Auditory Embryology

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Purpose of Auricle: Collect sound, helps in sound localization
11 parts of the Pinna: Helix
Anti-helix
scaphoid fossa
triangular fossa
crus of the helix
tragus
anti-tragus
intertragal notch
lobule
cymba concha
cavum concha
resonant frequencies: -pinna enhances sound in 4500 Hz range
-EAM enhances sound - 2700 Hz
-Middle ear resonance freq = 1500Hz

overall outer & middle ear provide themost gain in range of 1500-5000
Humans have how many auditory afferent nerve fibers?: co,000
The resonant frequency of the EAM: ~2700 Hz
The outer ear & EAM together contribute ____of gain to the TM: 10-15dB
The characteristic impedance of the ossicular chain is: Since the TM is considerable larger in area than the stapes footplate, energy from vibrations arriving at the eardrum is concentrated on the smaller stapes with an increase in applied force.
-an additional force is realized because the ossicular chain works as a lever in conducting sounds to the cochlea.
-This overall increase in energy is necessary because of an imbalance mismatch between the outer ear (low impedance, air) and the cochlea (high impedance , fluid)
-together with the buckling effect of the curved membranous structure of the TM increases energy at the oval window of 46:1 or 33 dB increase
The key process in mechanics of the cochlea is: the generation of the traveling wave
-(movement of the cochlear partition) the pressure wave that originates at the oval window travels the entire spiral of the cochlea, transferring the fluid movement from the scala vestibuli to the scala tympani at the helicotrema
The conductive mechanism of the ear: Sounds enter the ear as acoustic energy, at the middle ear they are converted from acoustic to vibratory or mechanical energy and then in the inner ear to hydraulic energy to a neural representation of the acoustic signal at the cochlea and beyond into the central auditory nervous system
Without a special mechanism to match impedances between air & fluid, what would happen to acoustic energy of sound?: most of the acoustic energy would be reflected back by high impedance of fluid in the inner ear
Outer hair cells: -3-5 rows
-12,000 OHCs
-150 stereocilia on each cell
-connect to tectorial membrane at Hardesty's membrane
-cylindrical in shape
-5% of afferent fibers innervate OHCs
Efferent fibers synapse directly to base of cells
-Electromotility- cells expand (when BM pushes down) and contract (when BM goes up)to aid in amplification
For low to mid intensity sounds the OHCs act as ?: amplifiers
-The OHC located at the characteristic frequency for a given sound will expand and contract in response to the basilar membrane movement
the active component of the cochlear amplifier is? Does what?: Improves sensitivity for soft sounds

improves frequency resolution
Deflection of stereocilia bundle toward kinocillium results in?: Depolarization
Deflection away from Kinocillium results in: Hyperpolarization
BM movement is ___ _____ to frequency and intensity: non linear
Stapes moves inward causes what kind of wave in the Scala vestibuli? Stapes pulls outward causes ?: a compression wave

a rarefaction wave
When stereocilia are deflected K+ in endolymph flows...: through the hair cells into perilymph through the spiral ligament and the stria vascularis and then back into the endolymph

Ions also flow from endolymph through Reisners membrane then through Stria vascularis & back into endolymph of scala media
Inner Hair Cells: Flask shaped
~3500 of them
What two ducts contribute to the fluid systems of the cochlea?: 1. Vestibular Aqueduct -endolymph, bony channel that encompases endoymphatic duct courses to the endolymphatic sac which is thought to help regulate endolymphatic pressure and to avoid the effects of over production or under absorption of endolymph. Dysfunction implicated in hydrops
-Cochlear aqueduct - perilymph, courses from the basal turn of the scala tympani to the subarachnoid space near the juggular fossa. Allows transfer of CSF. By age 50 may be closed
Fluids of the inner ear: Perilymph - in the scala vestibuli & Scala tympani- high in Na+ low in K = low voltage 0-5mV
Endolymph - in scala media (and semicircular canals)high K+ low Na+ have high positive voltage 80 mV (highest resting potential in the body
Cortilymph - lacated in the extracellular spaces of tunnel of corti - similar to perilymph
the sensory cells are bathed in cortilymph and the tops of cells cilia are in Endolymph
transduction is dependent on maintenance of this balance
Endolymph: Has high K+ concentration & low Na+
-it is believed that this concentration is maintained by an active process in the stria vascularis
What is Prestin?: -it is the motor protein of mammalian OHCs cover surface of OHC's not present on IHCs
There are afferent nerves that the brain receives input from: 20,000
The middle ear is classified as which kind of device? a)acoustic b)mechanical c)both d)neither: C) the middle ear is both acoustic & mechanical
-acoustic in that it collects sound energgy like the outer ear
-mechanical in that sound energy causes the TM to vibrate which moves the ossicular chain
The inner ear is classified as which kind of device? A)mechanical B)hydraulic C)biological D)all of the above: D) all of the above
(biological in that it analyzes incoming sound energy for properties including frequency , intensity and transmits energy along the AN to the brain
The maximum amount of amplitude transfer will occur when?: when the frequency of the driving object is equal to the resonant frequency of the receiving object
The changes in sound from a source caused by structures of the head and outer ear called?: HRTF or head related transfer function
Length of the basilar membrane?: 25 to 35 mm

apical end = .36mm wide

basal end = .04 mm wide
When using manual pure-tone audiometry, as part of a hearing loss prevention program, the frequency that should be tested?: 500,1k,2k,3k,4k,6k,8k
Extended high frequency audiometry when used for specialized purposes is done at frequencies of: 9000 Hz to 16,000 Hz
Why is it important to have an understanding of embryology?: -aids in the formation of hypotheses regarding the hearing status of a client
-child with small jaw, may also have ossicular chain malformation
What 2 factors control embryonic development?: Genetic factors - genes found on chromosomes control development

