hearing Flashcards
what are the functions of the auditory system?
- Hearing: distinguishing different sounds + localization of sounds
- Language: production + comprehension.
what are the main sections of the ear?
outer
middle
inner
what is the mode of operation of the outer ear?
air vibration
what is the function of the outer ear?
o Protects middle and inner ear.
o Helps us to localise sound, especially high frequency sound.
o Sound energy amplification - air vibration can be channelled over a large area to the inner ear.
what is the tympanic membrane?
eardrum
name the ossicles
malleus
incus
stapsus
what is the mode of operation of the middle ear?
mechanical vibration
what are the functions of the middle ear?
mechanical vibration transmission
pressure equalisation
impedance matching
inner ear stimulation
what structure allows for pressure equalisation in the middle ear?
Eustachian tube
what is impedance matching?
difference in area between eardrum and oval window compensates for impendence mismatch between the air and the cochlear fluid
o (sound travels differently in fluid vs. air and is compensated
what is the mode of operation of the inner ear?
mechanical, hydrodynamic and electrochemical
what are the functions of the inner ear?
sound filtering
signal transduction
what is the mode of operation of the central auditory nervous system?
electrochemical
what is the function of the central auditory nervous system?
information processing
what is the pinna and what is its function?
outer ear
provides a dish shape to collect sound over 5cm2
what is the function of the external auditory meatus?
sound energy travels through this from the pinna
what does the tympanic membrane do?
allows sound to be transferred through the inner ear through the ossicles
which nerve controls the ossicles?
facial nerve
what is the function of the ossicles?
allow transmission of sound energy from eardrum into the oval window of the cochlea
what does the tensor tympani muscle do?
controls the movement of the malleus
what is the stapedius?
muscle which controls the motion of the stapes
what does the Eustachian tube do?
prevents a standing pressure difference on either side of the eardrum that would compromise function
how is the inner ear stimulated?
arrangement of ossicles transmits vibrational energy to the oval window, preventing it from being reflected away from the surface of the inner ear
what is the function of auditory reflexes?
Exist to prevent damage to the person and to the ear + distinguish sounds from background
what are the types of auditory reflexes?
attenuation reflex and startle reflex
what is the delay of the attenuation reflex
40-80ms
why is the attenuation reflex needed?
Dampens out low frequency sound – allowing better discernment of speech
how does the attenuation reflex work?
- Relies on the activation of the middle ear muscles
* Activated by own voice or loud sounds
what happens if the ossicles are damaged?
person has hyperacuisis
what is hyperacuisis?
caused by damage increased sensitivity to certain frequency and volume ranges
what structures are involved in the startle reflex?
facial nerve and reticular formation
what is the startle reflex?
- Protects the back of the neck as well as the eye in response to sudden and loud/threatening noise.
- causes you to duck down and blink
describe how the cochlea looks?
Resembles a snail shell and spirals (around 2 and ¾ turns) around the bony column
where does vibrational displacement pass into the cochlea?
the oval window
what is the function of the cochlea?
transform vibrations from the into a neural signal
what are the 3 canals in the cochlea?
o Scala Vestibule – directly connected to the oval window. Sound energy enters through here
o Scala Tympani – Sound energy exits via this through the round window
o Scala Media – cochlear duct
what separates the canals in the cochlea?
membranous structures
what separates the scala media and the cochlear duct from the scala tympani?
basilar membrane
what structure sits above the basilar membrane?
Organ of Corti
what innervates the organ of Corti?
the cochlear branch of the vestibulocochlear nerve
what is sound transduction?
conversion of mechanical signals into electrical signals
what does the organ of corti do?
transduces sound
what cells make up the organ of corti?
inner and outer hair cells
what are inner and outer hair cells?
sensory receptors which detect movement
what are stereocilia?
projections from the hair cell on their apical side which are connected to the tectorial membrane - allow us to hear
what are inner hair cells and how are they arranged?
arranged in one row: these have a role in auditory discrimination and convert the vibrations within the cochlea into a nerve signal. Not located in the tectorial membrane.
what are outer hair cells and how are they arranged?
these improve frequency discrimination. Stereocilia are embedded in the tectorial membrane in a horshoe shape.
what are the types of fluid in the cochlea and describe their composition
o Perilymph – similar to ECF. High conc of Na+ and low conc of K+
o Endolymph – high conc of K+ and low conc of Na+
what is trypA1?
mechanical channel in the stereocilia in the PNS. Needed in the transduction of cold, painful stimuli.
when are trypA1 channels open?
opened and gated in the deformation of stereocilia
explain how the organ of Corti transduces sound?
