Notes

Key parts:

• External Ear (pinna): it collects sound waves and channels them into the ear canal, where the sound is amplified.
• Tympanic Membrane
• Ossicles: the ossicle chamber is filled with air. Ossicles are malleus, incus and stapes. These three ossicles connect the tympanic membrane to the inner ear allowing for transmission of sound waves.
• Eustachian tube: equalizes the pressure
• Cochlea: it is filled with liquid. It transforms changes in pressure in electrical signals.

The Middle Ear

The whole job of these ossicles is to transmit the vibration of the tympanic membrane to the cochlea. The middle ear matches the impedance difference, which is due to the change from the air environment (tympanic membrane & ossicles) to the liquid environment of cochlea. The muscles in the middle ear limit the range of motion of the ossicles to protect from high intensity stimuli. That's why when you go to a concert then you feel like your auditive capabilities are reduced, because these muscles become stiff and reduce your sensibility. In particular, we can notice that one of the three ossicles, the stapes, presses onto the oval window of the cochlea to transmit changes in air pressure.

The Inner Ear

The key element of the inner ear is represented by the Organ of Corti, it is contained in the middle canal. This organ can be thought of as the retina in the visual system, it contains the nerve cells that will translate changes in pressures to electrical signals.

Tympanic Membrane & Ossicular System

Sound stimuli pass through pinna and exterior auditory canal to strike Tympanic Membrane (TM), causing it to vibrate. The Ossicular System conducts sound from the TM through the middle ear to the cochlea. The faceplate of the stapes pushes forward on the cochlear fluid (oval window) every time the TM and malleus move inward. Impedance matching is provided by the ossicular system between sound waves in air and sound vibration in the cochlear fluid (fluid has a greater inertia than air). Most amplification occurs because the area of the TM is 17x greater than the stapes/oval window surface area.

The Mechanics of the Basilar Membrane

The basilar membrane is inside the cochlea and vibrates to sound waves, it will vibrate differentially along its length depending on the frequency of the stimulus. It is broader and thinner at the end, while it is smaller and thicker at the beginning. In particular, low frequencies can be found at the end, while high frequencies are at the beginning. Hair cells at different positions respond to different frequencies (only because of their location) (Mechanical Fourier Analysis).

The Organ of Corti is the location where vibrations are traduced into electrical signals. It is situated on top of the basilar membrane and contains hair (auditory receptor) cells, these generate nerve impulses in response to vibration of the basilar membrane. When the basilar membrane is vibrating, the tectorial membrane changes position, which makes hair cells move back and forth.

• Inner Hair Cells: single row, provide fine auditory discrimination. 90% of auditory nerve fibers innervate these cells.

• Outer Hair Cells: three rows, detect the presence of sound. (Less important for audition).

The hair cells contain stereocilia, which protrude into the overlying tectorial membrane.

Auditory Transduction The up-and-down motion of the basilar membrane causes the Organ of Corti to vibrate up-and-down, which, in turn causes the stereocilia to bend back-and-forth.

Polarization of the Stereocilia

• (B) When the Organ of Corti moves upward, the stereocilia bend away from the limbus and they depolarize.
• (C) When the Organ of Corti moves downward, the stereocilia bend toward the limbus and they hyperpolarize.

Transduction at Hair Cells

• High concentration of K+ outside the cell.
• Then the cannel opens.
• Causing a change in the membrane potential.

Receptor Potential The hair cells are depolarized by the movement of K+ ions into the cell:

• The endolymph contains a high K+ and is electrically positive. The hair cells also contain a high K+ but are electrically negative (NA/K pumps) because their concentration is less than the outside. Hence, driving force for K+ into cells.
• When the stereocilia bend away from the limbus, they cause K channels to open. K+ then flows into the cell and the hair cell depolarizes.
• When the stereocilia bend towards the limbus, they cause K channels to close and the hair cell hyperpolarizes.

Release of Synaptic Transmitter

• When the hair cell depolarizes, a Ca channel opens, allowing calcium to enter the cell. Calcium initiates the release of synaptic transmitter, which stimulates the auditory nerve fiber.
• The cell bodies of the auditory nerve fibers are located within the spiral ganglion. Their axons join those from the vestibular apparatus to form the vestibulocochlear nerve.

The picture to the right shows the minimal sounds intensity for single units in the cochlear nerve to react. Individual fibers show frequency preferences.