Hearing Flashcards
What are the two aspects of hearing?
Distinguishing between different sounds
Localisation (locating source)
Two aspects of language?
Production
Comprehension
Purpose of outer ear
- Detects air vibration
- Functions in protection, localisation and amplification
Middle ear functions
- Detects mechanical vibrations
- Functions in impedance matching
- Pressure equalisation
- Inner ear stimulation
What is impedance matching?
Matching the air vibration to the vibrations in the cochlear
How is pressure equalisation done?
Via Eustachian tube
How is inner ear stimulation done?
Via stapes
What are the functions of the inner ear?
- Detects mechanical, hydrodynamic and electrochemical signals
- Functions in sound filtering and signal transduction, converting the signal to send it to the brain
- Maintain balance by detecting position and motion
What is the anatomy of the outer ear?
(Structures between outer ear and the tympanic membrane)
- cartilages
- concha
- external acoustic meatus
What is the concha?
depression in middle that acts to direct sound into external acoustic meatus
What is the external acoustic meatus?
- a sigmoid shaped canal
- extends from concha to tympanic membrane
- supported by cartilage and temporal bone
What is the basic anatomy of the middle ear?
(Tympanic membrane to beginning of the cochlea)
- tympanic membrane
- umbo
- malleus (1/3 of the middle ear ossicles)
- incus (1/3 of the middle ear ossicles)
- stapes (1/3 of the middle ear ossicles)
- start of the eustachian tube
What is the tympanic membrane?
Sheet of thin, fibrous tissue that vibrates when it is hit with sound waves (i.e. ear drum)
What is the umbo?
A depression in the middle where the malleus attaches and extends upwards from.
Where does the malleus attach?
At the umbo on the TM and then on the incus
What 3 bones make up the middle ear ossicles?
Malleus
Incus
Stapes
What is the Eustachian tube?
- Cartilaginous and bony tube
- Drains into nasopharynx
- Functions to equalise pressure of the middle ear to match that of the external acoustic meatus
What is the basic anatomy of the inner ear?
(Beginning of cochlear to internal acoustic meatus)
- found within petrous part of temporal bone
- ‘bony labyrinth’ formed of:
- bony cavities
- vestibule
- cochlea
- semi-circular canals
- ‘membranous labyrinth’
Describe the bony cavities of the bony labyrinth of the inner ear
Series of bony cavities filled with periosteum and containing a fluid called perilymph
Describe the vestibule of the inner ear
- Central part
- Connected to both cochlea and semi-circular canals
- Contains the oval window where the middle ear connects to the inner ear via the stapes
Describe the cochlea of the inner ear
- Houses the cochlear duct
- Twists upon itself around a central portion of bone
- Produces a cone shape that points in the anteriolateral direction
- Contains 3 canals
- Contains the organ of Corti
What is the cochlear duct?
The auditory part of the ear
What are the 3 canals in the cochlea?
Cochlear duct (scala media)
Scala vestibuli
Scala tympani
What is the organ of Corti?
The receptor responsible for hearing
What are the semi-circular canals?
- anterior, lateral and posterior canals
- they contain the semi-circular ducts responsible for balance
What is responsible for balance?
Semi-circular ducts in the semi-circular canals of the inner ear
What is responsible for hearing?
The organs of Corti in the cochlea
What is the membranous labyrinth of the inner ear?
- lies within the bony labyrinth and filled with a fluid called endolymph
- made up of 3 cochlear canals
What are the 3 cochlear canals of the membranous labyrinth?
- saccule
- utricle
- semi-circular ducts
Where is the organ of Corti found?
Between the Scala tympani (canal of the cochlea) and the vestibule
What is the structure of the organ of Corti?
- Composed of mechano-senosry cells known as hair cells
- On basilar membrane, 3 rows of outer hair cells and 1 row of inner hair cells
What is the role of the inner hair cells in the organ of Corti?
Auditory discrimination
What are the cilia like in the inner hair cells of the organ of Corti?
They are not embedded in the tectorial membrane
What is the role of the outer hair cells in the organ of Corti?
Frequency tuning and amplification of sound signals
What are the cilia like in the outer hair cells?
They are embedded in the tectorial membrane.
How are the sound waves converted?
Vibration to electrical signals via electrical transduction
Describe the process of electrical transduction
- Inner ear splits sound in simple components and amplifies the signals through the outer hair cells
- Converts mechanical signals of the hairs moving into electrical signals
How does the inner ear convert mechanical signals to electrical signals?
By opening (depolarisation) or closing (hyperpolarisation) potassium channels, depending on direction of hair movement
What is the apex of the cochlea?
The centre of the coil
What is the oval window?
CT membrane-covered opening that is the intersection of the middle ear with the inner ear.
Describe the basilar membrane of the cochlea
Thin and side at apex and thick and narrow at the base.
Which parts of the cochlea membrane respond to which sounds?
Base - vibrates most strongly to high freq
Apex - low freq
How to specific hair cells communicate with the brain?
Hair cells have own connections to the brain - each wired to produce different note depending on relative position along cochlea.
What is the function of the outer hair cells?
They amplify the basilar membrane vibrations, enhancing sensitivity and frequency selection.
How is volume created?
Intensity of vibrations
Describe how a louder sound results in a sound being perceived as louder in the brain
Greater amplitude of vibrations = greater vibrations in basilar membrane = more signals sent to brain = sound perceived as louder
What parts of the ear are filled with air?
