Anatomy & Neurophysiology: Test #1

What is Hensen’s Stripe?

A prominent anatomical bulge that protrudes downward from the underside of the tectorial membrane

• Positioned directly above the inner hair cells in the organ of Corti within the cochlea

What does the Reticular Lamina do?

• Outer hair cells project from the reticular membrane to the tectorial membrane

• Provides a barrier that prevents endolymph of the scala media from entering the intercellular spaces around the hair cells

• Acts as an ionic barrier or electrical resistance

What is the middle pillar of bone [in the cochlea] called?

The modiolus

What is the innervation of the inner hair cells?

There are about 20 fibers connected to each inner hair cell (divergent relationship)

• Think: divergent = “one-to-many”

What is the innervation of the outer hair cells?

There are about 10 outer hair cells to each auditory nerve fiber (convergent relationship)

• Think: convergent = “many-to-one”

What neurons are the inner hair cells associated with?

Associated with afferent fibers of the Auditory nerve

• 95% of the afferent nerves go to the brain

• 5% go somewhere else

What neurons are the outer hair cells associated with?

Associated with efferent fibers of the Auditory nerve (small portion of afferent)

What is the % of inner and outer hair cells?

• 3-5 rows of outer hair cells (~25,000)

• 1 row of inner hair cells

If you lost all of your outer hair cells, you’d have a roughly ___ dB SNHL

~40 dB SNHL

Tell me about the tonotopic organization of the cochlea (i.e. What comes first? What comes last?)

• High frequency— base

• Low frequency— apex

What are the muscles located in the middle ear? What cranial nerves innervate them?

1. Stapedius muscle— innervated by CN VII

2. Tensor tympani muscle— innervated by CN V

What is the volume size of the ear canal?

1.0 - 1.2

What are OAEs? What do they generate?

• Generated by the electro-motility of the Outer Hair Cells (OHCs)

• Demonstrate the function of the OHCs

How long is the cochlea?

~35mm long

What did Békésy discover? (i.e. what did he come up with in the 1960s?)

Békésy discovered the physical mechanics of sound stimulation inside the cochlea:

• Traveling Wave Theory

• Basis of Tonotopicity (frequency mapping)

What is the Traveling Wave Theory?

• Mechanics of the Cochlear Partition: Békésy demonstrated that sound-induced movement of the stapes footplate creates fluid displacement that sets up a traveling wave (TW) along the basilar membrane

• Direction of Wave Movement: The traveling wave always propagates from the base of the cochlea (near the oval window) toward the apex

Tympanic membrane: thickness

0.1 mm

Tympanic membrane: surface area

55 mm² - 90 mm²

Tympanic membrane: diameter

9 mm

The Pars Tensa takes up about ___ of the TM

3/4

The Pars Flaccida takes up about ___ of the TM

1/4

What is the ionic composition of the hair cells versus endolymph?

Endolymph

• Ionic Makeup: Unlike typical extracellular fluids, endolymph contains a very high concentration of Potassium (K+) and a low concentration of Sodium (Na+)

• Electrical Potential: It maintains a strong positive charge of +80 mV (the endo-cochlear potential), which is actively generated and maintained by ion pumps in the stria vascularis

Hair Cells

• Resting Potential: Inside the hair cell body, the intracellular resting potential is strongly negative—ranging from –40 to –50 mV in Inner Hair Cells (IHCs) and around –70 mV in Outer Hair Cells (OHCs)

• Surrounding Fluid: Beneath the reticular lamina, the cell bodies of hair cells are bathed not in endolymph, but in cortilymph/perilymph, which is high in sodium, low in potassium, and carries a neutral charge of 0 mV

What is the average length of an adult EAC?

• 3.5 cm

• or 2 - 3 cm

• or 1 - 1 ½ inches

What is the average diameter of an adult EAC?

• 5 (or 7) - 9 mm (about a pencil)

• or ¼ inch

The outer ear causes an increase in the level of about __ to __ dB

10 - 15 dB

What is the resonance of the pinna? (i.e. what frequencies are enhanced by it)

1.5 kHz - 7 kHz

What is the ear canal concha-resonance? (i.e. What are the resonant peaks of the concha?)

4.5 - 5 kHz

What is the resonant peak of the EAC?

2.5 kHz

What is the helicotrema?

• A region of the cochlea that opens up between the Scala Tympani & the Scala Vestibuli (i.e. where they connect)

• Those two chambers contain perilymph

How many spins/turns are there in the cochlea

2 ¾ turns

What is the cough reflex?

When you go in too deep

What muscle opens up the eustachian tube?

Tensor veli palatini

Cochlear Microphonic

• Place a wire in the auditory nerve & connect it to an amplifier/loud speaker in another room

• Present a sound → sound can be produced w/ great fidelity

  • Essentially, the cochlea functions as a microphone

  • The CM seems to mimic the sound stimulus or displacement of the Basilar membrane

What is the neurotransmitter of the inner hair cells?

T-glutamate

What protein is inside outer hair cells that make them different from the inner?

Prestin

What is the resting cochlear potential?

The total resting potential difference across the apical (top) membrane of a cochlear hair cell is approximately 140 to 150 mV

• You have ionic differences between inner & outer hair cells → add them together = that’s your potential)

The stapedius muscle is innervated by which cranial nerve?

CN VII (Facial)

The tensor tympani muscle is innervated by which cranial nerve?

CN V (Trigeminal)

What are the layers of the TM? (3)

1. Squamous cell epithelial layer (outer)

2. Fibrous layer (middle)

3. Mucosal layer (inner)

   • Surrounds the entire ME cavity

The electrical potential in the scala media is approximately ___ mV

+80 mV