📚 L35 Ear 2

📚 Lecture Overview

This lecture covers the physiology of hearing, focusing on sound transmission and mechanoelectrical transduction within the inner ear's organ of Corti. It details how the basilar membrane encodes frequency and loudness, traces the neural auditory pathway to the cerebral cortex, and explains the mechanics of sound localization. Finally, it outlines the pathophysiology of conduction versus sensorineural hearing loss and the clinical diagnostic utility of audiometry.

🎯 Key Concepts & Definitions

• Organ of Corti: The cochlear mechanoreceptive organ located on the basilar membrane containing auditory sensory hair cells.
• Inner Hair Cells: Single row of ~3,500 sensory receptor cells responsible for converting sound vibrations into neural action potentials.
• Outer Hair Cells: Three to four rows of ~12,000 motor/receptor cells embedded in the tectorial membrane that adjust hearing sensitivity and frequency tuning.
• Place Principle: The theory that different sound frequencies maximally vibrate specific regions along the basilar membrane based on local fiber stiffness and length.
• Duplex Theory: Mechanism of encoding intensity and frequency, where loudness is determined by the number of action potentials per burst and pitch is determined by burst frequency.
• Endocochlear Potential: The strong electrical potential gradient (~120–140 mV total difference) between the endolymph (+80 mV) and the inside of hair cells (-66 mV).
• Word Deafness (Auditory Aphasia): The inability to interpret the meaning of heard sounds caused by lesions in auditory association cortex (Area 22).

📖 Main Content

1. Structure of the Organ of Corti & Hair Cell Transduction

The organ of Corti sits atop the flexible basilar membrane beneath the elastic tectorial membrane.

Hair Cell Organization & Potential Difference

• Inner Hair Cells: ~3,500 cells in 1 row; primary sensory transducers that signal to the brain.
• Outer Hair Cells: ~12,000 cells in 3–4 rows; stereocilia embedded in the tectorial membrane.
• Resting Potentials: Hair cell interior is -66 mV, while endolymph is +80 mV, creating a net driving potential of 120–140 mV across the apical membrane to maximize sensitivity.

Mechanics of Hair Cell Transduction

1. Sound pressure causes shearing forces between the basilar and tectorial membranes.
2. Cilia bend in the direction of the tallest stereocilium.
3. Filament tip-links tug outward and mechanically open 200–300 cation channels.
4. Rapid influx of Potassium ions ($K^+$) depolarizes the hair cell.
5. Depolarization increases neurotransmitter release onto afferent dendrites of the spiral ganglion.
6. Bending of stereocilia in the opposite direction causes hyperpolarization.

Sound Waves → Fluid Movement → Basilar Bending → Cilia Shearing → K+ Influx → Cell Depolarization

2. Basilar Membrane Dynamics & Pitch Analysis

Sound transmission travels from the stapes moving the oval window, through cochlear fluid, and dissipates through the round window (which acts as a pressure release valve).

Stapes movement → Oval window inward → Perilymph traveling wave → Basilar deflection → Round window outward

The Place Principle

The structural characteristics of the basilar fibers vary along the cochlea:
- Base (near Oval Window): Fibers are short, thick, and stiff → Vibrate best at high frequencies.
- Apex (helicotrema): Fibers are long, thin, and flexible (100-fold less stiff) → Vibrate best at low frequencies.
- Very low frequencies (< 200 Hz) are compressed onto a limited section at the apical end.

3. Discrimination Mechanisms & Sound Localization

Pitch and Loudness Encoding

• Pitch Discrimination: Determined by the specific region of the basilar membrane maximally excited (Place Principle).
• Loudness Discrimination: Determined by the amplitude of basilar membrane displacement, leading to greater action potential firing frequency in sensory nerve fibers and recruitment of additional fibers.

Sound Localization Cues

1. Time Difference: Sound reaches one ear up to ~1 msec earlier than the other ear. Used primarily for low frequencies.
2. Intensity Difference: The head creates a sound shadow, reducing sound intensity on the far side. Used primarily for high frequencies (e.g., 2:1 difference at 1,000 Hz vs 100:1 at 10,000 Hz).
3. Pinna Echoes: Small curves of the outer ear create complex echoes used to locate vertical (up/down) and midline (front/back) sound sources.

4. The Auditory Neural Pathway

The central pathway preserves tonotopic organization throughout its projection levels.