External and middle
What part(s) of the ear is/are filled with fluid?
Inner
Why must impedance matching occur?
So that sound can be transferred and heard correctly, despite different areas of the ear being filled with different mediums with different resistances.
Where does impedance matching occur?
Middle ear
What is the purpose of the ossicles in impedance matching?
Their relative sizes ensure the movement causes the same amount of vibration to be transferred from the air to the fluid. Stapes transfers more vibration than the malleus relatively.
What two mechanisms combine to give impedance matching?
- TM larger than oval window so that more vibration transferred to the cochlear than was initially transferred to the TM.
- Ossicles transfer vibration - stapes relatively more to combat increased resistance (?)
What is the function of an acoustic reflex?
To prevent damage to the ear structures and distinguish sounds from the background noise.
What are the two acoustic reflexes?
Attenuation reflex
Startle reflex
What is the attenuation reflex activated by?
Own voice or loud sounds
What is the effect of the attenuation reflex?
Contraction of muscles (such as stapedius) to dampen transmission from middle to inner ear.
What are the weaknesses of the attenuation reflex?
- Preference for low frequencies
- 50-100ms delay so not much protection from sudden loud noises
How can damage to the attenuation reflex occur?
Facial nerve (CN VII) damage as this supplies stapedius muscle
What does damage to the attenuation reflex cause?
Hyperacuisis - increased sensitivity to loud noises
What is the startle reflex?
Defensive response against sudden or threatening stimuli
What does the Startle reflex involve?
facial nerve and reticular formation
What is the function of the startle reflex?
to protect the back of the neck (whole body startle) and eyes (eyeblink)
Purpose of the ventral cochlear nucleus
Fast and precise
Encodes intensity by stellate cells
Encodes timing by bushy cells
Placement of the ventral cochlear nucleus
Projects to superior olivary nucleus
Purpose of the dorsal cochlear nucleus
More complex responses
Encodes the quality of sound
Analyses pitch by detecting minor pitch differences
Placement of the dorsal cochlear nucleus
Projects to the lateral lemniscus
What is the pathway of the vestibulocochlear nerve?
- cochlear nerve goes to cochlear nucleus in the medulla, entering brainstem at ponto-medullary junction
- then superior olivary nucleus in pons
- … or straight to inferior colliculus in midbrain
- next, medial geniculate nucleus in thalamus
- finally, primary auditory cortex in the superior temporal gyrus of the temporal lobe
Where is the primary auditory cortex?
In the superior temporal gyrus of the temporal lobe
Where does sound localisation occur?
In the superior olivary nucleus
How does sound localisation work?
Uses the interaural time differences (difference between the times sounds reach the two ears) and interaural level differences (differences in loudness in each ear) to localise sound cues.
When are interaural time or level differences more useful?
- Interaural time differences more useful at low frequencies
- Interaural level differences more useful at high frequencies
What are the two classifications of deafness?
- conduction
- sensorineural
What is conduction deafness?
- Hearing loss due to sounds not being able to pass freely into the inner ear
- Sounds become quieter but not distorted
- Can be temporary or permanent
What causes conduction deafness?
blockage (excess wax or fluid), anatomical abnormality of the outer ear, ear canal or middle ear or ruptured tympanic membrane
What is sensorineural deafness?
- Hearing loss due to damaged hair cells or cochlear nerve
- Sounds become quieter and the quality of speech is also reduced
- Damage often irreversible
What causes sensorineural deafness?
Normal ageing but also prolonged exposure to loud noises, ototoxic drugs, infectious diseases such as Rubella, head injury or benign tumours
Name 2 tests for hearing
Rinne’s and Weber’s
Describe Rinne’s test and it’s purpose
- Vibrating tuning fork places on mastoid process and then next to ear canal
- Compares air conduction to bone conduction
Describe and explain the results of Rinne’s test
Normal = sound heard louder and for longer next to ear (air conduction)
Conduction deafness = bone conduction better than air on affected side
Sensorineural deafness = air conduction better on both sides but louder in unaffected ear
Describe Weber’s test
Tuning fork placed in the middle of the forehead
Describe the results of Weber’s test
Normal = sound equal on both sides
Conduction deafness = sound louder in affected ear
Vibrations reach ear by both air and bone conduction but are out of sync so will interfere and actually seem louder
Sensorineural deafness = louder in unaffected ear
Name 3 middle ear disorders
- TM perforation
- Otosclerosis
- Otitis media
What is Otosclerosis?
progressive deafness due to overgrowth of bone near the middle ear – mainly affects the stapes
What is Otitis media?
inflammation of the middle ear caused by infection
What can TM perforation cause?
conductive hearing loss and ear infections
Name 2 tumours affecting the ear
Schwannoma and Meningioma
What is a schwannoma?
- Benign neoplasm arising from the myelin sheaths of cranial nerves
- Most common in the vestivular nerve
- Unilateral hearing loss and tinnitus
What is a meningioma?
- Arises from the arachnoid layer of the meninges
- Can affect any of Cn V, VII, VIII, IX, X and XII
What is the effect of damage to Wernicke’s area?
Fluent but meaningless speech
What is the effect of damage to Broca’s area?
Abbreviated, ungrammatical but meaningful speech
What is the effect of damage to the arcuate fasciculus?
Conduction aphasia - fluent speech production but poor repetition
What is the effect of damage to the angular gyrus?
Alexia (can’t read) with agraphia (can’t write) but can comprehend speech and speak themselves.
