Bio 50B - Lec 4

What are the primary functions of the outer ear (pinna and external auditory canal)?

To collect sound waves, direct them to the tympanic membrane, and selectively amplify certain sound frequencies (especially 2,000–5,000 Hz).

How does the middle ear overcome the impedance mismatch between air and fluid in the cochlea?

• Area ratio difference: The tympanic membrane has a much larger area than the oval window, concentrating force.

• Ossicular lever action: The arrangement of the malleus, incus, and stapes creates mechanical leverage to amplify pressure.

What is the function of the attenuation reflex (acoustic reflex)?

Contraction of the tensor tympani and stapedius muscles stiffness the ossicular chain to protect the inner ear from damage due to loud, low-frequency sounds.

What are the three main fluid-filled compartments (scalae) of the cochlea, and what fluids do they contain?

Scala vestibuli – Perilymph (low K+, high Na+)Scala media (cochlear duct) – Endolymph (high K+, low Na+)Scala tympani – Perilymph (low K+, high Na+)

What generates the endocochlear potential, and what is its magnitude?

The stria vascularis actively pumps potassium (K+) into the scala media, producing an endolymphatic potential of approximately +80 mV relative to perilymph.

Explain the concept of tonotopic organization in the basilar membrane.

• Base (near oval window): Narrow, stiff, responds best to high-frequency sounds.

• Apex (near helicotrema): Wide, flexible, responds best to low-frequency sounds.

How do hair cell stereocilia movement trigger electrical depolarization?

Bending towards the tallest stereocilium pulls tip links open, allowing K+ ions from the endolymph to enter down their electrochemical gradient, depolarizing the hair cell.

How does repolarization occur in auditory hair cells?

Potassium (K+) exits the basal region of the cell into the perilymph (which has low K+) through voltage-gated potassium channels down its concentration gradient.

What is the distinct functional role of Outer Hair Cells (OHCs) versus Inner Hair Cells (IHCs)?

• IHCs: Primary sensory receptors that transmit auditory signals to the brain via type I afferent fibers.

• OHCs: Act as "cochlear amplifiers" through electromotility (motor protein prestin) to sharpen frequency tuning and boost sensitivity.

What protein powers the fast length changes (electromotility) of outer hair cells?

Prestin.

What is phase-locking in auditory nerve firing?

The tendency of auditory nerve fibers to fire action potentials at a precise phase of a sound wave's cyclical waveform (primarily active for sound frequencies below 3–4 kHz).

Which brainstem structures process binaural sound localization cues, and what specific cues do they use?

• Medial Superior Olive (MSO): Interaural Time Differences (ITD) for low-frequency sounds.

• Lateral Superior Olive (LSO) & Medial Nucleus of the Trapezoid Body (MNTB): Interaural Level Differences (ILD) for high-frequency sounds.

Trace the central auditory pathway from the cochlea to the primary auditory cortex.

Cochlear nerve → Cochlear Nuclei → Superior Olivary Complex → Lateral Lemniscus → Inferior Colliculus → Medial Geniculate Nucleus (MGN) of thalamus → Primary Auditory Cortex (A1 / Brodmann area 41).

What is conductive hearing loss vs. sensorineural hearing loss?

• Conductive: Impedance of sound transmission in outer or middle ear (e.g., earwax, otitis media, otosclerosis). Weber test lateralizes to affected ear.

• Sensorineural: Damage to inner ear hair cells or auditory nerve (e.g., noise exposure, presbycusis, acoustic neuroma). Weber test lateralizes to healthy ear.

What are the expected results of the Rinne test in a normal ear vs. conductive hearing loss?

• Normal / Sensorineural: Air Conduction > Bone Conduction (Positive test).

• Conductive loss: Bone Conduction > Air Conduction (Negative test).

What does External Ear do?

Hearing only

What does the Middle Ear do?

Hearing only

What does the internal ear do?

Hearing and Equilibrium

Mechanoreceptors:

STIMULI of Mechanical

Force, with many examples including:

- Vibration in Ear … “Special Senses”

What happens as vibration moves through fluid?

the Hairs move and trigger Depolarization…

- Vestibule + Semicircular Canals = Equilibrium

- Cochlea = Hearing

What is sound?

a “Pressure Disturbance” where a Vibrating Object pushes on the surrounding

- Media, i.e. Air or Liquid

- Ear has both Air and Liquid regions…

What does alternating high-low vibrational energy create?

creates high-low “Pressure Pushes” on the surrounding Media molecules.

