Basilar Membrane: Lining & Function

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  • 0:04 Definition
  • 1:18 The Organ of Corti
  • 2:37 Distinguishing Volume & Pitch
  • 3:27 Lesson Summary
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Lesson Transcript
Instructor: Katie Chamberlain

Katie has a PhD in Microbiology and has experience preparing online education content in Biology and Earth Science.

The basilar membrane is a key player in the process of hearing. To 'hear' more about it, we think that completing this lesson 'sounds' like a good idea. We have also 'heard' good things about the quiz.

Definition of Basilar Membrane

The basilar membrane is a structure within the inner ear that is moved by incoming sound waves and is essential for the sense of hearing. The ear as we see it is only the outer ear (or pinna). If we shrank and travelled into the ear canal we would soon have to cross the eardrum (or the tympanic membrane), and then we would enter the middle ear. Here, we would find three tiny bones (malleus, incus, and stapes).

Sound waves enter the ear through the air and then transmit movement to these little bones. One of those bones, the stapes, acts like a door knocker, tapping on another membrane called the oval window which separates the middle ear and the inner ear. The oval window is the start of a coiled structure (like a snail) called the cochlea, where a fluid called perilymph moves when the oval window is struck by the stapes.

As the sound waves enter the perilymph of the cochlea at the oval window, they travel all the way to the apex. They then zip around a hairpin turn and travel back through the coiled cochlea. You can imagine that their return-tunnel is parallel to the tunnel they took towards the apex. Along the length of this return-tunnel is the basilar membrane.

The Organ of Corti

The basilar membrane has hair cells attached to its surface (on the side that does not face the perilymph). We don't mean hairs like the kind on your head. Hair cells are just the name of a type of mechanoreceptor, which means that they are stimulated by movement. Deep in the ear, surrounded by a fluid called endolymph (similar to the perilymph) they do not move except by sound waves. As a sound wave moves through the perilymph, the basilar membrane moves, too. Check out this diagram:

cochlea

The coiled cochlea is shown here as if it were unrolled. It also shows how the basilar membrane differs in width and the different frequencies detected at each end.

At the top of the hair cells are teensy tiny little cilia. These tiny little hair-like extensions gently touch against a sturdy ceiling-like structure called the tectorial membrane. Together, the basilar membrane, the hair cells/cilia, and the tectorial membrane make a structure called the Organ of Corti.

As the basilar membrane moves, the hair cell's cilia are brushed gently against the surface of the tectorial membrane. This bending movement triggers the hair cells to fire a neural impulse, which means that a sound wave was detected. Voila, hearing!

Distinguishing Volume and Pitch

The basilar membrane moves in accordance with the size of the sound wave. A louder sound will make the membrane move more. A quieter sound wave will make it move less. This is how the body senses volume, which is determined by the height of the sound waves.

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