Environmental factors -
Differentiation is?: -the process that leads cells to become specialized for specific purposes

leads to cell layers
-ectoderm
-endoderm
-mesoderm
What is migration?: the process whereby tissues move into the proper location
Pregnancy trimesters?: 1st trimester
-development of all majoy body systems
2nd trimester
- fetus grows dramatically
-can distinguish anatomical structures
-can detect abnormalities via ultra sound
3rd trimester
- if born premature, the fetus can survive
-at 35 wks considered mature & weighs approx 2500 grams
The embryonic stage is ? The Fetal stage is?: 2nd to 8th week gestation

3rd to 9th month
The pre embryonic segment is a ___ or ____period in that...: all or none in that
-before the 14th day, the embryo either:
- repairs itself & recovers fully
0r
- doesn't repair & dies, miscarriage
The embryonic period is very vulnerable to ?: external insult
- the time of most congenital abnormalities

90% of 4500 sturctures develop in the Embryonic period
0-3 wks the embryo ..... 3-8 wks the embry susceptable to ... 8-38 wks disorders of ____ more likely: embryo death

to abnormality

disorders of function more likely
What is the Organogenetic Period?: the developmental period when the fetus is most susceptible to congenital abnormalities
Congenital primary abnormality: -can be traced back to anomaly in development
-gene aberration
-chromosomal aberation
-mulitfactorial - several genes and can be affected by the environment
Congenital secondary abnormalities: Interruption to normal development
-teratogenic elements
--infections
--rubella
--CMV
--toxoplasmosis
Chemical
--Thalidomide
--lithium
Trauma
Classification of congenital anomalies Deformation is?: change caused by exogenous mechanical effect on normal organs/structures
Aplasia: Failure of a tissue or organ to develop
Dysplasia: Abnormal organization of tissue cells
-osteogenesis imperfecta
Agenesis: Absense of an organ due to failed development
Sequence: - a single factor results in multiple secondary effects

Potters sequence - not enough amniotic fluid
Syndrome: Group of anomalies traced to a common origin

- trisomy 21
The pharyngeal arches develop from ? composed of? How many arches ? Visable by end of week___: Neural crest cells
composed of :
-endoderm
-ectoderm
-mesenchyme
there are 4 pairs visable by end of week 4
1st arch called: develops from? Involved in ? cartilage name, developes into?: mandibular arch
developes from neural crests separating into mesenchyme
-involved in facial development
-maxillary prominence
-mandibular prominence
1st arch cartilage = meckel cartilage
-dorsal end ossifies to create malleus & incus
Factors influencing fetal growth: -malnutrition
-cigarettes
-alcohol
-drugs
-multiple pregnancies
-obstructed blood flow
- extreme hypotension(renal disease)
-deficiency of the placenta - decreases the amount of nutrients
Significance of week 21-25: Premature fetus may survive with intensive care but may die during infancy because of respiratory system is not fully developed.
Maternal hormone effect on fetal development?: -maternal hormones & growth factors don't typically pass through the placenta
-byt may have altered metabolism in the placenta
-alcohol and tobacco will affect fetus

Diabetes - may affect
An embryo becomes a fetus at ____ weeks: nine weeks
Primary function of outer ear: -localizaton
-protection
amplification
The pinna developes from: the 1st & 2nd branchial arches (6 hillocks of HIS)
A congenital malformation that results from suppressed development of the 6 hillocks is?: Microtia
The embryonic outer ear forms from the a)mesoderm b)ectoderm c)endoderm d)all of these: B) from the ectoderm, the outer embryonic layer that is the 1st to develop
-skin, teeth & nervous system tissue also develop from ectoderm
-the ear canal forms from the 1st pharyngeal cleft (ectodermic)
The outer ear developes out of :: -a few skin tubercles, which grow to form the external ear
-there is a clear connection developmentally between skin, tongue , jaw and ear
Dr. Alfred Tomatis - believed that there is a direct relationship between ?: the skin and the ear
- the ear preceded in the nervous system & the sensory cells found in the skin are actually differentiated cells of corti
-supports the hypotheses that we not only take in sound with our ear but with our bones, tissues and skin
The Branchial grooves or clefts the 1st groove persists and becomes: externally separate each arch

the external auditory meatus
Week 8: Pinna is almost fully shaped

embryonic period ends at the end of week 8
Complete lack of auricle =: Anotia
Week 5: Middle ear
-primary auditory meatus begins
-outer ear changes occur in the ridges and furrows of the vestigial gills on either side of the head
At what week do the 6 hillocks of His develop?: week 6