Spiral Ganglion → Cochlear Nuclei → Superior Olivary Nuclei → Inferior Colliculus → Medial Geniculate Body → Auditory Cortex

Functional Specialization of Pathway Structures

• Spiral Ganglion: Contains ~30,000 afferent cell bodies whose axons form the cochlear nerve (CN VIII).
• Cochlear Nuclei (Upper Medulla):
• Receives unilateral input only.
• Medial neurons: Process high frequencies.
• Lateral neurons: Process low frequencies.
• Superior Olivary Nucleus:
• Receives bilateral inputs; essential for sound localization via time and intensity comparisons.
• Connects to the reticular formation for brain arousal.
• Sends descending olivocochlear fibers to hyperpolarize outer hair cells, dampening sensitivity to select frequencies.
• Inferior Colliculus: Center for audiospinal reflexes.
• Medial Geniculate Body (MGB): Thalamic processing and relay center for auditory information.
• Primary Auditory Cortex (Areas 41 & 42): Processes pitch, sound direction, and timbre.
• Auditory Association Cortex (Area 22): Interprets sound meaning and integrates sound with other sensory modalities.

5. Types of Hearing Loss & Clinical Correlations

Lesion Effects Summary

• Unilateral Cochlea/Cochlear Nerve/Nucleus Lesion: Ipsilateral complete deafness in that ear.
• Unilateral Primary Auditory Cortex Lesion: Minor hearing reduction mainly in the contralateral ear; severely impairs sound localization (due to bilateral projection pathways).
• Supranuclear Lesions (above Cochlear Nucleus): Do not cause total deafness in either ear due to bilateral pathway redundancy.
• Bilateral Cortical Destruction: Total cortical deafness.
• Area 22 Lesion: Auditory aphasia / Word deafness (can hear sound, but cannot interpret word meaning).

📊 Visual Learning

Diagram 1: Mechanoelectrical Transduction in Hair Cells

Diagram 2: Structure of the Auditory Neural Pathway

💡 Important Points to Remember

• Depolarizing Ion: Hair cells depolarize via $K^+$ (Potassium) influx, not $Na^+$, due to high $K^+$ concentration in the endolymph and a steep electrical gradient.
• Basilar Frequency Map: Base = High frequency (short/thick fibers); Apex = Low frequency (long/thin fibers).
• Cochlear Nucleus Mapping: Medial = High pitch; Lateral = Low pitch.
• Bilateral Representation: Supranuclear auditory pathways carry signals from both ears, predominantly contralateral. Thus, a unilateral cortical lesion does not cause complete deafness.
• Sound Localization Mechanics: Timing differences for low frequencies; Intensity differences for high frequencies; Pinna curves for vertical/midline sources.
• Ototoxic Antibiotics: Kanamycin, Neomycin, and Gentamycin damage hair cells in the organ of Corti, causing sensorineural deafness.
• Eustachian Tube Closure: Creates negative middle ear pressure, leading to conduction deafness.
• Bone Conduction Audiometry: Vibrations placed on the forehead bypass external and middle ears to evaluate inner ear function directly.

⚠️ Common Exam Questions & Traps

MCQ Tricks & Traps

1. The Ion Trap: Examiners will ask which ion causes hair cell depolarization.
   - Trap: Selecting $Na^+$ or $Ca^{2+}$.
   - Fact: Hair cell depolarization is driven by $K^+$ influx.
2. The Cortical Lesion Trap: Examiners ask the effect of a unilateral lesion of the primary auditory cortex.
   - Trap: Selecting "complete unilateral deafness."
   - Fact: Unilateral cortex lesions cause impaired sound localization and mild contralateral hearing reduction, but NOT total deafness, because auditory signals project bilaterally.
3. Frequency Mapping Inversion:
   - Base of Cochlea = High Frequency | Medial Cochlear Nucleus = High Frequency.
   - Apex of Cochlea = Low Frequency | Lateral Cochlear Nucleus = Low Frequency.
4. Area 22 vs Areas 41/42 Function:
   - Area 41/42 damage affects pitch/localization.
   - Area 22 damage causes Word Deafness (hearing is intact, but speech sounds lose all meaning).

Common Exam Traps

• Conduction vs Sensorineural Total Deafness: Conduction deafness can never be complete/total. Only sensorineural loss or complete bilateral pathway destruction can cause total deafness.
• Outer Hair Cell Function: Descending olivocochlear fibers hyperpolarize outer hair cells to decrease sensitivity/tune specific frequencies, not to stimulate sound perception directly.

📝 Quick Review Checklist

I can state the functions and cell counts of inner vs outer hair cells in the organ of Corti.
I can explain the mechanism of hair cell depolarization, including the specific ion responsible ($K^+$).
I can describe the Place Principle and how basilar fiber anatomy changes from base to apex.
I know the difference between time lag and intensity difference in sound localization.
I can trace the auditory pathway from the spiral ganglion to Areas 41, 42, and 22.
I understand why a unilateral primary auditory cortex lesion does not cause total deafness.
I can differentiate between conduction and sensorineural deafness by cause and bone conduction results.
I know the effect of damaging Area 22 (Auditory Association Cortex).