL = air and bone conduction reduced
