IB Biology Neurology Unit Option E The Ear and Hearing IB Biology Neurology Unit Option E

The Ear Functions: Utilizes mechanoreceptors Hearing Equilibrium (balance) Utilizes mechanoreceptors

Anatomy of the Ear Three areas Outer (external) Ear Middle Ear Inner Ear

The External Ear Function: Hearing only Structures: Pinna (auricle) External auditory canal Figure 8.12 Slide 8.22 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Pinna Collects soundwaves

The External Auditory Canal Narrow chamber in the temporal bone Lined with skin Contains ceruminous glands Ends at the tympanic membrane Slide 8.23 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Mastoid Process Bony ridge behind the pinna Provides support to the external ear and posterior wall of the middle ear cavity

The Middle Ear Air-filled cavity within the temporal bone Function: Hearing only Separated from outer ear by tympanic membrane Slide 8.24a Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Tympanic Membrane Thin membrane Forms boundary between outer and middle ear Vibrates in response to sound Changes acoustical energy into mechanical energy

Bones of the Middle Ear Ossicles: Malleus (hammer) Incus (anvil) Stapes (stirrip) Figure 8.12 Slide 8.25a Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Ossicles Act as a lever system Action initiated from vibration of tympanic membrane A = malleus B = incus C = stapes

External and Middle Ear Functioning: An Overview Soundwaves (collected by pinna) travel through external auditory canal Soundwaves lead to tympanic membrane vibrations. Tympanic membrane vibrations move malleus Malleus contacts incus Incus contacts stapes Stapes movement initiates inner ear response!!

Eustachian Tube Lined with mucus membrane; connects middle ear to back of the throat (nasopharynx) Equalizes air pressure Normally closed except during yawning or swallowing Not a part of the hearing process

INNER EAR FUNCTIONS: Hearing! Balance! Fluid-filled (perilymph) Figure 8.12 Slide 8.26a Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Structures of the Inner Ear Cochlea Vestibule Semicircular Canals

Cochlea snail-shaped organ with a series of fluid-filled tunnels converts mechanical energy into electrical energy

Structures of the Inner Ear (Cont.) Oval Window – located at the base of the stapes; when the stapes vibrates, the cochlear fluid (perilymph) is set into motion Round Window – functions as the pressure relief port for the fluid set into motion initially by the movement of the stapes in the oval window

Organ of Corti The end organ of hearing; contains stereocilia and hair cells.

Central Auditory System 8th Cranial Nerve or “Auditory Nerve” carries signals from cochlea to brain Fibers of the auditory nerve are present in the hair cells of the inner ear Auditory Cortex: Temporal lobe of the brain where sound is perceived and analyzed

How Sound Travels Through The Ear... Acoustic energy, in the form of sound waves, is channeled into the ear canal by the pinna. Sound waves strike the tympanic membrane, causing it to vibrate like a drum, and changing it into mechanical energy. The malleus, which is attached to the tympanic membrane, starts the ossicles into motion. (The middle ear components mechanically amplify sound). The stapes moves in and out of the oval window of the cochlea creating a fluid motion. The fluid movement within the cochlea causes membranes in the Organ of Corti to shear against the hair cells. This creates an electrical signal which is sent via the Auditory Nerve to the brain, where sound is interpreted!

Vestibular System Consists of three semi-circular canals Shares fluid with the cochlea Controls balance No part in hearing process