Auditory System

Question 1

CNVIII carries two special sensory components- whatare these?

Answer 1

Cochlear division and the vestibular devision.

Question 2

What is the external ear?

Answer 2

Part attached to lateral aspect of the head , auricle & the canal leading inwards

Question 3

What is the middle ear?

Answer 3

A cavity in the petrous part of the temporal bone bounded laterally, and separated from the external canal by the tympanic membrane and connected internally to the pharynx by a narrow tube

Question 4

What is the inner ear?

Answer 4

Series of cavities within the petrous part of the temporal bone between the middle ear laterally & the internal acoustic meatus medially

Question 5

What is the tympanic membrane- name the parts!

Answer 5

• Translucent membrane

• Umbo – central depression
• Attached to manubrium (handle) of malleus
• Anterior & posterior mallear folds
• Cone of light in anteroinferior quadrant

Question 6

What is the innervation of the tympanic membrane?

- note external surface and internal surface are different

Answer 6

• External surface = trigeminal (V3) nerve

* Internal surface = Glossopharyngeal (IX) nerve

Question 7

Describe the MIDDLE EAR TYMPANIC CAVITY

Answer 7

• Air filled compartment
• Mucous membrane lined

Basic function is to transmit vibrations of the tympanic membrane across the cavity of the middle ear to the internal ear. It accomplishes this through 3 interconnected but moveable bones that bridge the space between the tympanic membrane & the internal ear.

Ossicles –MALLEUS, INCUS AND STAPES (smallest bones in our body)
• The footplate of stapes is attached to the internal ear at the OVAL WINDOW
• Amplification of sound → greater force per unit area at the oval window than at tympanic cavity

Question 8

The middle ear connects with two things:

Answer 8

1. Auditory or eustachian tube – equalize pressure between middle ear & posterior pharynx
   a. Normally a closed tube, swallowing opens it up
2. Mastoid cells (located more posteriorly)

Question 9

What are the muscles of the Middle Ear/Tympanic Cavity

- what are the two things innervated by?

Answer 9

TENSOR TYMPANI
o Attached to the handle of malleus
o Trigeminal (V3) nerve
o Contraction pulls the handle medially → tenses the tympanic membrane, reducing the force of the vibrations in response to loud noises

STAPEDIUS MUSCLE
o Attached to the neck of stapes
o Facial (VII) nerve
o Contracts in response to loud noises – pulls the stapes posteriorly & prevents excessive oscillation

Question 10

What is the funciton of these muscles?

Answer 10

• These muscles decrease the transmission of vibrations through the ossicles – protective function
o i.e. if you are talking in a noisy environment, they modify the vibration so you aren’t exposed to as much background noise

Question 11

What is on the posterior wall of the tympanic cavity

Answer 11

prominence of : lateral semicircular canal & facial canal
Tensor tympani muscle
Promontory of Glossopharyngeal nerve -> lesser petrosal nerve, tympanic branch of glossopharyngela nerve
Oval window
Round window

Question 12

Describe the course of the chorda tympani in the middle ear

Answer 12

Course in middle ear
• Branch from facial (VII) nerve as it runs in the facial canal (before CN VII exits skull at stylomastoid foramen)

Both CN VII & VIII run through the internal acoustic meatus

• Close to the tympanic membrane
• Exits at base of skull to inferotemporal fossa

Question 13

Describe the composition of the chorda tympani?

Answer 13

Composition of chorda tympani
• Taste fibers to anterior 2/3 of tongue
o Joins the lingual nerve to innervate taste anterior 2/3 of tongue

Preganglionic parasympathetic to submandibular ganglion
o To submandibular & sublingual salivary glands

Question 14

What is in the INNER EAR

Answer 14

Fluid filled compartment. Consists of a series of bony cavities (the bony labyrinth) and membranous ducts and sacs (the membranous labyrinth) within these cavities. All these structures are in the petrous part of the temporal bone with the middle ear laterally and the internal acoustic meatus medially.
• Perilymph & endolymph are important for auditory transduction & survival of auditory hair cells

Question 15

Describe the Bony Labrynth,
- where is it located?
what kind of lymph does it contain?

Answer 15

• In petrous part of temporal bone
• Irregularly shaped space inside the bone suspended in fluid
• The vestibule (which contains the bony window in its lateral wall) is the central part of the bony labyrinth
• Contains perilymph

o Low in K+, high in Na+ (composition similar to extracellular fluid)

Question 16

Describe the membranous labrynth
- what is in the membranous labrynth
What lymph does it contain

Answer 16

WITHIN MEMBRANOUS LABRYNTH
• Saccule, cochlea duct and nerve, utricle, 3 semicircular ducts
• Endolymphatic duct & sac

• Contains endolymph - High in K+, low in Na+ (composition similar to intracellular fluid)

Question 17

What does the bony labrynth have?

Answer 17

Semicircular Canals: Posterior, lateral and Anterior
Common bony limb
Ampullae
Vestibule
Cochlear

Question 18

Describe the COCHLEA

What are the parts of the cochlea?

Answer 18

• The cochlea projects in an anterior direction from the vestibule
• Bony structure that twists onto itself around a central column of bone (the MODIOLUS)
• This arrangement produces a cone-shaped structure with a base of cochlea near the internal acoustic meatus

Parts of the cochlea
• There are two canals within the cochlea, separated by the cochlear duct
1. Scala vestibule – adjacent to stapes at oval window
2. Scala tympani – ends at the round window

Question 19

COCHLEA

What is the Scala Media

Answer 19

Scala media – space in the chochlear duct (part of the membranous labyrinth)

Question 20

COCHLEA

What is the Helicotrema

Answer 20

• Helicotrema – small opening joining the scala vestibule & scala tympani
So when the oval window vibrates, the fluid is disturbed in the scala vestibuli first & then the scala tympani

Question 21

COCHLEAR DUCT is ?

