Lec 19: Balance and Hearing Flashcards by William Nguyen

External and Internal Ears

External & Middle Ear – for hearing only
Internal Ear for balance and hearing

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External Ear

Portion of the ear that includes the auricle and external acoustic meatus
Terminates at the eardrum

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Auricle

Part of the external ear that protrudes from the side of the head

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External Acoustic Meatus

Passageway that leads from the outside of the head to the tympanic membrane of each ear

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Tympanic Membrane

Also known as the eardrum
Cellular membrane that separates the external from the middle ear (boundary)
Vibrates in response to sound waves

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Middel Ear

Air-filled space within the temporal bone
Contains auditory ossicles
Between the external and internal ear

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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

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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

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Inner Ear

Bony Labyryinth

Part of the inner ear
Contains the membranous labyrinth that forms the cochlea, vestibule, and semicircular canals
filled with perilymph

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Inner Ear

Membranous Labyrinth

Membranous structure within the inner ear consisting of the cochlea, vestibule, and semicircular canals
filled with potassium-rich endolymph

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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

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Saccule

leads into the cochlea

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Utricule

leads into the semicircular canals

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Semicircular Canals

3 canals that each define two-thirds of a circle and lie in one of the three planes of
space

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Ampula

is the swollen end of each canal, it houses equilibrium receptors in a region
called the crista ampullaris
Respond to angular head movements

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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

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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