ANP 1106 Chapter Notes - Chapter 15: Ear Canal, Bony Labyrinth, Semicircular Canals

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ANP1106 C
Dr. Jackie Carnegie & Dr. W Staines
1. Special Sense: Balance and Hearing
P 574-588, 500 Vestibulocochlear Nerve
The Ear: Hearing and Balance
Structure of the Ear – divided into external, middle, and inner ear; external and middle =
hearing, inner – equilibrium and hearing (complex)
External Ear – collects sound waves and passes them inwards; consists of:
Auricle (pinna): shell-shaped protection surrounding opening of external acoustic meatus, functions to
funnel sound waves into the external acoustic meatus
omade of elastic cartilage covered with thin skin and hair
omany animals have large auricles and can be oriented in different directions
External auditory canal (acoustic meatus): short curved tube connecting the auricle to eardrum
oits composed of elastic cartilage near auricle, the rest is embedded in temporal lobe
ohas hairs, sebaceous glands and sweat glands that secrete earwax (cerumen)
Tympanic membrane (eardrum): thin CT covered in skin creating boundary btwn outer and inner ear
osound waves strike here and causes vibrations and transfers the sound energy to tiny bones of
middle ear causing them to vibrate
Middle Ear (tympanic cavity) – small air-filled mucosa-lined cavity in the temporal bone
Ear Ossicles in the tympanic cavity contains three small bones: malleus – secured to the
eardrum, incus – articulate with malleus laterally and stapes medially, stapes – its base fits in
the oval window
otransmits vibratory motion of the eardrum to the oval window which sets the
fluids of the internal ear into motion and excites the hearing receptors
otensor tympani and stapedius are tiny muscles that contract reflexively upon loud
sound to limit ossicle vibration and minimize damage to hearing receptors
example of CNS control over sensory input
oprotects the auditory receptor cells; innervated by cranial nerves
sound amplification overcomes the impedance matching problem
Inner Ear (labyrinth; complicated shape) – deep in temporal bone behind eye socket, provides secure site for all
delicate receptor machinery; two division:
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1) bony labyrinth - tortuous channels worming their way through the temporal lobe (a cavity); filled with
perilymph (fluid similar to cerebrospinal fluid; conduct vibrations and respond to mechanical forces during changes
in body position and acceleration); contains:
vestibule – central egg-shaped cavity of the bony labyrinth, suspended in perilymph are two sacs, saccule
and utricle, they house equilibrium receptors maculae, and respond to gravity and report on changes in
the position of the head
semicircular canals – three canals that each define 2/3 of a circle and lie in the anterior, posterior, and
lateral planes
omembranous semicircular ducts line each semicircular canal, communicates w utricle anteriorly
ampulla is an enlarged swelling at one end of each canal and
houses equilibrium receptors in a region called the crista
ampullaris – thede receptors respond to angular movements of
the head
cochlea – spiral, conical, bony chamber that extends from the anterior vestibule, coils around a bony pillar
modiolus; contains membranous cochlear duct ending at cochlear apex, cochlear duct houses organ of
corti (hearing receptor)
odivided in three chambers – scala vestibule (superior to cochlear duct), scala media (cochlear
duct), scala tympani (terminates at round window, inferior to cochlear duct)
scala media is part of membraneous labyrinth, its filled with endolymph
scala tympani and vestibule are part of bony labyrinth and are filled with perilymph;
they are continuous with each other via the helicotrema (hole in the spiral)
ofloor of cochlear duct is made of bony spiral lamina, and the flexible basilar membrane which
supports the organ of Corti – its critical for sound reception
ocochlear branch of the vestibular nerve (VIII) runs from the organ of Corti to the brain
2) Membraneous labyrinth – continuous series of membranous sacs and ducts in the bony labyrinth
ofilled with potassium-rich fluid (endolymph) to conduct sound vibrations involved in hearing and
respond to mechanical forcers occurring during changes in body position and acceleration
Physiology of Hearing
outer ear – sounds set up vibration in air that beat against the eardrum (via pinna nad auditory canal)
middle ear – vibrations push a chain of tiny bones (malleus, incus, and stapes) that press fluid in the
internal ear (via oval window)
inner ear – fluid presses against membranes that set up shearing forces pulling on tiny hair cells that
stimulate nearby neurons giving rise to impulses that travel to the brain which interprets them
Properties of Sound - sound travels much slower than light; Sound: a pressure disturbance (alternating areas of
high and low pressure) originating from a vibrating object and propagated by molecules of the medium
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