Answer 21

THE HEARING ORGAN!!

Question 22

Cochlear duct ->

Answer 22

The cochlear duct has a central position in the cochlea of the bony labyrinth, dividing it into two canals

It is maintained in this position by being attached centrally to the lamina of the modiolus, which is a thin lamina of bone extending from the modiolus (central bony core of the cochlea) & peripherally to the outer wall of the cochlea.

Question 23

Boundaries of the Cochlear Duct

Answer 23

MODIOLUS – core of spongy bone
BASILAR MEMBRANE on which the organ of Corti sits
- Floor of the organ - Extends from the free edge of the lamina of modiolus to an extension of the spiral ligament
- Organ of Corti = hearing organ
VESTIBULAR MEMBRANE
- The roof, separated endolymph of the cochlear duct with perilymph of the scala vestibule
STRIA VASCULARIS – epithelium that synthesizes & secretes endolymph only within the cochlear duct. Endolymph is reabsorbed in the endolymphatic sac via endolymphatic duct

Question 24

What is Meniere’s Disease?

Answer 24

Meniere’s disease:
• Defective circulation or absorption of endolymph → swelling of the membranous labyrinth
• Transient attacks of vertigo, Ringing in ears (tinnitus)

Question 25

The ORGAN of CORTI is in the ..?

Answer 25

COCHLEAR DUCT -> Hearing!

Question 26

Thee ORGAN OF CORTI has three types of cells

Answer 26

STEROCILIA OF HAIR CELLS - long microvili - rigid, due to crosslinking acting of actin fillaments - Embedded in tectorial membrane: difference in height- taller ones are embedded in the tectorial membrane Tectorian membrane = a gelatinous deposit present over the hair cell in contact with the longer sterocilia of the hair cell Inner Hair cells: Less numerous, more heavilly innervated by CN8: motor neurons terminating on them than into outer hair cells. BUT all axons terminate together to give the COCHLEAR nerve

Question 27

Explain Auditory TRANSDUCTION ***

Answer 27

• A sound wave enters the external acoustic meatus & strikes the tympanic membrane, moving it medially. Because the ossicles are connected laterally to the tympanic membrane & medially to the oval window, this is transmitted as a medial push against the oval window. • Vibration at the oval window is transmitted through perilymph • Through scala vestibule → displacement of vestibular & basilar membrane → through scala tympani → round window (secondary tympanic membrane) Displacement of the vestibular and basilar membrane • Tip links connect stereocilia • They have mechanosensitive cation channels present at the end of tip links • So you have these mechanically activated channels, and deflection of the hair will influence if channel opens/shuts. • Direction of stereocilia deflection determines whether the cell depolarizes or hyperpolarizes o Basilar membrane movement – deflection of hairs as apical attachment is tight & floor is moving • Mechanically gated cation channels allow K+ to enter the cell → opening of voltage gated Ca++ channels → glutamate release, binds to receptors on afferent fibers

Question 28

Explane the Tonotopic map of sound frequency in the basilar membrane

Answer 28

Specialisation occurs because you have changes in the property of the basilar membrane. • At threshold, the base of the basilar membrane responds to high frequency sound; • The apex of basilar membrane response to low frequency sound • Difference in sensitivity due to width & mechanical property of basilar membrane; o Apex is wider & less rigid; as you approach the apex there is less connectivity in the membrane; not as rigid, so sensitivity to low frequency of vibration

Question 29

Describe the physical properties of sound

Answer 29

INTENSITY (loudness) - decibels | FREQUENCY *pitch - hertz

Question 30

Where does CN8 enter the brainstem

Answer 30

Central process of the spiral ganglion form the vestiubulocochlear nerve

Question 31

Processing Auditory information

Answer 31

Input from one side is processed bilaterally: spiral ganglion-> cochlear nuclei -> superior olivary nucleus (small, caudal pons, close to facial motor nucleus) -> decussating fibers contribute to the trapezoid body -> lateral lemniscus -> inferior colliculus -> medial geniculate body - > primary auditory cortex (transver temporal gyrus)

Question 32

What would happen if i had a unilateral lesion rostral to the cochlear nuclei

Answer 32

Difficulty in localising sound | Does not cause deafnessm diminished hearing

Question 33

WAHT IS CONDUCTIVE HEARING LOSS?,

Answer 33

When there is a disruption of the sound reaching the inner ear through the normal method • Air-borne vibrations are unable to reach the Organ of Corti – eg blockage in external ear, middle ear infection • Hears better by bone conduction • Rinne’s test

Question 34

WHAT IS SENSORINEUREAL HEARING LOSS?

Answer 34

When inner ear is damaged • Impairment in hair cells, cochlear nerve, central pathways o If your hair cells die your hearing dies! These cannot be replaced • Impaired hearing whether vibrations are delivered via air or bone conduction

Question 35

Describe RINNES TEST

Answer 35

* Determines the relative sensitivity of ear to air & bone conduction * Mastoid process – bone conduction, bypass middle ear * Lateral to tragus of ear – air conduction * Air conduction is more sensitive than bone conduction * Conductive hearing loss is impaired via air conduction NORMAL test is when a patient can hear the fork easier when 1cm from the ear rather than on bone. IF can hear better on bone -> called conductive hearing loss

Question 36

WEBERS TEST

Answer 36

* Tuning fork placed on vertex of the skull * Conductive hearing loss: vibration is perceived to be louder on affected side * Sensorineural hearing loss: vibration is perceived to be louder on the normal side