CorpusHuman Anatomy
Special SensesMembrane

Basilar membrane

Lamina basilaris

Also known as: basilar lamina, cochlear partition

The fibrous membrane forming the floor of the cochlear duct and supporting the organ of Corti; its graded width and stiffness make each place along the cochlea resonate best at a particular frequency (tonotopy).

Show on body map Ear — Coronal Section

Overview

Width≈0.1 mm (base) → ≈0.5 mm (apex)
Stiffness gradient≈100-fold, stiffest at base

The basilar membrane stretches from the tip of the osseous spiral lamina to the spiral ligament. It contains radially oriented collagen fibres ("basilar fibres") embedded in ground substance, covered on the scala tympani side by mesothelial cells. Its width increases from ≈0.1 mm at the base to ≈0.5 mm at the apex, while the bony cochlear canal narrows — and its stiffness decreases ≈100-fold from base to apex.

This gradient underlies place coding of frequency (Georg von Békésy, Nobel Prize 1961): a sound sets up a travelling wave that peaks where the membrane's local resonant frequency matches the sound — near the base for high frequencies, near the apex for low frequencies. Passive mechanics alone give broad tuning; in the living cochlea the wave is sharply peaked and amplified (≈40–50 dB) by outer hair-cell electromotility (the cochlear amplifier), producing exquisite frequency selectivity.

Vertical displacement of the basilar membrane causes a shearing motion between the reticular lamina of the organ of Corti and the overlying tectorial membrane, which deflects the hair-cell stereocilia.

Location

Floor of the cochlear duct, between the osseous spiral lamina and the spiral ligament, along the full 32–35 mm of the cochlea.

Function

  • Mechanically separate sound frequencies along the cochlea (tonotopy)
  • Support the organ of Corti and convert fluid pressure waves into hair-cell shearing
Physiology

Base: narrow (≈0.1 mm), stiff → ≈20 kHz. Apex: wide (≈0.5 mm), compliant → ≈20 Hz. The travelling wave takes a few milliseconds to reach the apex. Loss of outer hair cells reduces sensitivity by ≈40–60 dB and broadens tuning — reducing speech clarity even when amplified.

Clinical significance

High-frequency vulnerability

Basal turn hair cells (high frequency) are the first damaged by ageing, noise and ototoxic drugs — the typical audiogram slopes down at high frequencies.

Test yourself

Where along the cochlea are high-frequency sounds detected, and why?
  1. At the apex, where the basilar membrane is narrow and stiff
  2. At the base, where the basilar membrane is narrow and stiff
  3. At the apex, where the basilar membrane is wide and floppy
  4. Uniformly along the cochlea; frequency is coded only by firing rate

Answer: At the base, where the basilar membrane is narrow and stiff

The basilar membrane is ≈0.1 mm wide and stiff at the base (≈20 kHz) and ≈0.5 mm wide and compliant at the apex (≈20 Hz). This place (tonotopic) code is preserved throughout the auditory pathway; basal hair cells are lost first in presbycusis and noise damage.

Which statement about outer hair cells is correct?
  1. They change length via prestin and amplify basilar membrane motion
  2. They provide ≈95% of the afferent fibres of the cochlear nerve
  3. They form a single row on the modiolar side of the tunnel of Corti
  4. They lack stereocilia

Answer: They change length via prestin and amplify basilar membrane motion

The ≈12 000 outer hair cells (3 rows) are electromotile (prestin), forming the cochlear amplifier and generating otoacoustic emissions. The ≈3 500 inner hair cells (1 row) receive ≈90–95% of afferent fibres.

More special senses questions

Compiled from standard anatomy and physiology references — see the Special Senses sources. For education only; not medical advice.