Flashcards in Hearing Deck (23):
What is the function of the ossicles?
To match the impedence of the air to impedence of the fluid within the inner ear
How is the reflection of sound which normal occurs when passing from air to fluid overcome by the structure of the ear?
Tympanic membrane has a pressure 200x greater than the inner ear, and is significantly larger than the oval window (20:1)
The lever action of the ossicles results in greater pressure on the stapes (1.3:1)
What are the 3 chambers in the cochlear and the intervening membranes?
What is the role of the pinna of the ear?
For localisation of sound in the vertical domain
Describe the structure of the basilar membrane
Wider at the apex than the base
Stiffer base than apex
What are the relative roles of the apex and base of the basilar membrane in hearing?
Apex responds to low frequencies
Base responds to high frequencies
What are the auditory receptors and where are they located?
Sandwiched between basilar membrane and reticular lamina
What are the 2 types of hair cells? What are their roles?
Inner (95%): project to brainstem nuclei
Outer (5%): efferent inputs from the superior olivary complex (for amplification of movements of the basilar membrane and therefore sound, particularly quiet sound, so that a smaller range of hair displacement is required for a large range of sound pressures)
How is sound transduced into a neural signal by the hair cells?
Sound-induced vibration of the basilar membrane causes the tectorial membrane to exert a shearing force on the stereocilia of the hair cells
Deflection of the hair bundle towards the tallest stereocilium (the kinocilium) causes opening of mechanically gated K+ channels leading to depolarisation of the cell (opposite occurs for movement away from the kinocilium)
Depolarisation results in the opening of voltage-gated calcium channels and release of glutamate
Does the hair cell experience larger depolarisations or hyperpolarisations?
Describe the cycling of K+ in the cochlear
Endolymph in the scala media is high in K+ to enable depolarisation of the hair cells following K+ channel opening
Once it has entered the cells, the K+ empties into the perilymph in the scala tympani via basolateral K+ channels
It is then recycled via the stria vascularis into the endolymph (so that the perilymph typically does not contain K+)
What stimulates contraction of the outer hair cells?
Depolarisation of the inner hair cells (hyperpolarisation causes elongation)
How do antibiotics cause hearing loss?
Via loss of outer hair cells
How much do outer hair cells amplify basilar membrane movement by?
Describe the nuclei and pathways involved in the transmission of auditory informtion
Ear: sound vibration causes activation of hair cells and subsequent transmission of a neural signal via CNVIII and the spiral ganglion
Brainstem: several nuclei involved including cochlear nucleus (medulla) and superior olive, information carried by lateral lemniscus
Midbrain: inferior colliculus
Thalamus: medial geniculate nucleus
Temporal lobe: auditory cortex
What are the main components of the superior olivary complex and what are their roles?
Lateral superior olive: localisation of sound by detecting intensity differences
Medial superior olive: localisation of sound by measuring time delay
Trapezoid body: medial nucleus also involved in intensity differences
Which is more important for the localisation of low frequency sounds: time delay or interaural intensity difference?
Which is more important for the localisation of high frequency sounds: time delay or interaural intensity difference?
Interaural intensity difference
What is columnar organisation in the auditory cortex?
Cells in same column are tuned to the same frequency
Describe the pattern of input to the auditory cortex
Alternating regions of input from both ears (excitatory from one ear and inhibitor from the other)
In what hemisphere are speech, environmental and music sounds processed?
What is sudden sensorineural hearing loss?
Sudden loss of hearing of at least 30db within 3 hours to 3 days