4 Diagnostic Aud II - Electrocochleography

Question 1

In thinking about the role of the Organ of Corti and its component structures, how is frequency coded and what is the role of the cochlear amplifier?

Answer 1

• Frequency is coded both by place on the BM and the rate of firing of the afferent neurons.
• Cochlear amplifier makes hearing an active rather than passive process and its primary function is to control tuning and sensitivity to transduction processes.

Question 2

How are endolymph and perilymph different and why is that important in the acoustic transduction process?

Answer 2

• Perilymph is typical extracellular fluid (like CSF).
• Endolymph is different from other extracellular fluid (High in K+ & low in Na+)
• Endolymph is electrically + relative to perilymph by ~80mV
• HCs have a normal intracellular potential of ~-50mV
• Potential difference on 130mV is set up over the top of the HC, which powers the transduction process.

Question 3

What is the difference between the AP and the CAP and which is recorded in ECochG?

Answer 3

• The AP is from a single nerve fiber and the CAP is the summed fields generated by all afferent nerve fibers.
• The CAP is recorded in ECochG

Question 4

Why do you suppose that people with hair cell damage have CAP tuning curves that are more broad than those with normal hearing or those with other types of HL?

Answer 4

The OHCs are important in determining the sensitivity and tuning properties of the cochlea. As long as the OHCs are healthy that function should remain intact.

Question 5

What happens to the CAP/SP ratio in patients with endolymphatic hydrops? Can you speculate about why that is the case?

Answer 5

• The SP/CAP amplitude ration increases.
• CAP amplitude decreases
• The SP amplitude increased due to the increased pressure in the cochlear duct due to hydrops.