1. The Inner Ear
SPA 4302
Summer 2004

2. Sensory Endorgans encased within very dense bone
INNER EAR
Sensory Endorgans encased within very dense bone
Two Halves:
Vestibular--transduces motion and pull of gravity
Cochlear--transduces sound energy
(Both use Hair Cells)

4. Subdivision into spaces containing endolymph (blue), and spaces containing perilymph (red)

5. Cochlea is Divided into 3 “Scala”
Scala Vestibuli
Reissner’s Membrane
Scala Media
Basilar Membrane
Scala Tympani
Helicotrema - the opening between 2 outer Scala

6. Fluids filling the Inner Ear
Perilymph- in S. Vestibuli and S. Tympani
High Sodium / Low Potassium concentrations
Low Voltage (0 to +5 mV)
Endolymph- in S. Media
High Potassium / Low Sodium concentrations
High Positive Voltage (85 mV)

7. Cross-Section of the Cochlea
Third Turn
Second Turn
First Turn

9. A Cross Section Shows the 3 Scala

11. Within S. Media is the Organ of Corti

13. I = Inner Hair Cells P = Pillar Cells
O = Outer Hair Cells D = Deiter’s Cells

14. The Stereocilia on IHCs and OHCs
OHCs (at top)
V or W shaped ranks
IHC (at bottom)
straight line ranks

15. Stereocilia bent toward tallest row
Potassium flows into cell
Calcium flows into cell
Voltage shifts to a less negative value
More neurotransmitter is released

16. Cochlear Afferent (Sensory) Neurons
Type I (95%)
synapse w/ IHC
1 IHC to 20 neurons
well myelinated
Type II (5%)
Synapse with OHCs
Many OHCs to 1 neuron
unmyelinated

17. Cochlear Functions
Transduction- Converting acoustical-mechanical energy into electro-chemical energy.
Frequency Analysis-Breaking sound up into its component frequencies

18. Development of the Inner Ear
Beginning in week 3, Auditory placode develops,
Invaginates to form auditory pit,
Which then closes off, leaving an auditory capsule.

19. Capsule divides into saccular, utricular divisions
Saccular: cochlea (begins forming in week 6)
Utricular: semicirc. canals, endolymphatic sac & duct
Organ of Corti forms from week 10
Inner ear fully formed by 25 weeks

20. Hearing Loss and Disorders of the Inner Ear
Vast majority of SNHL results from inner ear disorder.
Cochlear pts often hear, but cannot distinguish what they hear very clearly: dysacusis.
Arises from frequency and other distortions associated with changes in inner ear function.

21. Inner Ear Disorders: Prenatal Causes

22. Anatomical Anomalies Often seen as Bony malformations Examples:
Mondini
(incomplete cochlea)
Enlarged Duct
(shown here)

23. Age Effects

24. Noise Damage Temporary Threshold Shift (TTS)
Permanent Threshold Shift (PTS)
Duration, Timing and Intensity influence
Typical “Noise Notch” often seen between 3 and 6 kHz first.
Notch widens and deepens over time, with hearing loss spreading to adjacent frequencies, and increasing in degree.

25. Ototoxicity Substances poisonous to the ear Medicines
Aminoglycocide antibiotics
Chemotherapy drugs for CA
Aspirin
Industrial Chemicals (tolulene, etc)
May affect balance
Degree of hearing loss varies
Synergy with noise or other ototoxic agents