- Alternating high-low Pressure makes Waves

Depending on ____, waves will have different _____ and _____ making _____?

Vibration

Wavelength

Amplitude

Point of Highest Pressure

Compression, and represents “Crest” of Soundwave

Point of Lowest Pressure

Rarefication, and

represents “Trough” of Soundwave

Wavelength

Distance between each “Crest”

- Frequency = # passed Wavelengths / Time

- Frequency corresponds to Pitch, i.e. Notes

Amplitude

Change between high-low Pressure

- Amplitude = Strength of Wave

- Amplitude corresponds to Loudness (Volume)

What is Auricle (Pinna)

Basically, the “Ear” that we think about…it is actually a Funnel

to collect Soundwaves

- Helix: cartilaginous rim

- Lobule: fleshy “earlobe”

What is External Acoustic Meatus

a short tube lined with skin, hairs, glands

- Transmits Soundwaves to Ear Drum

- Tympanic Membrane (Ear Drum) then Vibrates in response to Soundwave

What is Tympanic Membrane

the boundary between External/Middle Ear, CONVERTS Sound to Vibrations

What does skin covered outer surface do?

RECEIVES Soundwaves via External Environment

What does the thin / translucent connective tissue do?

Vibrates in response to Sound

What does Mucosa inner surface do?

TRANSFERS the Vibration Energy to connected Bones of Middle Ear

Malleus

Hammer joined to Ear Drum

Incus

Anvil intermediary…

Stapes

Stirrup fits into Oval Window

Where is vibration transferred to?

through Bones to Oval Window and into Inner Ear

What do Vibrations cause?

The Oval Window to PUSH INWARD into Inner Ear

What happens to the fluid in the inner ear?

it gets DISPLACED by Vibrational Pressure

What happens after displacement from vibrational pressure?

Round Window then PUSHES OUTWARD back into Middle Ear
Allows Fluid Movement in Inner Ear

• Prevents Pressure Buildup

Membranous Labyrinth:

the membrane sacs and ducts contained in Bony Labyrinth

- filled with Endolymph Fluid

- K+ - rich, w/other ions

Cochlea

small spiral, conical,

Bony Chamber

- looks like a “Snail”

Scala Vestibule:

abuts Oval Window,

contains Perilymph

Scala Tympani:

terminates at the

Round Window; contains Perilymph

Perilymph Chambers

are continuous with each other and Bony Labyrinth, and merge in the Helicotrema, at the Apex

Scala Media (Cochlear Duct)

houses

Spiral Organ; contains Endolymph

“floor” contains Basilar Membrane, which SUPPORTS the Spiral Organ

- Soundwaves will VIBRATE the

Basilar Membrane, to STIMULATE

Sensory Receptors in Spiral Organ

Spiral Organ:

contains Hair Cells, that arrange between Tectorial Membrane

AND Basilar Membrane

Inner Hair Cells (ONE ROW):

detect Sound Vibrations coming through the

Basilar Membrane

- Sterocilia “Tips” are enmeshed in

Tectorial Membrane, and PIVOT

while Basilar Membrane VIBRATES,

causing Depolarization that is then

relayed to Cochlear Nerve

Outer Hair Cells

can contract and stretch, which change

the stiffness of Basilar Membrane

1. Helps “fine-tune” Inner Hair Cells by

AMPLIFYING the motion of Basilar

Membrane

2. Protect Inner Hair Cells from loud

noises by DECREASING motion of

Basilar Membrane

Resonance:

movement of different areas of Basilar

Membrane in response to a Particular Frequency…

Fibers near Oval Window…

are short and stiff, and

Resonate with waves of High-Frequency

Fibers near cochlear apex…

are longer and floppier, and

Resonate with waves of Lower-Frequency

What are the 3 areas with sensory receptor neurons in inner ear?

1. Vestibule

2. Semicircular Canals

3. Cochlea

Maculae

Sensory Receptor Organs which will

monitor “Static Equilibrium”

Crista Ampullaris

has both Support Cells

and Hair Cells that extend into gel-mass

called Ampullary Cupula

• - Dendrites of Vestibular Nerve Branch

encircle BASE of Hair Cells

• Conflicting Information causes…?

Motion Sickness, triggering excess salivation,

pallor, rapid deep breathing, profuse

sweating, vomiting…