Anatomy of the Hearing Mechanism - Topic 2: Development of the Ear and Outer Ear Anatomy and Physiology

What happens during the period of the zygote?

rapid cell division, ends 2 weeks later when the zygote is implanted in the wall of the uterus

what happens during the period of the embryo?

body parts are formed during this period, embryo rests in the amnion filled with amniotic fluid, umbilical cord joins embryo to placenta

when is the period of the embryo?

3-8 weeks after conception

when does the period of the zygote begin?

when the egg is fertilized in the fallopian tube

When is the period of the fetus?

from 9 weeks after conception to birth

What happens during the period of the fetus?

increase in size and systems begin to function

What is the age of viability during the period of the fetus

22-28 weeks

What are teratogens?

a drug or other substance capable of interfering with the development of a fetus, causing birth defects

What about nutrition is important in prenatal development?

getting adequate amount of food, protein, vitamins and minerals

What does stress do to the mother and fetus during prenatal development?

decreases oxygen to fetus and weakens mother's immune system

What should be noted about the mother's age during prenatal development?

if the mother is too young or too old, the fetus is at a higher risk

What are some of the potentially dangerous drugs to prenatal mothers and the fetus?

potentially dangerous drugs not limited to cocaine, but include alcohol and caffeine

How do diseases reach the fetus?

they attack through the placenta and directly attach the fetus (TORCH infection)

Why are environmental hazards dangerous?

We are often unaware of their presence

How do teratogens influence prenatal development?

Not universally harmful. But particular structures at a particular point in development in particular animals.

How are prenatal diagnoses found?

ultrasound, amniocentesis, and chorionic villus sampling can detect physical deformities and genetic disorders

What treatment is there for prenatal disorders?

Fetal medicine and genetic engineering but are experimental

How can we assess a newborn?

APGAR to assess the newborns' health, NBAS for a comprehensive assessment of infants

What are the 4 primary states of the newborn?

alert inactivity, waking activity, crying, sleeping

How much of the newborn's sleep is REM

half

How can am mother prevent SIDS

have the baby sleep on it's back

When is the APGAR score given?

a score is given for each sign at 1 minute, 5 minutes, and -- if there are still problems after 5 -- 10 minutes

What is a normal APGAR score?

7-10

What APGAR score indicates a need for observation?

4-7; might requires some resuscitative measure

What APGAR score indicates critical condition?

3 and below; requiring immediate resuscitation

What does APGAR stand for?

Activity (muscle tone), Pulse, Grimace (Reflex Irritability), Appearance (skin color), Respiration

What is anoxia and what can it lead to?

lack of oxygen; can lead to the surgical removal of the fetus (C-section)

Premature vs Small-for-date

Prematurity is a baby that is born before the 3rd trimester is finished, small-for-date means the child is smaller than they should when born on the correct date. Prematurity is less serious than small-for-date

What birth weight is considered very low?

1500 grams (3 lbs, 4 ounces) only 1.5% of babies

What can cause a premature birth?

intrauterine growth restriction: problems with the placenta, poor maternal health, birth defects

When is birth considered premature?

less than 37 weeks in gestation

which race often has a lower birth weight?

African American babies, due to lack of health care, younger mothers, and lack of nutritional resources

What can cause an infant to be very low birthweight?

race, age (teen or advanced maternal age, geriatric pregnancy after 35), multiple births, poor mother's health

What are the complications and risks associated (after birth) with very low birthweight (VLBW) babies?

• Low oxygen levels of birth

• Inability to maintain body temperature

• Difficulty feeding and gaining weight

• Infection

• Breathing problems such as respiratory distresssyndrome (immature lungs)

• Neurological problems (bleeding in the brain)

• Gastrointestinal problems

• Sudden infant death syndrome (SIDS)

What are some long term problems an infant could face due to VLBW?

- cerebral palsy

- blindness

- hearing loss

- mental retardation

When does prenatal hearing loss occur?

conception to fetal stage (22-28 weeks or 7 months of pregnancy)

When does perinatal hearing loss occur?

22 weeks of pregnancy to 7 days after birth?

When does postnatal hearing loss occur?

after 7 days of birth

What are the main components of the external ear?

Pinna (auricle) and the external auditory canal

What are the two parts of the external auditory canal?

lateral 1/3 --> cartilage

medial 2/3 --> bone

What is the "conveyor belt shedding" processing in the external ear?