- When a sound wave enters the ear, tension is applied to the stereocilia – trypA1 channels are opened
- K+ is allowed to enter and depolarize the cell
- Calcium channels activated calcium enters the cell
- Vesicles of neurotransmitter fuse with the membrane and are released
- Afferent nerve ending that innervates the organ of Corti is depolarized sends an AP to the brain
what causes tension to be applied to the stereocilia?
When a sound wave enters the cochlea and gives rise to upwards and downwards deflections of the basilar membrane. There is a displacement and a force applied to the stereocilia which are connected to the tectorial membrane
how is loudness measured?
decibels
what is a decibel?
• A decibel is a ratio – 20 log[P/Pr]
o P = Pressure of incoming sound; Pr = Pressure of a reference pressure (pressure of a just audible sound)
what is the auditory threshold?
0 decibels - sound that is just audible
how is sound created?
variations in air pressure/time
what is the curve of the threshold for hearing sensitivity determined by?
by the transmission resonances of the outer and middle ear
what is the most sensitive range for hearing sensitivity?
1-4kHz
when is the auditory pathway used?
• Used in sound localization and discrimination of pitch and quality of sound
where is the superior olivary nucleus found and what is it used for?
pons - important for spatial localisation
where is the cochlea nucleus found and what is it used for?
medulla
o Ventral Cochlea Nucleus: fast, precise, projects to superior olivary nucleus. Encodes the intensity/timing of noise.
o Dorsal Cochlea Nucleus: complex responses. Project to lateral lemniscus and encode the quality of sound.
how can sound be localised through timing and loudness?
ITD and ILD
interaural time difference
interaural level difference
what is interaural time difference?
the difference between the times sounds reach the ears
what is interaural level difference?
difference in sound pressure level reaching the ears.
how can the sound location be detected through timing?
- Occurs at the level of the superior olivary nuclei
- Sound reaches the left ear first
- AP begins travelling to the MSO
- Sound reaches the right ear a little later
- AP from right ear begins travelling towards MSO
- APs converge on an MSO neuron that responds more strongly if their arrival is coincided
how is location detected through sound intensity?
- Stronger stimulus to the left ear excites left LSO
- This stimulus also inhibits right LSO via MNTB interneuron
- Excitation from left side is greater than inhibition from right side, resulting in net excitation to higher centres
- Inhibition from left side is greater than excitation from right side, resulting in net inhibition on right and no signal to higher centres.
- Bc there is crossed inhibition, the olive receiving the greater input is going to inhibit the contralateral olive, which allows us to discern what side the sound is coming from
how can you test for damage to the auditory pathway?
Tuning fork tests (Rinne’s test and Weber’s test)
what damage is tested for during the tuning fork tests
conduction deafness and sensorineural deafness
what is conduction deafness?
problem conducting sound from one area to the other
what is sensorineural deafness?
cause lies in the inner or sensory organ
describe the rinne’s test method?
vibrating tuning fork is placed on the mastoid process. Can hear the sound being conducted through the bone. Allow the vibrations to become diminished so you can no longer discern the bone sound. Hold the tuning fork next to the ear – should be able to hear.
what is the normal response to the rinne’s test?
Sound is heard louder and longer (>15s) by air conduction as sound energy dissipates quickly while travelling through bone. If sound ceases within 15s it suggests a middle ear problem.
describe how the weber’s test is carried out?
tuning fork placed in middle of forehead and ask patient in which ear is tone heard.
what is the normal response to the weber’s test?
Sound heard equally well on both sides
what is the response to the weber’s test in someone with conduction deafness and why?
sound is louder in affected ear due to fact that vibrations reach cochlea by both air and bone but these are slightly out of phase and so interfere with each other in the more sensitive ear
what is the response to the weber’s test in someone with sensorineural deafness?
sound is louder in unaffected area
why can weber’s test not be interpreted in isolation?
bc it only detects the relative difference between the 2 ears
what is otitis media?
build up of mucus in the middle ear. No longer allows movement of the ossicles + prevents sound pressure equalization across the tympanic membrane.
name middle ear disorders
- Perforation of the tympanic membrane
* Otosclerosis
what are the functions of the auditory cortex?
- To identify complex auditory sounds
- To detect changes in auditory environment.
- To learn about behaviourally relevant sounds
- To integrate attention and auditory processing
- Higher functions: language and musical appreciation.
what does damage to wernicke’s area cause?
fluent meaningless speech – severely impaired speech understanding
what does damage to broca’s area cause?
abbreviated, ungrammatical but meaningful speech – speech understanding impaired where syntax conveys meaning
what does damage to the arcuate fasciculus cause?
conduction aphasia
what does damage to the angular gyrus cause?
alexia (cant read), with agraphia (cant write) but can comprehend speech and speak
