The Auditory Pathway Flashcards
Label the following structures surrounding the ear location
What are the 3 divisions of the ear?
Label the diagram
- External ear
- middle ear
- inner ear
What bone forms the bony part of the external ear?
Temporal bone
the mastoid process and styloid process are visible
What is the role of the external ear?
what is its composition like?
The external ear receives sound waves
the lateral 1/3 is cartilaginous and the medial 2/3s are bony
What is the role of the middle ear?
What is its composition like?
It is an air-filled space where sound waves are converted to mechanical energy
it contains ossicles
these are bones that mechanically vibrate
What is the role of the inner ear?
What is its composition like?
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
What is the role of the tympanic membrane?
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
What is the composition of the middle ear like?
Which body part does it communicate with?
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)
What bones are found within the middle ear?
What is their function?
The ossicles:
- Malleus
- Incus
- Stapes
they vibrate to transmit / amplify sound waves into the cochlea
What happens to the ossicles with age?
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
What is the role of the Eustachian tube?
what are its 3 functions?
It connects the nasopharynx to the middle ear
- it drains the middle ear to prevent accumulation of fluid
- it ventilates the middle ear
- it equalises pressure between atmospheric pressure and pressure within the ear
How is the Eustachian tube different in adults and children?
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
Label the middle ear
What are the main connections shown?
- 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
For which 5 reasons is the middle ear known as a high risk space?
- 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
What is meant by pulsatile tinnutis?
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
Where is the inner ear located?
What is its composition like?
Located within petrous part of temporal bone
it is composed of 2 special sense organs
- Vestibular system - balance & equilibrium (posture)
- Cochlear system (hearing)
What is meant by the cochlea being tonotopically organised?
Different parts of the cochlea detect different frequencies of sound
the apex detects lower frequency sounds
the base detects higher frequency sounds
What nerve emerges from the cochlea?
How is it formed?
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
Label the inner ear
What are the 2 fluid-filled cavities within the inner ear?
What do they contain?
Bony (outer) labyrinth:
- this contains perilymph
- this is similar to extracellular fluid
Membranous (inner) labyrinth:
- this contains endolymph
- this is similar to intracellular fluid
what are the components of the bony (outer) labyrinth?
Vestibular system:
- vestibule
- semicircular canals (SCC)
cochlear system:
- cochlea
these contain perilymph
What is the composition of the membranous (inner) labyrinth like?
Vestibular system:
- vestibule structures
- semicircular ducts
cochlear system:
- cochlear duct
these contain endolymph
How does the cochlear duct separate the cochlea?
It separates the cochlea into two chambers:
- Scala vestibuli
- Scala tympani
these two chambers are continuous at the apex of the cochlea through a narrow slit (helicotrema)
Label the components of the cochlea
What are the roles of the oval window and round window?
- 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
Why is it important for the ossicles to amplify sound waves?
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
What is contained within the cochlear duct?
The spiral organ of Corti
What membranes are located either side of the spiral organ?
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
What is significant about the hair cells embedded within the tectorial membrane?
Axons that are in contact with the hair cells fuse to form the cochlear nerve
this is a division of the vestibulocochlear nerve
What happens when vibrations in the inner ear deform the basilar membrane?
- 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
What stimulates the spiral organ?
What does this lead to?
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
What are the characteristics of the auditory pathway?
- 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
Label the components of the auditory pathway
What is the pathway taken by the 1o auditory fibres?
- 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 2o neurones in the dorsal & ventral cochlear nuclei (ICP / CS)
What are the roles of the cochlear nuclei?
What do they receive and project?
- 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)
Label the pathway from the cochlear nuclei
What information does the superior olivary nucleus receive and transmit?
- SON receives BILATERAL auditory information from the cochlear nuclei
- sends ascending fibres to inferior colliculus via the lateral lemniscus BILATERALLY
Where are the inferior colliculus and MGN located?
How are they connected?
- 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
What components of the auditory pathway involve the inferior colliculus and medial geniculate nucleus?
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
What is another name for the primary auditory cortex?
Heschl’s gyrus
(superior temporal gyrus)
What is meant by tonotopic organisation of the cochlea & auditory cortex?
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
Low frequency - anteroLateral part
high frequency - posteromedial part
What are the 2o auditory areas and where are they found?
The left hemisphere houses the 2o auditory areas
Broca’s area:
- involved with motor functions like production of words
Wernicke’s area:
- involved with sensory functions like understanding
what type of stroke leads to non-fluent (Broca’s aphasia)?
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
What type of stroke leads to fluent (Wernicke’s) aphasia?
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
What is the purpose of the descending auditory pathways?
They are involved in responding to auditory information that we receive
What is the reflex that prevents damage during loud noise?
- 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
What is the purpose of reflex head and eye movements?
Which nerves are involved?
Involves CN III, IV and VI
this is movement in response to sound
e.g. Looking at someone who calls your name
