Auditory Pathway Flashcards
Which skull bone does the ear sit around?
Temporal bone
What 2 parts can the temporal bone be divided into?
- Squamous; lateral, flat portion
- Petrous; rough portion
The ear is divided into 3 portions. What are they?
- External
- Middle
- Inner
The external ear can be divided functionally and structurally into two parts. What are they?
- Auricle (or pinna)
- External acoustic meatus
In the middle of the auricle is a hollow depression. What is it called? What does it continue into the skull as?
Concha - continues into the skull as the external acoustic meatus
Function of the concha/auricle?
funnels sound waves into the ear canal via the external acoustic meatus
Where does the external acoustic meatus end?
Tympanic membrane
Function of the
a) external ear
b) middle ear
c) inner ear?
a) receives sound waves
b) transmits sound waves into mechanical waves
c) transmits sound waves into electrical waves
Describe the composition of the external acoustic meatus
- Lateral 1/3rd of the canal is cartilaginous
- Medial 2/3rds of the canal is bony, made up of temporal bone
In the middle ear, how are sound waves transmitted into mechanical waves?
Vibration of ossicles
What are the ossicles? Names?
3 bones of the middle ear are the auditory ossicles that link the tympanic membrane to the oval window of the internal ear.
- Malleus (most lateral)
- Incus
- Stapes (most medial)
What are the smallest bones in the body?
Ossicles
What is the middle ear? Location?
Air filled space in the petrous part of the temporal bone
What separates the middle from the external ear?
Tympanic membrane
Which ossicle is in contact with the middle ear?
Stapes
Function of the ossicles?
- Vibrate to transmit/amplify sound waves into cochlea ear
- Bones vibrate to amplify sound
Ossicles can can calcify with age, becoming less flexible and less able to vibrate. What can this cause?
This can cause conductive hearing loss. May be progressive or complete.
How is the middle ear in contact with the nasopharynx?
Via the Eustachian (auditory tube)
What is the Eustachian tube? What does it connect?
Tube that connects nasopharynx to the middle ear
3 major functions of the Eustachian tube?
- Drains fluids into nasopharynx
- Ventilates the middle ear
- Equalise pressure between external environment and pressure on the inside
Why are infants more prone to ear infections than adults?
As they have a smaller and more horizontal Eustachian tube –> more direct route for infections to access the tube and therefore the middle ear
As we grow, how does our Eustachian tube change shape?
Becomes more vertical with age
Is the Eustachian tube typically held open or closed?
Open
How is the Eustachian tube affected when there is a big change in atmospheric pressure e.g. airplane, scuba diving?
In certain situations when there is a big change in atmospheric pressure, then the tube is held more tightly closed e.g. airplane, scuba diving (why your ears hurt)
Why does swallowing/opening mouth relieve ear pain in airplane/scuba diving?
Swallowing, opening mouth, or closing nose and expelling air can forcefully open the Eustachian tube
Where does the Eustachian tube drain fluid into?
Nasopharynx
The middle ear is a high risk space for infection. Why is this?
Is connected to nasopharynx via Eustachian tube –> prone to infection
What is infection of the middle ear called?
Otitis media
What 4 major infections can a middle ear infection lead to?
- Connect to mastoid air cells (of mastoid process) –> infection may spread across these structures to middle cranial fossa and potentially brain
- Internal jugular vein lies inferior to middle ear –> infections of middle ear can spread to IJV and damage lining leading to thrombosis risk
- Internal carotid artery lies anterior and inferior –> link to pulsatile tinnitus
- Traversed by chorda tympani and facial canal – infection risk of facial muscles, tongue, stapedius muscle, lesser petrosal nerve and parotid gland
What is pulsatile tinnitus?
Perception of a sound without external sound, sound corresponds to heart rate and can be caused by increased blood flow through arteries in the middle ear.
Infection of which vessel can lead to pulsatile tinnitus?
Internal carotid artery
Where is the inner ear located?
In the petrous part of the temporal bone
The inner ear is composed of 2 special sense organs. What are they? Function of each?
- Vestibular system; balance and equilibrium
- Cochlear system; hearing
Shape of cochlea?
Snail shaped
Frequencies of sound are detected by different areas of cochlea.
a) where are lower frequency sounds detected?
b) where are higher frequency sounds detected?
a) apex of cochlea
b) base of cochlea
What is the range of frequencies that humans can hear?
between 20-20,000 Hx
The inner ear has two main components. What are they?
- Bony (outer) labyrinth
- Membranous (inner) labyrinth
These are fluid filled cavities.
Describe the bony labyrinth. What structure is it composed of? What fluid does it contain?
- Consists of a series of bony cavities within the petrous part of the temporal bone.
- It is composed of the cochlea, vestibule and three semi-circular canals.
- All these structures are lined internally with periosteum and contain a fluid called perilymph.
Describe the membranous labyrinth. What is it composed of? What fluid does it contain?
- Membranous labyrinth – lies within the bony labyrinth
- It consists of the cochlear duct, semi-circular ducts, utricle and the saccule.
