Ear TO 3

General Anatomy of the Cochlea

  • The cochlea is the primary structural unit of the inner ear responsible for the sense of hearing.

  • It contains the specialized organ of hearing known as the Organ of Corti.

  • Morphologically, the cochlea is described as being shaped like the coiled shell of a snail.

  • Anatomically, the cochlea is composed of two primary labyrinthine structures:

    • Osseous labyrinth

    • Membranous labyrinth

The Osseous Labyrinth and Fluid Compartments

  • The osseous labyrinth of the cochlea is divided into two distinct functional compartments by a thin bony shelf.

  • These compartments contain a fluid known as perilymph.

  • Upper Compartment:

    • Referred to as the vestibular duct or scala vestibuli.

    • Originates at the oval window and terminates at the apex of the spiral.

    • Contains perilymph fluid.

  • Lower Compartment:

    • Referred to as the tympanic duct or scala tympani.

    • Originates at the apex and extends to the round window.

    • Contains perilymph fluid.

The Membranous Labyrinth and the Cochlear Duct

  • The membranous labyrinth is the subdivision of the cochlea that contains the cochlear duct.

  • Cochlear Duct (Scala Media):

    • This is the specific portion of the membranous labyrinth situated between the scala vestibuli and the scala tympani.

    • It begins at the oval window and ends at the apex of the spiral.

    • Critically, the cochlear duct contains a different fluid called endolymph.

  • Defining Barriers of the Cochlear Duct:

    • Vestibular membrane: This membrane serves as the ceiling of the cochlear duct.

    • Basilar membrane: This membrane serves as the floor of the cochlear duct.

The Organ of Corti and the Physiology of Hearing

  • Organ of Corti:

    • Positioned within the cochlear duct, specifically located above the cochlear membrane.

    • It houses the hearing receptor cells, which are referred to as hair cells.

  • Tectorial Membrane:

    • This membrane projects over the Organ of Corti.

  • Mechanism of Hearing Reception:

    • Hearing is initiated when vibrations move the basilar membrane.

    • As the basilar membrane vibrates, the hair cells (hearing receptor cells) brush up against the tectorial membrane.

    • This mechanical interaction is what allows the receptor cells to detect sound stimuli.

Questions and Learning Objectives

At this stage, students should be able to fulfill the following learning requirements:

  • Identify and describe the cochlea.

  • Identify and describe the function of the Organ of Corti.

  • List the two parts of the cochlea.

  • Describe the Osseous Labyrinth.

  • Name and identify the upper chamber and state what fluid is found there.

  • Name and identify the lower chamber and state what fluid is found there.

  • Identify and give the function of the following structures:

    • Membranous labyrinth

    • Cochlear duct/scala media (including identification of the fluid found here)

    • Vestibular membrane

    • Basilar membrane

    • Organ of Corti

    • Tectorial membrane