A self-cleaning mechanism of the ear canal that move debris outward

Which animals have pinnae?

only mammals have pinnae, though their form varies greatly

What is the function of movable pinnae in some mammals?

mammals with high-frequency hearing can move their pinnae to help localize sounds?

Do humans have functional muscles to move their pinnae?

no -- humans have no useful muscles for moving pinnae

What gives the pinna rigidity?

cartilage

how is the skin of the pinna attached?

skin and underlying tissues are firmly attached to the cartilage

What part of the pinna has no cartilage?

the earlobe

What lines the external ear?

skin

about how long is the external auditory canal?

2.5 cm

Where does the external auditory canal end?

at the tympanic membrane

What is the diameter and volume of the concha?

Diameter: 10-20 mm; Volume: 2.5 cm³.

What is the "concha effect"?

A resonance boost at 4000-5000 Hz.

Why is the concha acoustically important?

It is the most acoustically important part of the external ear (helps collect and funnel sound).

What are the two parts of the concha?

Cavum concha and Cymba concha.

What is the helix?

The outer rim of the pinna.

What is the antihelix?

An elevation running close to the center of the pinna.

What is the triangular fossa?

A depression between the two crura (legs) of the antihelix.

What is the crus of the helix?

The "leg" of the helix that connects the pinna to the skull.

What is the tragus and where is it located?

A small, triangular protrusion on the anterior portion of the pinna.

What does the word "tragus" mean in Greek?

"Goat's beard."

What notable feature is often found on the tragus of older men?

Bristly hair.

What happens when the tragus is pressed closed?

It can cut out/block sound.

What is the antitragus?

A protrusion of hard cartilage above the earlobe.

What is the intertragal notch?

A depression between the tragus and antitragus.

What is the lobule?

The earlobe. The bottom portion of the pinna, free of cartilage.

What is the primary function of the pinna?

To collect and funnel sound into the external auditory canal.

How do the pinna and head help with sound localization?

They provide directionality cues that assist in determining where a sound comes from.

What factors affect horizontal-plane sound localization?

- Wavelength of the sound

- Diameter of the head

What is the "head shadow effect"

The head blocks or reduces sound from reaching the far ear, especially for high frequencies (>1500 Hz).

How does sound diffraction relate to the head shadow effect?

Sounds with frequency >1500 Hz bend around the head (diffraction), affecting horizontal localization.

How are front-to-back localization cues affected by frequency?

Sounds coming from behind the head are reduced in intensity for frequencies between 3000-6000 Hz.

What is notable about the external auditory canal (EAC) in terms of development?

The EAC is related to many structures that fuse together during development

What is the external auditory meatus?

The opening to the external auditory canal; diameter 5-7 mm.

What are the dimensions of the adult external auditory canal (EAC)?

Diameter: ~7 mm; Length: 23-30 mm (longer at bottom, shorter at top)

What is the resonance frequency of the external auditory canal?

2,600-3,500 Hz.

What is notable about the outer 1/3 of the EAC?

- Made of quasi-independent pieces of cartilage

- Contains ceruminous (wax) glands

- Contains sebaceous glands (fat-producing) and apocrine glands

- Lined with hair pointing up and out

What is the composition of the medial 2/3 of the external auditory canal?

Osseous (bony) with thin skin covering.

How has the medial EAC been described anatomically?

As a cylinder of temporal bone at the base of the skull.

Which walls are formed by the tympanic portion of the medial EAC?

Anterior and inferior walls, plus the lower part of the posterior wall.

What other portions of the temporal bone contribute to the medial EAC?

Squamous portion and mastoid portion.

What is the osseous-cartilaginous junction of the external auditory canal?

The point where the outer 1/3 (cartilaginous) meets the inner 2/3 (osseous); also called the isthmus.

What is the primary function of the external auditory canal (EAC)?

Efficiently transmits sound to the tympanic membrane.

What is the resonance effect of the EAC?

Approximately 10 dB boost at ~3000 Hz.

How does the EAC protect the tympanic membrane and inner ear?

Protects from injury, extreme temperatures, and foreign bodies.

What is the self-cleansing mechanism of the EAC?

Cerumen, hair, and epithelial migration ("conveyor belt" shedding) help remove debris naturally.

How thick is the skin over bony (osseous) portions of the ear?

Approximately 0.2 mm thick.

How thick is the skin over cartilaginous portions of the ear?

0.5-1.0 mm thick.

What structures make up the apopilosebaceous unit in cartilaginous skin?

- Hair follicle

- Erector pili muscle

- Apocrine or eccrine gland

- Sebaceous gland