ANATOMY THE EAR Dr. J.K. GERALD, (MD, MSc.)

Main Components of the Ear Divided into 3 main parts: Outer Ear -Pinna, ext. auditory canal Middle Ear / tympanic cavity -TM, auditory ascicles, ET Inner Ear / labyrinth -Oval window, SM, vestibule, cochlea

Structures of the Outer Ear Pinna (Auricle) Gathers sound waves Aids in localization Amplifies sound waves

External Auditory Canal: Approx. 1 inch (2.5cm) long “S” shaped Outer 1/3 surrounded by cartilage; inner 2/3 by mastoid bone Allows air to warm before reaching TM Isolates TM from physical damage Cerumen glands moisten/soften skin Presence of some cerumen is normal

MIDDLE EAR A closed air filled cavity in the petrous part of the temporal bone 6 sided box Lateral wall Medial wall Anterior wall Posterior wall Roof Floor

The temporal bone

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

The Ossicles Ossicular chain = malleus, incus & stapes Malleus Incus TM attaches at Umbo Incus Connector function Stapes Smallest bone in the body Footplate inserts in oval window on medial wall The ossicles focus/amplify vibration of TM to smaller area, enables vibration of cochlear fluids

Eustachian Tube (“The Equalizer”) Mucous-lined, connects middle ear cavity to nasopharynx “Equalizes” air pressure in middle ear Normally closed, opens under certain conditions May allow a pathway for infection

Stapedius Muscle Attaches to stapes Contracts in response to loud sounds; (the Acoustic Reflex) Changes stapes mode of vibration makes it less efficient and reduce loudness perceived Absent acoustic reflex could signal conductive loss or marked sensorineural loss

Middle ear – applied anatomy Otitis media

Inner Ear

Semicircular canals: Superior Posterior Lateral Utricle Saccule Vestibulocochlear nerve Cochlea Stapes

Vestibulocochlear nerve (CNVIII) The vestibulocochlear nerve (auditory vestibular nerve), transmits sound and equilibrium (balance) information from the inner ear to the brain. It consists of the cochlear nerve (auditory n.), carrying information about hearing, and the vestibular nerve, carrying information about balance. 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 hit the tympanic membrane and cause it to vibrate, like a drum, changing it into mechanical energy. The malleus, which is attached to the tympanic membrane, starts the ossicles into motion. The stapes moves in and out of the oval window of the cochlea creating a fluid motion, or hydraulic energy. The fluid movement causes membranes in the Organ of Corti to shear against the hair cells. This creates an electrical signal which is sent up the Auditory Nerve to the brain. The brain interprets it as sound!

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