- The membranous labyrinth is filled with fluid called endolymph.
Fluid of cochlea vs cochlear duct?
Cochlea - perilymph
Cochlear duct - filled with endolymph
What is the cochlear duct?
a closed tube within the cochlea
What does the cochlear duct separate the cochlea into?
2 chambers (both filled with perilymph):
- Scala vestibuli (SV)
- Scala tympani (ST)
Where are the 2 chambers of the cochlea continuous?
These two chambers are continuous at the apex of the cochlea through a narrow slit (helicotrema)
The inner ear has two openings into the middle ear, both covered by membranes. What are they?
- The oval window; lies between the middle ear (stapes) and the vestibule
- The round window; separates the middle ear from the scala tympani (part of the cochlear duct).
Where does the fluid in the ear begin? How are vibrations transmitted through this?
- Past the oval window there is fluid.
- Vibrations are transmitted into fluid through the perilymph at the level of the scala vestibuli which is then continuous with the scala tympani at the helicotrema. Vibrations are transmitted to the scala tympani here.
- Movement of perilymph causes deformation of fluid inside cochlear duct (endolymph)
Amplification of sound by ossicles necessary for transfer to fluid filled medium. What does vibration of stapes cause?
- Vibration of the stapes creates hydraulic pressure in the perilymph of the scala vestibuli, passing to the apex (helicotrema) and descend via the scala tympani, through the round window.
- As the fluid (perilymph) moves around the bony cochlea, it deforms the endolymph inside the cochlear duct.
- Scala vestibuli (SV)
- Scala tympani (ST)
Part of bony or membranous labyrinth?
Bony
What is the spiral organ of corti? Where is it?
Is a strip of sensory epithelium within the cochlear duct made of hair cells which acts as the sensory receptors of the inner ear.
What are the 3 components of the spiral organ of corti?
- Tectorial membrane; found above
- Hair cells; actual receptors
- Basilar membrane; forms base
What are the hair cells of the spiral organ of corti connected to?
Cochlear nerve fibres (the axons of the hair cells fuse to form the cochlear nerve CN VIII)
Describe how sound is transmitted through/from the spiral organ of corti
- Upon movement of the cochlear duct, the tectorial membrane moves up and down.
- The hair cells are in contact with the membrane and detect movement and ion channels open.
- APs travel in axons which travel to cochlear nerve
Fluid pressure is converted into electrical signals via the cochlear nerve.
Summary slide part 1:
- Cochlea contains perilymph.
- The cochlear duct is a closed tube within the cochlea. It contains endolymph
- The cochlear duct separates the cochlea into two chambers:
- Scala vestibuli (SV)
- Scala tympani (ST)
- These two chambers are continuous at the apex of the cochlea – helicotrema
- Vibrations of stapes create hydraulic pressure created in the perilymph of the SV
- Such vibrations of the stapes pass to the apex (helicotrema), where SV and ST meet
- Vibrations then descend via the ST to the round window
- Stapes → perilymph → SV → helicotrema→ ST → Round window
- As the fluid (perilymph) moves around the bony cochlea it deforms the fluid (endolymph), inside the cochlear duct.
- The cochlear duct contains the Spiral organ of Corti.
- The spiral organ of Corti is stimulated by the deformation of the cochlear duct by the perilymph in the surrounding SV and ST
- Converts fluid pressure into electrical signals via cochlear nerve (CN VIII)
What is the spiral organ of Corti stimulated by?
The spiral organ of Corti is stimulated by the deformation of the cochlear duct by the perilymph in the surrounding SV and ST
Briefly outline the auditory pathway checkpoints
Cochlear nerve –> cerebellopontine angle (cochlear nuclei) –> mid pons (superior olivary nucleus) –> caudal midbrain (inferior colliculus) –> medial geniculate nucleus –> auditory cortex
What type of neurons are the 1ary auditory fibres?
Bipolar neurons; 2 processes extending from cell body (1 connects to hair cell and other to cochlear nuclei in brainstem)
Where are the cell bodies of the 1ary auditory fibres located?
Cell bodies located in spiral ganglion of the cochlea
Describe the route of the 1ary auditory cells from hair cells to brainstem
- Hair cells are depolarised, action potentials are formed and are transmitted down the bipolar neurones.
- Axons of bipolar neurons form the cochlear division of the vestibulocochlear nerve CN VIII
- CN VIII enters the brainstem at the cerebellopontine angle
- Synapses with 2nd order neurones in dorsal and ventral cochlear nuclei, which are in the lateral aspect of the open medulla.
Where does CN VIII enter the brainstem?
Cerebellopontine angle
What do the axons of the 1ary auditory fibres form?
the cochlear division of the vestibulocochlear nerve CN VIII
Where are the dorsal and ventral cochlear nuclei located?
in the lateral aspect of the open medulla
After synapsing with 2nd order neurons at the cochlear nuclei, where are fibres sent?
Cochlear nuclei project bilaterally to the superior olivary nucleus (SON) where they synapse with 3rd order neurons
- One projection to ipsilateral SON
- Another projection to contralateral SON
Where is the SON located?
