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CAM304 INN C Lectures > A7 Auditory System > Flashcards

A7 Auditory System Flashcards

1
Q

Describe the parts of the Aurice (outer ear)

A 
Helix (top curvature)
Antihelix (next curvature)
Tragus -> Antitragus is opposite
Lobule
Concha (beneath Antihelix)
External Auditory Meatus -> conducts soundwaves to the tympanic membrane
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2
Q

Describe the major components of the tympanic membrane (as viewed by an otoscope)

A

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

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3
Q

Innervation of Tympanic Membrane

A

Internal Surface: Trigeminal (V3) -> Mandibular Division

External Surface: Glossopharyngeal (IX)

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4
Q

What are the limits of the outer, middle and inner ear?

A

Outer = Auricle-inner surface tympanic membrane

Middle= Inner surface tympanic membrane-Oval Wintow (tympanic cavity)

Inner=Oval window inwards (includes nerves)

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5
Q

Structures of & connections with the middle ear

A

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
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6
Q

Describe locations, innervations and functions of the 2 muscles of the middle ear

A

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

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7
Q

Which nerves are present in middle ear, and what is their function?

Where abouts do they run?

A

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)

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8
Q

Describe the course of Chorda Tympani in the Middle Ear

A

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 ->

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9
Q

Composition of Chorda Tympani

A

Arising from Facial Nerve

Taste (SS) fibres to Anterior 2/3 of tongue
Preganglionic PSNS fibres to Submandibular Ganglion

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10
Q

Describe the Structures of the Inner Ear

A

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.

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11
Q

Describe the composition & contents of the Bony Labyrinth

A

Bony Labyrinth formed from Petrous Temporal Bone

Contains Perilymph: similar to ECF: High Na, Low K

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12
Q

Describe contents of Membranous Labyrinth & its function

A 
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
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13
Q

Describe the Parts of the Cochlea:

A

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.

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14
Q

Describe the Boundaries and Components of the Cochlear Duct

A

Basilar membrane (inferiorly) houses the Organ of Corti

Vestibular (Reissner’s) Membrane (superiorly)

Stria Vascularis on the lateral wall - synthesises and secretes endolymph

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15
Q

Where is endolymph produced/secreted and then reabsorbed?

A

Synthesised and secreted in Stria Vascularis of Cochlear Duct (part of Scala Media)

Reabsored in Endolymphatic Sac

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16
Q

What is Meniere’s Disease? Symptoms?

A

Defective circulation and/or absorption of endolymph
Causes swelling of the membranous labyrinth
Symptoms: Transient attacks of vertigo & Ringing in the ears (tinnitus)

17
Q

Describe the structure of the Organ of Corti

A

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)

18
Q

Describe the process of Auditory Transduction

A
  • 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
19
Q

On the cellular level, what happens when direction of stereocilia deflection is towards taller stereocilia?

A

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

20
Q

Describe the tonotopic map of the cochlea. What accounts for the differences in sensitivity?

A

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.

21
Q

What are the physical properties of sound, and what is the human ear most sensitive to?

A 
Intensity = loudness (decibels)
Frequency = pitch (measured in Hz)

Human ear most sensitive to sounds at ~3000Hz

22
Q

What & where is the Spiral Ganglion?

A

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)

23
Q

Describe the course of information from hair cells to auditory cortex

*Where does CNVIII enter brainstem?

A

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

24
Q

Unilateral lesion rostral to the cochlear nuclei would cause…

A

Difficulties localizing sound

Would not cause deafness in either ear

25
Q

Describe the two major types of hearing loss & the appropriate test for each

A

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

26
Q

Describe Rinne’s Test

A

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

27
Q

Describe Weber’s Test. Results?

A

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

28
Q

What are ‘Brainstem Auditory Evoked Responses ‘ (BAER)?

How is it measured? What do results indicate?

A

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.

29
Q

Lesions to the Primary Auditory Cortex on one side will cause what defecits?

Which artery supplies this area?

A

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

30
Q

Lesions in the Secondary Auditory Cortex lead to what defecits?

A

Inability to interpret sound

31
Q

Bilateral lesions to PAC will cause what defecit?

Which artery involved?

A

Deafness

MCA

32
Q

What is the average normal Rinne’s test result?

A

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

33
Q

Major causes of conductive hearing loss

A

Blockage of external ear (otosclerosis)
or damage to ossicles (otitis media)
Prevents efficacy of vibrations transmitting from tympanic membrane to oval window

34
Q

Describe normal test result from Weber’s hearing test.

A

Tuning fork plaed on vertex of skull

Sound should be equal in both ears

35
Q

Hearing the tuning fork louder in the LEFT ear, during a Weber’s test, indicates what?

(two options, remember**)

A

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.

36
Q

How do you determine if someone has conductive vs sensorineural hearing loss?

A

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

CAM304 INN C Lectures (8 decks)

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