1. Peripheral auditory mechanisms
Domina Petric, MD

2. Auditory function
Auditory system transduces sound waves into distinct patterns of neural activity that are integrated with other sensations.
Sound waves are collected and amplified by physical structures in the external and middle ear for transfer to neural elements in the inner ear.
Biomechanichal properties of the inner ear decompose complex sound waves into sinusoidal components.

3. Auditory function
Frequency, amplitude and phase are encoded in the firing of the receptor cell.
Tonotopy is the systematic representation of sound frequencies.
Tonotopy is preserved in the inner ear and throughout central processing stations.
In the brainstem the auditory information is first processed and divided into several parallel pathways.

4. Auditory function
Brainstem centers relay the information to the midbrain (inferior colliculus).
Midbrain projects to the auditory thalamus (medial geniculate complex).
Auditory cortex recieves inputs from the thalamus and processes more complex aspects of sounds (for example speech).

5. Amplitude is the intensity of sound.
Frequency is the pitch of the tone (sound).

6. External ear
(concha, pinna)
is collecting sound
frequencies from
the environment.
Sound is collected in the external auditory canal.
Stapes is connected with
the oval window.
Sound energy is 200 fold amplificated
from the point of tympanic membrane via
the middle ear ossicles (malleus, incus, stapes).
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7. Biomechanical function of cochlea
is to decompose complex sounds
into their component frequencies.
The neural function of the cochlea
is to transduce the mechanical
energy into neural signals.
Vestibular part
Auditory part

8. Outer hair cells have
biomechanical function:
they act as motor units 
that amplify the movement
of the basilar membrane
in response to a stimulus
and some of this added
energy is transmitted back
through the middle ear,
where it can be recorded
as an otoacoustic emission.
Cilia of the inner hair cells are in contact
with tectorial membrane.
Sensory cells (inner hair cells)
are between the basilar
membrane and tectorial
(roof) membrane.
Cell bodies of the
cochlear nerve are located
in the spiral ganglion.

9. Inner ear Stapes makes contact with the oval window.
Deformation of the oval window (vibration) is transduced into the fluids in the Scala vestibuli and Scala tympani.
The pressure on the oval window is relaxed with the outward bulge of the round window.
Fluids then cause the vibration of the hair cells cilia.

10. Inner ear
Base of the basilar membrane is tuned for high frequencies.
Apex of the basilar membrane is tuned for low frequencies.
Tip of the cochla is Helicotrema.
Movement of the basilar membrane against the tectorial membrane causes the inner hair cell cilia to move

11. Inner ear
Movement of hair cells stereocilia causes opening of the ion channels, ion influx and generation of the graded potential in the inner hair cells.
In the Scala vestibuli and Scala tympani there is perilymph: relatively low concentration of potassium.
Scala media is filled with endolymph: high concentration of potassium (+80 mV).
Endolymph is rich with potassium because of the activity of the cells in Stria vascularis: highly vascularised structure with high metabolic activity.

12. Inner ear The cells of the Stria vascularis secrete potassium ions.
On the tips of inner cells stereocilia are potassium channels.
Protein called TIP LINK connects potassium channels of one cilia to potassium channels of other cilia.
When the stereoicilia move in the direction from the smallest stereocilia towards the largest one, tip link protein is streched and the potassium channels are opened.
When potassium ions influx into the inner hair cell, there is depolarisation.

13. Inner ear
Graded potential of inner hair cell opens the voltage gated calcium channels on the lateral sides of the inner hair cell.
Calcium rushes in through the basal lateral aspects of the inner hair cell.
Then occurs calcium dependent exocytosis of synaptic vesicles.
Neurotransmitter is released from the vesicles and makes contact with receptors at the peripheral end of the spiral ganglion processes.

14. Stereocilia move in the direction from the smallest towards the largest.
Tip link protein streches and opens the potassium channels: influx of potassium ions
causes the depolarisation of the inner hair cell.
Calcium channels open: calcium
ions influx causes the exocytosis
of vesicles. Released neurotransmitter
generates the action potential
in the axons of the cochlear nerve.

15. That causes hyperpolarisation of the inner hair cell (-45 mV).
Inner ear
When stereocilias then move back in the direction from the largest to the smallest stereocilia, tip link protein relaxes and then the potassium channels close.
That causes hyperpolarisation of the inner hair cell (-45 mV).

16. Auditory nerve
Human ear hears from 20 Hz to 20 kHz.
Helicotrema: 20 Hz.
Base of the basilar membrane: 20 kHz.
Different axon of the auditory nerve has its own BEST frequency: at that specific frequency has the low treshold for action potential.
Tonotopic map from the basilar membrane is preserved as the tonotopic map in the auditory nerve.

17. Literature
neuroscience: Leonard E. White, PhD, Duke University
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