ENT (Otorhinolaryngology) · MU Sofia

2. Physiology of the external and middle ear.

Covers how the outer ear collects, localises and amplifies sound, including the resonance peak around 2,500 to 4,000 Hz, and how the canal protects itself through cerumen, hairs and skin migration. Then explains middle ear impedance matching through area ratio, ossicular lever action and membrane buckling, followed by the stapedial reflex with a table comparing the stapedius and tensor tympani. Closes with the Eustachian tube's pressure-equalising, protective and drainage roles, how it opens, and the effects of its dysfunction.

Inside: 2 sections · 3 labelled pictures · 1 table · 14 flashcards · 6 min

Topic 2 of 52 in the MU Sofia syllabus for ENT (Otorhinolaryngology) (Medicine, Year 4). The free AceMed note covers it in 2 sections, about 802 words (6 min of reading), with 14 flashcards.

In this note

  1. Physiology of the External and Middle Ear
  2. Eustachian Tube (ET) Physiology: Pressure Equalization

Physiology of the External and Middle Ear

1. External Ear Physiology: Sound Collection and Amplification - acts as a passive sound collector and resonator.

A. Sound Collection and Localization

  • Auricle (Pinna):
  • Acts as a funnel to collect sound waves and direct them into the External Auditory Canal (EAC).
  • The complex folds (concha, helix, etc.) are crucial for sound localization, especially for determining a sound source's elevation (up or down). These folds introduce subtle time delays and frequency-dependent filtering that the brain uses for spatial mapping.

Mechanism of localization:

  • Interaural time difference (ITD) for low frequencies (<1500 Hz).
  • Interaural intensity difference (IID) for high frequencies (>1500 Hz).
  • The unique folds of the pinna produce directional filtering, helping identify where a sound is coming from directionally.

B. Acoustic Amplification (Resonance)

  • EAC functions as a closed-end tube resonator.
  • It selectively amplifies sounds, peaking at frequencies between 2,500 Hz and 4,000 Hz. This is a critical frequency range for understanding human speech.
  • This resonance effect provides an amplification of 10–15 dB at the tympanic membrane (TM).
  • Clinical correlation:
  • Canal obstruction → reduced amplification → conductive hearing loss.

C. Protection and Clearance

  • Cerumen and Hairs: The hairs and cerumen (earwax) in the cartilaginous part of the EAC provide protection against foreign bodies, dust, and infection due to the cerumen's slightly acidic.
  • Epithelial Migration: The skin of the TM and bony EAC migrates outward, carrying cerumen and debris out of the canal. This is a crucial self-cleaning mechanism.
  • Physical Protection: S-shaped curve prevents direct access, Hair in lateral canal prevents entry of insects & debris.

2. Middle Ear Physiology: Impedance Matching = Efficient transfer of sound from air → fluid-filled cochlea.

Impedance Mismatch: Sound travels efficiently in air but is reflected when it hits the fluid in the cochlea. Without a mechanism to overcome this, 99.9% of sound energy (30db) would be lost. The middle ear system provides a gain that overcomes this loss.

A. The Mechanism: Pressure Amplification

The middle ear overcomes this impedance mismatch via two main mechanisms:

1. Area Ratio (Hydraulic Principle)

  • The surface area of the TM (55 mm²) is significantly larger than that of the Stapes Footplate in the oval window (3.2).
  • Principle: By concentrating the force from a large area (TM) onto a small area (oval window), the pressure is magnified.
  • Magnification: The ratio of the effective area of the TM to the area of the stapes footplate is approximately 17:1.

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