Meanwhile...
inner ear the semicircular canals begin, the utrical and saccule are present
The embryonic middle ear forms from? a) endoderm b)mesoderm c)both d) neither: c)from both the endoderm and the mesoderm
-middle ear cavity, eustachian tube
-the ossicles form from the 1st pharyngeal arch (maleus and incus) and 2nd pharyngeal arch (stapes) the arches are formed from mesenchyme or cells of the mesoderm
At what week to the inner and middle ear start to develop: Week 3
The ME cavity begins from: the tubotympanic recess
What does mesoderm give rise to?: -to muscles, bones & connective tissue, bone marrow, cardiovascular system, reprodictive organs excretory organs
What does Endoderm give rise to?: -the internal organs and Gut (gastrointestinal tract)
Week 2-3: In Middle ear - development begins with tubotympanic recess

in Inner ear - placode pit --> will eventually become membranous labyrinth
The tympanic cavity & structures & EAM begin The extermal ear also begins around: around the 4th week

the 4th week and is complete by 7th week
Week 4: -eustachian tube begins
1st pharyngeal pouch extends lateral ward to form tybotympanic recess
-Inner ear
otocyst , otic pit
vestibular cochlear division
The primary contributers to outer & ME development are: the 1st & 2nd arches
endoderm and ectoderm come together and the 1st pair of pharyngeal pouches to yield: the tympanic membrane
Middle ear development: -6 pairs of pharyngeal arches separated by endodermal lined pouches and ectodermally lined clefts
-come around to the front of the embryo and are responsible for face, neck, external and middle ears
-also responsible for laryngeal cartilage and pulmonary artery
Week 9: 3 tissue layers of TM present
week 15: middle ear
-cartilage stapes
-ossicles are nearly adult size and begin to ossify
week 16 (4montns): middle ear- ossification of malleus and incus
week 18: ossification of stapes
week 21: all development from here on is in middle ear
-meatal plug disintegrates exposes TM
Week 32 (8 months): Malleus and incus complete
middle ear complete by?: 36th week
begins 4th -->8th
Rests 9-->12
resumes 13th-->36
The embryonic inner ear forms from the a)ectoderm b)mesoderm c)endoderm d)all of the above: a)the ectoderm or outer embryonic layer
-otic placodes are thickened , ectodermal depressions that form into otocysts which differentiate into 2 compartments
1. the cochlear pouch
2. the vestibular pouch
What does ectoderm do?: Wraps aroud everything & forms the skin
-but because of invagination, it also forms neural tissue & the neuroepithelium of the retina and the organ of corti
The inner ear begins to develop when?: 3rd week- continues to grow and differentiate until about week 25
Inner ear development: 3rd wk - membraneous labrynth starts as a thick mass of epithelial cells on the surface ectoderm-->otic placode
-it invaginates into the mesoderm & becomes a cyst -->otocyst or otic vessicle
-6th to 8th wk
- otocyst develops into an anterior and posterior portion
The anterior portion of the otocyst becomes: the saccule, the cochlear duct
-the membranous labyrinth fills with endolymph
The posterior portion of the otocyst becomes?: differentiates first yielding
- endolymphatic duct
-the utricle
-semicircular canals
Week 7: Auricles move dorsal laterally

inner ear
-1 cochlear coil
-sensory cells
-utricle & saccule
9th week -inner ear devel: -bony labyrinth forms originally from the mesoderm & differentiates into a cartilagionous structure
-This structure then ossifies with the petrous portion of the temporal bone
-the space between the membranous labyrinth & bony lab fill with perilymphatic fluid
Week 10-11: inner ear - 2 1/2 cochlear coils(nearly adult form)
-VIII nerve attaches to cochlear duct
-differentiation of organ of corti from base to apex
Week 12: Inner ear
-sensory cells in cochlea
-membranous labyrinth complete
-otic capsule begins to ossify
Week 20-22: outer ear- auricle is adult shape (will continue to grow until age 9)

inner ear - maturation of inner complete
adult size not necessarily turned on yet
At what week is maturation of inner ear complete?: week 24
Causes of abnormal development of inner ear: Genetic factors
-chromosomal abnormalities
-gene mutations

Exogenous factors
-hypoxia
-radiation
-drugs
-infectious insult
Malformation fo the osseous and membranous labyrinths: - complete labyringhine aplasia (michel)
-cochlear anomalies - cochlear aplasia, cochlear hypoaplasia
-incomplete partition (mondini only 1.3 turns of cochlea)
-common cavity - detectable radiographically
Membranous labyrinth malformations: - Cojmplete membranous labrinth dysplasia
limited membranous labyrinth dysplasia
-cochleosaccular dysplasia (scheibe)
-Cochlear basal turn dysplasia (familial high frequency SNHL
Week 30: pneumatization of tympanum
-EAC continues to mature until 7 yrs of age
week 34: mastoid air cells develop
Week 35: antrum pneumatized

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Created By josyg