PHY238Y Lecture 12 The human inner ear Physics of hearing (IV) References: Hallett et al.: Physics for the life sciences, 4 th ed., Ch.2 (2.6) Some of the pictures were taken from Hyper Physics: Hyper Physics: Thanks to dr. Rod Nave for the permission to use the above resource

PHY238Y Lecture 12 The inner ear contains specialized sense organs for transducing sound into neural impulses: - the semicircular canals which serve as the body's balance organ - the cochlea which serves as the body's microphone, converting sound pressure impulses from the outer ear into electrical impulses which are passed on to the brain via the auditory nerve.

PHY238Y Lecture 12 The semicircular canals are the body's balance organs, detecting acceleration in the three perpendicular planes. These accelerometers make use of hair cells which detect movements of the fluid in the canals caused by angular acceleration about an axis perpendicular to the plane of the canal. The canals are connected to the auditory nerve.

PHY238Y Lecture 12 Cochlea has three fluid filled sections. : The perilymph fluid in the canals (almost identical to spinal fluid) the endolymph fluid in the cochlear duct. The fluids differ in terms of their electrolytes and if the membranes are ruptured so that there is mixing of the fluids, the hearing is impaired. The organ of Corti is the sensor of pressure variations.

PHY238Y Lecture 12 Cochlea is a coiled tube (10 mm diameter; 35 mm long) bisected by cochlear partition and supporting the basilar membrane Peak frequencies along the basilar membrane: high frequencies close to the base; low frequencies at the apex end.

PHY238Y Lecture 12 Traveling waves of different frequencies along the basilar membrane (Fig. 6.5 from K. Bogdanov: Physics in Biology)

PHY238Y Lecture 12 The basilar membrane of the inner ear plays a critical role in the perception of pitch The theory that won the Nobel prize for Physiology or Medicine (1961): Traveling Waves propagate along the basilar membrane (Georg von Bekesy) Basilar membrane does not respond to vibrations in a simple way but to sinusoidal vibrations and traveling waves: - High frequencies peak near base - Low frequencies peak near apex

PHY238Y Lecture 12 Organ of Corti is situated on the basilar membrane in one of the three compartments of the cochlea. It contains four rows of hair cells which protrude from its surface. Above them is the tectorial (tectoral) membrane which can move in response to pressure variations in the fluid - filled tympanic and vestibular canals. Movement of tectorial membrane relative to basilar membrane causes hair cells to tilt. A lilt of ~ 0.3 nm can be converted into a nervous signal

PHY238Y Lecture 12 Taking electrical impulses from the cochlea and the semicircular canals, the auditory nerve makes connections with both auditory areas of the brain.

PHY238Y Lecture 12 Cochlear implants: electronic devices designed to provide sound information in people with intact auditory nerve, but injured cochlea. 22 electrodes are surgically inserted inside the inner ear; They receive harmonics from a microphone + speech processor system They send signals to the corresponding nerve terminals from the inner ear