Notes

The Outer Ear The outer ear consists of the pinna, the ear canal and the tympanic membrane. The funnel-shaped pinna "collects" air pressure fluctuations from the environment and directs them through the ear canal to the tympanic membrane (eardrum), which constitutes the transition to the middle ear. The tympanic membrane vibrates in response to sounds (air pressure changes), and this vibration is transmitted to three ossicles situated in the middle ear (malleus, incus and stapes). The length and shape of the ear canal plays an important role in sound amplification.

The Middle Ear The middle ear is separated from the outer ear by the tympanic membrane (eardrum), and is responsible for the conversion of air pressure fluctuations into mechanical energy. The transmission of sounds from the outer to the middle ear takes place through the deflection of the tympanic membrane. The deflection of the tympanic membrane results in vibrations of the three ossicles (malleus, incus and stapes), which in turn convey energy to the fluid-filled cochlea in the inner ear.

The Inner Ear: Cochlea The cochlea is a fluid-filled tube which is divided into two major compartments by the basilar membrane, namely the scala vestibuli and tympani. The movement of the ossicles transfers the mechanical energy to the oval window of the cochlea, which sets the fluid in motion, from the oval to the round window (pressure compensation). The two liquid-filled tubes that run along the cochlea are separated by the basilar membrane. The basilar membrane moves up and down in response to incoming sound waves, which are converted into traveling waves on the basilar membrane. The Corti organ, which is situated on the basilar membrane, is responsible for the transduction of auditory signals into action potentials. The location-specific deflection of hair cells in the Corti organ activated calcium and potassium channels which lead to action potentials.

The Inner Ear: Tonotopic Organization of the Cochlea Basilar membrane vibration to sinusoids varies with frequency because its mechanical properties vary along its length. The basilar membrane is wider at the apex (most responsive to low frequencies) and stiffer at the base (most responsive to high frequencies). The mechanical properties of the basilar membrane are the initial source of tonotopic organization (frequency coding).

Recap

The pinna catches sound waves and deflects them into the external ear canal. Waves are amplified and directed to the eardrum, causing it to vibrate, which in turn vibrates ossicles. Ossicles amplify and convey vibrations to the oval window. Vibration of oval window sends waves through cochlear fluid causing the basilar and tectorial membranes to bend, which in turn cause cilia of outer hair cells, embedded in the tectorial membrane, to bend. This bending generates neural activity in hair cells.