Pons
The fibres that cross over to the contralateral side at the level of the SON form what?
a thick bank of white matter called the trapezoid body
What is the purpose of the auditory pathway being bilateral?
allows retained hearing if one-sided lesion
The SON receives bilateral auditory information. Where are fibres then sent?
Sends ascending fibres to the inferior colliculus via the lateral lemniscus bilaterally –> synapse with 4th order neurons at the inferior colliculus
What is the lateral lemniscus?
Lateral lemniscus is a tract of axons in the brainstem that carries information about sound from the cochlear nucleus to various brainstem nuclei, and ultimately the contralateral inferior colliculus of the midbrain
Where does the auditory pathway reach after the SON?
The inferior colliculus (via the lateral lemniscus)
Where are the inferior colliculi found?
Two inferior bumps on the dorsal aspect (tectum) of the midbrain
Function of the inferior colliculus?
The inferior colliculus is a part of the midbrain that serves as a main auditory centre for the body. It acts as the channel for almost all auditory signals in the human body.
What does that inferior colliculus connect to?
Connects to the medial geniculate nucleus on the thalamus (MGN) via the brachium of the inferior colliculus
Function of the brachium of the inferior colliculus?
carries auditory afferent fibres from the IC to the MGN
Where do afferent auditory fibres pass after the inferior colliculus?
medial geniculate nucleus on the thalamus via the brachium of the inferior colliculus (then synapse with 5th order neurons)
Does the inferior colliculus receive unilateral or bilateral fibres from the lateral lemniscus?
Bilateral
What is the commissure of the inferior colliculus? Function?
- A white matter structure consisting of myelinated axons that joins the paired inferior colliculi
- Means to preserve bi-laterality in case of lesion to reach the other inferior colliculus
How can the inferior colliculi exchange information?
via the inferior colliculus commissure
Where do auditory fibres pass after the medial geniculate nucleus of the thalamus?
ibres pass to the primary auditory cortex via the acoustic radiation
Where is the primary auditory cortex (also called Heschl’s gyrus)
Lies in the superior temporal gyrus of the temporal lobe
Which 2 parts of the auditory system are tonotopically organised?
Cochlea
Auditory cortex
How is the cochlea tonotopically organised?
- Different regions of the basilar membrane are sensitive to different frequencies
- Apex = low pitch
- Base = high pitch
How is the auditory cortex tonotopically organised?
- Low frequency = anterolateral part
- High frequency = posteromedial part
Where is the 2ary auditory cortex? Function?
- 2ary auditory cortex surrounds the 1ary auditory cortex
- Function: interprets sound and gives it meaning
Which hemisphere houses the 2ary auditory area?
Hemispheres have specific dominant functions: the left hemispheres (dominant) house the secondary auditory area, specialised for linguistic function.
What is the dominant hemisphere?
The cerebral hemisphere that controls language is called the dominant hemisphere. In most individuals, language functions are processed in the left hemisphere.
What is Wernicke’s area? Location? Function?
- A region of the brain important for language development
- Located in temporal lobe on left side of brain (posterior to 1ary auditory cortex)
- Responsible for comprehension (understanding) of speech
What is Broca’s area? Location? Function?
- Region in the frontal lobe of dominant hemisphere (usually left) anterior to the 1ary auditory cortex
- Function: speech production, motor
- Anterior
What is aphasia?
Aphasia is an inability to comprehend or formulate language because of damage to specific brain region.
How would damage to Broca’s area present?
Broca’s aphasia:
- Significantly affect the use of spontaneous speech and motor speech control. Words may be uttered very slowly and poorly articulated.
- Non-fluent but makes sense.
- People with Broca’s aphasia typically understand the speech of others fairly well.
E.g. Broca’s aphasia may say, “Walk dog,” meaning, “I will take the dog for a walk,” or “book book two table,” for “There are two books on the table.”
How would damage to Wernicke’s area present?
Wernicke’s aphasia:
- Fluent but impaired understanding i.e. sentences don’t make sense
- E.g. ‘In the pier, somebody is fishing. Somebody is playing into the water. The man with the kite has the jeans with the pocket deal.’
Damage to which vessel can lead to Broca’s aphasia?
Damage to the the anterior distribution of the left middle cerebral artery
Damage to which vessel can lead to Wernicke’s aphasia?
Damage to the posterior distribution of the middle cerebral artery
What is the acoustic reflex?
The acoustic reflex is the contraction of stapedius and tensor tympani muscles of the ossicles when presented with an intense sound stimulus.
Which cranial nerves are involved in the acoustic reflex?
- To stapedius muscle via CN VII
- To tensor tympani muscle via CN V3
At what level does the acoustic reflex occur?
at the level of the superior olivary nucleus
Which auditory reflex occurs at the level of the inferior colliculus?
Via some interneurons, the auditory information that has reached the IC can then reach nuclei of cranial nerves III, IV and VI which innervate the eye movements as well as spinal nerves to help you move head and neck (turn your head and look when someone calls your name)
Overall Pathway Overview
