The Auditory Pathway

Question 1

Label the following structures surrounding the ear location

Answer 1

Question 2

What are the 3 divisions of the ear?

Label the diagram

Answer 2

• External ear
• middle ear
• inner ear

Question 3

What bone forms the bony part of the external ear?

Answer 3

Temporal bone

the mastoid process and styloid process are visible

Question 4

What is the role of the external ear?

what is its composition like?

Answer 4

The external ear receives sound waves

the lateral 1/3 is cartilaginous and the medial 2/3s are bony

Question 5

What is the role of the middle ear?

What is its composition like?

Answer 5

It is an air-filled space where sound waves are converted to mechanical energy

it contains ossicles

these are bones that mechanically vibrate

Question 6

What is the role of the inner ear?

What is its composition like?

Answer 6

It converts mechanical energy to electrical energy

it contains the cochlea

there is a hollow cavity within the temporal bone through which CN III exits

Question 7

What is the role of the tympanic membrane?

Answer 7

It separates the external ear and middle ear

The external auditory canal transmits sound waves towards the tympanic membrane (eardrum)

it physically moves when sound waves hit it, and transfers these waves to the middle ear

Question 8

What is the composition of the middle ear like?

Which body part does it communicate with?

Answer 8

It is an air-filled space in the petrous part of the temporal bone

it communicates with the nasopharynx via the Eustachian tube (auditory tube)

Question 9

What bones are found within the middle ear?

What is their function?

Answer 9

The ossicles:

1. Malleus
2. Incus
3. Stapes

they vibrate to transmit / amplify sound waves into the cochlea

Question 10

What happens to the ossicles with age?

Answer 10

The ossicles become calcified with age

this means that they are less flexible and able to move and vibrate

this leads to conductive hearing loss

Question 11

What is the role of the Eustachian tube?

what are its 3 functions?

Answer 11

It connects the nasopharynx to the middle ear

1. it drains the middle ear to prevent accumulation of fluid
2. it ventilates the middle ear
3. it equalises pressure between atmospheric pressure and pressure within the ear

Question 12

How is the Eustachian tube different in adults and children?

Answer 12

In an infant, the tube is more horizontal

this means there is a more direct route for infection, so more ear infections are seen in young children

Question 13

Label the middle ear

What are the main connections shown?

Answer 13

• The facial nerve supplies the facial muscles
• the chorda tympani (branch of facial nerve) supplies the anterior 2/3 of the tongue
• the lesser petrosal nerve (branch of facial nerve) supplies the parotid gland
• the Eustachian tube leads to the nasopharynx
• the mastoid air cells of the mastoid process are connected to the middle cranial fossa

Question 14

For which 5 reasons is the middle ear known as a high risk space?

Answer 14

• It is connected to the nasopharynx, making it prone to infection
• it is connected to the mastoid air cells - infection may spread to the middle cranial fossa
• the internal jugular vein lies inferior - infection leads to thrombosis risk
• the internal carotid artery lies anterior - link to pulsatile tinnutis
• traversed by chorda tympani & facial canal - risk of infection spreading to regions supplied by the facial nerve

Question 15

What is meant by pulsatile tinnutis?

Answer 15

Tinnutis is the perception of sound when no external sound is present

it is “pulsatile” when the sound has the same rate as the heart rhythm

this occurs due to a change in blood pressure within the internal carotid artery

Question 16

Where is the inner ear located?

What is its composition like?

Answer 16

Located within petrous part of temporal bone

it is composed of 2 special sense organs

1. Vestibular system - balance & equilibrium (posture)
2. Cochlear system (hearing)

Question 17

What is meant by the cochlea being tonotopically organised?

Answer 17

Different parts of the cochlea detect different frequencies of sound

the apex detects lower frequency sounds

the base detects higher frequency sounds

Question 19

What nerve emerges from the cochlea?

How is it formed?

Answer 19

It is a snail-shaped cavity containing sensory receptors for sound

they synapse with spiral ganglia, whose axons fuse to form the cochlear division of the vestibulocochlear nerve (CN VIII)

this transfers sound information to the rest of the CNS

Question 20

Label the inner ear

Answer 20

Question 21

What are the 2 fluid-filled cavities within the inner ear?

What do they contain?

Answer 21

Bony (outer) labyrinth:

• this contains perilymph
• this is similar to extracellular fluid

Membranous (inner) labyrinth:

• this contains endolymph
• this is similar to intracellular fluid

Question 22

what are the components of the bony (outer) labyrinth?

Answer 22

Vestibular system:

• vestibule
• semicircular canals (SCC)

cochlear system:

• cochlea

these contain perilymph

Question 23

What is the composition of the membranous (inner) labyrinth like?

Answer 23

Vestibular system:

• vestibule structures
• semicircular ducts

cochlear system:

• cochlear duct

these contain endolymph

Question 24

How does the cochlear duct separate the cochlea?

Answer 24

It separates the cochlea into two chambers:

1. Scala vestibuli
2. Scala tympani

these two chambers are continuous at the apex of the cochlea through a narrow slit (helicotrema)

Question 25

Label the components of the cochlea

Answer 25

Question 26

What are the roles of the oval window and round window?

