Week 6 - Lecture 2b - alterations in hearing and balance Flashcards
three major areas of the ear
external (outer) ear - hearing only middle ear (tympanic cavity) - hearing only internal ear (inner) - hearing and equilibrium
receptors for hearing and balance respond to separate stimuli
are activated independently
External ear is composed of
auricle (pinna)
external acoustic meatus (auditory canal)
tympanic membrane
(eardrum )
auricle (pinna) is composed of
helix (rim); lobule (ear lobe)
funnels sound waves into auditory canal
external acoustic meatus (auditory canal) is composed of
short, curved tube lined with skin bearing hairs, sebaceous glands, and ceruminous glands (secrete wax, cerumen)
transmits sound waves to eardrum
protect against foreign bodies and environmental debris
Tympanic membrane is
boundary between external and middle ears
connective tissue membrane that vibrates in response to sound
transfers sound energy to bones of middle ear
Middle ear is referred to as the
tympanic cavity
middle ear is
a small, air filled, mucosa-lined cavity in temporal bone
The three small bones in the tympanic cavity are
malleus
stapes
incus
(stapes footprint pushes into the oval window)
what is the mastoid Antrum
located in the middle ear
canal for communication with mastoid air cells – mastoiditis
What is the Eustachian tube
part of the middle ear
it connects the middle ear to the nasopharynx
also known as the pharyngotympanic (auditory) tube
equalises pressure in the middle ear cavity with external air pressure
in children : shorter, more horizontal tubes – otitis media
What does the inner ear compose of
outer bony labyrinth
inner membraneous labyrinth
outer bony labyrinth
perilymph
inner membranous labyrinth
endolymph
- vestibule
- semicircular canals
- cochlea
sounds waves heard
sound waves vibrate tympanic membrane
ossicles vibrate and amplify pressure at oval window
cochlea fluid set into wave motion
pressure waves move through perilymph of Scala vestibuli
pg. 6
Cochlea
a spiral, conical, bony chamber
- size of a split pea
- extends from vestibule, coils around a bony pillar
cavity of cochlea divided into three chambers
1. scala vestibuli
next to the oval window, contains perilymph
2. scala media (cochlear duct)
contains endolymph
3. Scala tympani
terminates at round window; contains perilymph
scala tympani and vestibuli are continuous with each other at helicotrama (apex)
the cochlear duct houses the spiral organ (organ of corti)
The cochlear branch of CN VIII runs from spiral organ to the brain
hair cells in the spiral organ
cells or the spiral organ (hair cells) contain hairs that protrude into the endolymph (stereo cilia)
stereo cilia - longest hair is enmeshed in gel-like tectorial membrane
opens mechanical gated ion channels
inward K+ and Ca2+ current causes graded potential and release of neurotransmitter glutamate
cochlear fibres transmit impulses to brain
Equilibrium and orientation
vestibule (vestibular apparatus)
responsible for equilibrium
- vestibular receptors : monitor static equilibrium
- semicircular canal receptors : monitor dynamic equilibrium
vestibular receptors : macula located in the saccule and utricle wall - monitor the position of head in space -- necessary for control of head in space -respond to linear acceleration forces - - not rotation contain hair cells
stereo cilia are embedded in the otolith membrane studded with otoliths (CaCO3 stones)
maculae
maculae in utricle : respond to horizontal movements and tilting head side to side
maculae in saccule : respond to vertical movements
hair cells release neurotransmitter continuously - movement modifies amount they release
semicircular canals
three canals (anterior, lateral, posterior) that each define 2/3 circle - lie in three planes of space
membranous semicircular ducts line each canal and communicate with utricle
ampulla of each canal houses an equilibrium receptor region called the crista ampullaris (crista)
–receptors respond to angular (rotational) movements of the head
sensory receptor for rotational acceleration
- one in ampulla of each semicircular canal
- major stimuli are rotational movements
The crista ampullares (crista)
each crista has supporting cells and hair cells that extend into gel-like mass called ampullary cupula
dendrites of vestibular nerve fibres encircle base of hair cells
cristae respond to changes in velocity of rotational movements of the head
- bending of hairs in cristae causes depolarisation of nerve fibres
- bending of hairs in the opposite direction causes hyperpolarisation
thus brain informed of rotational movements of head
