Lec 19: Balance and Hearing Flashcards
External and Internal Ears
External & Middle Ear – for hearing only
Internal Ear for balance and hearing
External Ear
Portion of the ear that includes the auricle and external acoustic meatus
Terminates at the eardrum
Auricle
Part of the external ear that protrudes from the side of the head
External Acoustic Meatus
Passageway that leads from the outside of the head to the tympanic membrane of each ear
Tympanic Membrane
Also known as the eardrum
Cellular membrane that separates the external from the middle ear (boundary)
Vibrates in response to sound waves
Middel Ear
Air-filled space within the temporal bone
Contains auditory ossicles
Between the external and internal ear
Auditory Osicles
The three smallest bones in the human body
Contained within the middle ear space
Serves to transmit sounds from the air to the cochlea
Pharyngotympanic tube
The tube that runs from the middle ear to the pharynx
Also known as the Eustachian tube.
Protects and drains the middle ear
Inner Ear
Bony Labyryinth
Part of the inner ear
Contains the membranous labyrinth that forms the cochlea, vestibule, and semicircular canals
filled with perilymph
Inner Ear
Membranous Labyrinth
Membranous structure within the inner ear consisting of the cochlea, vestibule, and semicircular canals
filled with potassium-rich endolymph
Vestibule
central cavity of bony labyrinth; contains 2 sacs (utricule and saccule) suspended
in perilymph
monitor head position: they both contain equilibrium receptors called maculae
that respond to the pull of gravity
Saccule
leads into the cochlea
Utricule
leads into the semicircular canals
Semicircular Canals
3 canals that each define two-thirds of a circle and lie in one of the three planes of
space
Ampula
is the swollen end of each canal, it houses equilibrium receptors in a region
called the crista ampullaris
Respond to angular head movements
Cochlea (defenition)
The auditory portion of the inner ear
Spiral-shaped cavity in the bony labyrinth
The region of the inner ear that is responsible for hearing
Cochlea ( 3 chambers)
scala vestibuli – perilymph;
continuous with vestibule & begins at
oval window
* scala media – endolymph; cochlear
duct itself
* scala tympani – perilymph; links to
round window
Helicotrema
Opening at the apex of the cochlea through which the scala vestibuli and the scala tympani of the cochlea connect
Spiral Ear
Structure located on the inner surface of the basilar membrane of the cochlea containing hair cells that transmit sound vibrations to the nerve fibers within the cochlea
Also called organ of Corti
Transmission of Sound to the Inner Ear
Outer ear – pinna to acoustic meatus to tympanic membrane
Middle ear – malleus, incus, and stapes to the oval window
Inner ear – scalas vestibuli and tympani to the cochlear duct
stimulation of spiral organ and generation of impulses in the cochlear nerve
Sound
a pressure disturbance (alternating areas of high and low pressure) originating
from a vibrating object and propagated by the molecules of the medium
* composed of areas of compression and rarefaction that create sound waves
* energy transferred from molecule to molecule in the direction of the sound wave
but will decline with time and distance
Frequency =?
Amplitude =?
Frequency of sound waves perceived as pitch; loudness perceived as amplitude
Frequency
frequency range of human hearing is 20-20,000 hertz (Hz; waves per second)
* most sensitive to 1500-4000 Hz; can distinguish differences of 2-3 Hz in that range
and perceive them as differences in pitch
Amplitude
intensity of sound measured in decibels (dB) – a logarithmic scale, meaning an
increase of 10 dB = 10X increase in sound energy
* loudness is our perception of sound intensity and, interestingly, an increase of 10 dB
is perceived as only about a doubling of loudness
* normal conversation is ~50 dB; noisy restaurant ~70 dB; amplified concert ~120 dB
(prolonged exposure to 90dB considered danger zone for hearing loss)
Transmission of Sound to the Internal Ear
sound waves must travel through air, membranes, bones and fluids to
stimulate receptor cells in the spiral organ
* louder sounds cause increased deflection of the tympanic membrane
* only sounds in our hearing range are able to be transmitted through the
cochlear duct, vibrating the basilar membrane and activating the hair cells
Basilar Membrane
The part of the membranous labyrinth that borders the scala tympani
Sound Transduction
inner hair cells are key; their longest
stereocilia (= hairs; actually microvilli) are
embedded in tectorial membrane; hairs
are connected by fine tip link
Movement of the basilar membrane causes:
- bending of hairs → puts tension on tip links
- opens mechanically gated cation channels
(K+ and Ca++ enter) → receptor potential - hair cells release NT glutamate to excite
cochlear nerve - movement in other direction loosens tip
links, closes channels → repolarization
Outer Hair Cells
Outer row of hair cells that are involved in regulating the tension of the basilar membrane
Auditory Pathway
Balance
balance organs monitor head movements so we can maintain our orientation ad
balance in space; balance also depends on visual and proprioceptive information
Balance Receptors
located in the semicircular canals and the vestibule (utricle and saccule) = vestibular apparatus
receptors in vestibule
monitor static equilibrium (linear acceleration & position of head with respect to gravity
receptors in semicircular canals
monitor head rotation = dynamic equilibrium
Maculae
one macula in each of the utricle and saccule
each consists of supporting cells and
hair cells
each hair cell has stereocilia (these are
actually microvilli) and one kinocilium
(true cilium) embedded in the otolithic
membrane
Otolithic membrane
Jellylike mass studded
with tiny calcium carbonate crystals called
otoliths
Utricular hairs
respond to horizontal
movement or tilting the head (macule is
horizontal, making hairs vertical when head is
upright)
Saccular hairs
respond to vertical movement
(macula is nearly vertical meaning that the
hairs extend horizontally into the otolith
membrane
What happens at the Level of the Hair Cells?
When you start to move (think of starting to run), the otolithic membrane slides over
the hair cells, bending the hairs; this modifies the amount of NT they release and
either increasing or decreasing it acts as a signal
* When hairs bend toward the kinocilium
* depolarizes vestibular nerve fibers & increases the number of action potentials
* Movement away from the kinocilium:
* hyperpolarizes vestibular nerve fibers & reduces the number of action potentials
* From this information, the brain is informed of the changing position of the head
Crista Ampullaris (Crista)
*one in each semicircular canal allowing them to be located in all 3 planes of space
* stimulated primarily by rotational type movements; specifically, changes in velocity
of rotational movements
* support cells plus hair cells whose hairs are extend into a gel-like mass, the cupula
* as with the maculae, movement bends the hairs of the hair cells → signal
* dendrites of vestibular nerve fibers encircle base of each hair cell
Comparing Movement of Cupula during a Counter
Clockwise Rotation
Equilibrium Pathway to the
Brain
Needs to react quick