Answer 26

* Stapes transfers vibrations into the inner ear via the **oval window** * fluid vibrations travel through the scala vestibuli and scala tympani * fluid vibrations are then reabsorbed at the **round window**, allowing new sound waves to enter the system

Question 27

Why is it important for the ossicles to amplify sound waves?

Answer 27

There is a change in medium from air to fluid when sound waves enter the inner ear ossicles need to vibrate and amplify the sound to facilitate transfer into a fluid-filled medium

Question 28

What is contained within the cochlear duct?

Answer 28

The spiral organ of Corti

Question 29

What membranes are located either side of the spiral organ?

Answer 29

The spiral organ sits above the **basilar membrane** this contains **hair cells** which have their projections embedded into the **tectorial membrane** the tectorial membrane is above the spiral organ

Question 30

What is significant about the hair cells embedded within the tectorial membrane?

Answer 30

Axons that are in contact with the hair cells fuse to form the cochlear nerve this is a division of the vestibulocochlear nerve

Question 31

What happens when vibrations in the inner ear deform the basilar membrane?

Answer 31

* Deformation of the basilar membrane leads to movement of the tectorial membrane * hair cells detect the movement and open their ion channels * this causes an action potential in the fibres that go on to form the cochlear nerve

Question 32

What stimulates the spiral organ? What does this lead to?

Answer 32

It is stimulated by the deformation of the cochlear duct by the perilymph in the surrounding SV and ST it converts fluid pressure into electrical signals via cochlear nerve

Question 33

What are the characteristics of the auditory pathway?

Answer 33

* Polysynaptic as it contains many synapses * tonotopically organised * bilateral (so sound can be localised) - right & left sides project to both hemispheres * it compares 2 inputs - **timing and loudness**

Question 34

Label the components of the auditory pathway

Answer 34

Question 35

What is the pathway taken by the 1^o auditory fibres?

Answer 35

* Axons of bipolar neurones in the **spiral ganglion** form the **cochlear division** of the vestibulocochlear nerve (CN VIII) * CN VIII enters the brainstem at the **cerebellopontine angle** * synapse with 2^o neurones in the **dorsal & ventral cochlear nuclei** (ICP / CS)

Question 36

What are the roles of the cochlear nuclei? What do they receive and project?

Answer 36

* Receive afferent from **cochlear nerve** * they project **_BILATERALLY_** to the **superior olivary nuclei** * fibres travelling to the contralateral olivary nucleus enter the **trapezoid body** (thick band of white matter)

Question 37

Label the pathway from the cochlear nuclei

Answer 37

Question 38

What information does the superior olivary nucleus receive and transmit?

Answer 38

* SON receives **_BILATERAL_** auditory information from the cochlear nuclei * sends ascending fibres to **inferior colliculus** via the **lateral lemniscus _BILATERALLY_**

Question 39

Where are the inferior colliculus and MGN located? How are they connected?

Answer 39

* Inferior colliculus located in **tectum of midbrain** * Medial geniculate nucleus (MGN) located in **thalamus** * IC connects to MGN via the **brachium of the inferior colliculus**

Question 40

What components of the auditory pathway involve the inferior colliculus and medial geniculate nucleus?

Answer 40

**Inferior colliculus:** * receives information from **lateral lemniscus** * inferior colliculi may exchange auditory information via the **inferior colliculus commisure** * sends auditory information to **MGN** via the **brachium** of the inferior colliculus **Medial geniculate nucleus:** * fibres pass to the **primary auditory cortex** via the **acoustic radiation**

Question 41

What is another name for the primary auditory cortex?

Answer 41

Heschl's gyrus (superior temporal gyrus)

Question 42

What is meant by tonotopic organisation of the cochlea & auditory cortex?

Answer 42

Different regions of the basilar membrane respond to sounds of different pitch **apex** responds to low pitch and **base** responds to high pitch the pitch of sound is distributed to the auditory cortex **L**ow frequency - antero**L**ateral part high frequency - posteromedial part

Question 43

What are the 2^o auditory areas and where are they found?

Answer 43

The left hemisphere houses the 2^o auditory areas **Broca's area:** * involved with motor functions like production of words **Wernicke's area:** * involved with sensory functions like understanding

Question 44

what type of stroke leads to non-fluent (Broca's aphasia)?

Answer 44

This is where the person knows what they want to say but they cant find the words this is caused by a stroke in the **anterior distribution** of the **left middle cerebral artery** this affects Broca's area

Question 45

What type of stroke leads to fluent (Wernicke's) aphasia?

Answer 45

This is when someone speaks fluently but what they are saying makes no sense stroke in the **posterior distribution** of the **middle cerebral artery** this affects Wernicke's area

Question 46

What is the purpose of the descending auditory pathways?

Answer 46

They are involved in responding to auditory information that we receive

Question 47

What is the reflex that prevents damage during loud noise?

Answer 47

* Superior olivary nucleus sends fibres to: * **stapedius** via CN III * **tensor tympani** via CN V₃ * these muscles contract to lock the ossicles in place and stop them from vibrating * this protects the inner ear from damage due to loud noise

Question 48

What is the purpose of reflex head and eye movements? Which nerves are involved?

Answer 48

Involves CN III, IV and VI this is movement in response to sound e.g. Looking at someone who calls your name