What is being detected when we perceive sound?
Briefly describe how different elements of the anatomy of the auditory system are involved in sound perception?
Tympanic membrane moves ossicles
Ossicles move oval window
Cochlea tranforms physical motion of oval window into a neural response
What is the function of the ossicles?
Matches the impedance of air to that of fluid in inner ear
Describe the pressure that the ossicles must overcome?
Pressure 200 fold greater at tympanic membrane compared to inner ear
Size: tympanic memrane > oval window (20:1)
Lever action of ossicles: 1.3:1
Describe the three chambers of the cochlea?
Where are the basilar membrane and tentorial membrane located?
Describe the basilar membrane?
Wider at apex than base
Stiffer at base
Base responds to high frequencies, apex responds to low frequencies
(Like a flipper)
What is the function of the basilar membrane?
Vibrates in response to sound waves
(certain areas vibrate depending on frequency)
Describe the functional arrangement of the basilar membrane?
Tonotopic encoding of sound
Low frequencies at apex, high frequencies at base
Where is the Organ of Corti situated?
In middle of cochlea
Sandwiched between basilar membrane and reticular lamina
What does the Organ of Corti consist of?
Auditory receptors: inner and outer hair cells
Where do the inner and outer hair cells sit in relation to other inner ear structures?
Sit on top of basilar membrane
What is the function of the Organ of Corti?
Transduces soundwaves into neural signals
Describe the hair cells of the inner ear?
Inner and outer
Each hair cell has about 100 stereocilia
Describe the attachments of the sterocilia?
Hair cell to tectorial membrane
Describe the role of the stereocilia?
bending of stereocilia causes neuronal signalling
Describe how soundwaves initiate auditory transduction?
Movement causes basilar membrane to osciallte
Causes stereocilia to move
Shear force of stereocilia against tectorial membrane causes ion conductances
What is the tallest stereocilium in a hair bundle called?
Describe how opening and closing of K channels within a hair cell is achieved?
K channels partially open
Deflection of hair bundle towards kinocilium causes more opening of channels > depolarisation
Hyperpolarisation > channel closure
What is the effect of depolarisation in the hair bundles?
Depolarisation > opening of voltage-dependent Ca channels > glutamate release
How does membrane potential differ according to different movements of hair bundles?
Larger depolarisations than hyperpolarisations
Describe the difference in K within the chambers of the cochlea?
Scala media: high K
Scala tympani: low K
Describe the difference in function between the inner and outer hair cells?
Inner: for hearing
Outer: amplify movements of basilar membrane (depolarisation > contraction, hyperpolarisation > elongation)
Describe the effect of loss of OHCs?
Basilar membrane movement is 100 fold less > loss of hearing
What can cause loss of OHCs?
Antibiotics and other medications
Describe the auditory pathway?
How is sound from both ears integrated?
Extensive crossing across midline at brainstem level
Information from cochlear nuclei on either side communicate at superior olivary complex
Describe the superior olivary complex?
Conists of lateral and medial superior olives and trapezoid body
These structures are crucial for working out where sound is coming from
What are the functions of the medial and lateral superior olives?
MSO: localisation of sound by measuring time delay
LSO: localisation of sound by sensing intensity differences
Describe the duplex theory of sound localisation?
Provides an explanation for the ability of humans to localise sounds by time differences between the sounds reaching each ear and differences in sound level entering the ears.
1) Time that sound arrives at each ear
2) Interaural intensity differences
Which sound frequencies are involved in each part of the duplex theory?
1) Time that sound arrives at each ear > low frequencies
2) Interaural intensity difference > high frequencies
Describe how sound localisation is achieved using time differences?
Describe how interaural intensity difference is used to localise sound?
Info carried down to cochlear nuclei and down to LSO
Excitation to higher centres
At same time, processes to contralteral trapezoid body > communicates with inhibitory neuron in TB > TB sends inhibitory singal to ipsilateral LSO
At same time, same process is happening in opposite ear
So, brain gets relative amount of excitation and inhibition from each ear to determine where sound is coming from
Describe the orgnaisation of the auditory cortex?
Columnar organisation: cells in same column are tuned to the same frequency
Also, alternating regions of input from both ears (excitatory from one ear, inhibitory from other)
Describe the representation of sounds in the auditory cortex?
Speech sounds: left
Environmental sounds: both hemispheres
Describe the time frame within which sudden sensorineural hearing loss occurs?
Sudden loss of hearing of at least 30db within 3 hours to 3 days
What are the causes of sudden hearing loss?
10-15% have known cause:
Peripheral causes (meningitis, metastasis...)
Central causes (MS)
Cochlear causes (infection, autoimmune disease, trauma, vascular...)
Which structures in the superior olive are importnat in which components of the duplex theory?
MSO - time differences
LSO & TB - intensity differences