Transduction is the conversion of sound wave to voltage
Steps to cochlear transduction:
Sound wave causes fluid wave in cochlea
Cochlear fluid wave causes differential movement of different compartments of cochlear
This causes tectorial membrane movement relative to basilar membrane
This causes movement of fluid near steriocilia of hair cell
Fluid movement near cilia cause cilia to move
Cilia are mechanoreceptors
Voltage in hair cell changes
Hair cells do not fire action potentials. They synapse on spiral ganglion cells and those fire action potentials
Scalia media: contains endolymph
Scala vestibule and tympani: contains perilymph
Sound is quantified by amplitude and frequency
The ear drum is also known as the tympanic membrane
The surface area of the tympanic membrane is much larger than the oval window
scala media: contains endolymph
scala vestsibuli and tympani: contain perilymph
The three ventricles of the cochlea:
Scala vestibuli
scala media
scala tympani
low-frequency sounds excite basilar membrane motion near the apex
high-frequency sounds excite basilar membrane motion near the base
Tuning curves for cochlear hair cells: experimenter presents sound at each frequency at increasing amplitudes until the cell produces a criterion response.
Tuning curves for cochlear hair cells: plot stimulus frequency against stimulus intensity
Movement of the basilar membrane creates a shearing force that bends the stereocilia of the hair cells > beginning cell firing
Mechanoelectrical auditory transduction:
hair bundle deflected toward tallest stereocilium
cation-selective channels open near tips of stereocilia
K+ ions flow into hair cell down electrochemical gradient > depolarizing
voltage-gated Ca++ channels open
cell firess
Labeled line hypothesis holds that the CNS determines the type of stimulus based receiving input from all sensory cells activated by that stimulus
Tonotopy: the spatial arrangement of where sounds of different frequency are processed in the brain. Tones close to each other in terms of frequency are represented in topologically neighbouring regions in the brain
Tonotopic organization of the cochlea is maintained at successive levels in the brain
Phase-locked activity: spikes come at same spot in sound cycle (not necessarily every cycle)
Phase locking preserves temporal information
Phase locking does not occur for sounds >2-3 kHz
auditory temporal information is used for sound localization
Auditory nerve fibers carry information about stimulus:
frequency
timing
duration
Characteristic frequency: the preferred frequency of an auditory cell
types of responses to auditory stimuli:
phasic (transient spike)
tonic (sustained)
off (not firing)
spontaneous
Auditory afferent neurons:
Type I
Type II
Type I auditory afferent neuron: carry information from inner hair cells
Type II auditory afferent neuron: carry information from outer hair cells
Two types of auditory efferent neurons:
Lateral olivocochlear neuron
medial olivocochlear neuron
Lateral olivocochlear neuron: innervate type I dendrites