Name the parts of cochlea.

Overview of the Cochlea

The cochlea is a bony, spiraling tube located in the petrous part of the temporal bone. It is approximately 9mm long and makes 2.75 turns. It’s divided into three fluid-filled spaces: the scala vestibuli, scala tympani, and scala media.

External Structures

1. Cochlear Duct (Scala Media)

This is the central fluid-filled canal within the cochlea. It's separated from the scala vestibuli by Reissner’s membrane and from the scala tympani by the basilar membrane. It contains endolymph, a potassium-rich fluid crucial for hair cell function.

2. Scala Vestibuli & Scala Tympani

These are the two lateral fluid-filled spaces. They are filled with perilymph, a sodium-rich fluid similar to cerebrospinal fluid. The scala vestibuli connects to the oval window, while the scala tympani terminates at the round window. These windows allow for pressure equalization.

3. Helicotrema

This is the opening at the apex of the cochlea that connects the scala vestibuli and scala tympani, allowing fluid to flow between them.

4. Cochlear Nerve (Auditory Nerve Branch)

The cochlear nerve enters the cochlea through the internal auditory meatus and innervates the hair cells, transmitting auditory information to the brain.

Internal Structures

1. Basilar Membrane

This membrane runs the length of the cochlea and varies in width and stiffness. It’s narrow and stiff at the base (near the oval window) and wide and flexible at the apex. This gradient allows different frequencies of sound to stimulate different regions of the membrane – a phenomenon known as tonotopy.

2. Reissner’s Membrane (Vestibular Membrane)

This thin membrane separates the scala vestibuli from the scala media. It’s permeable to ions and helps maintain the ionic composition of the endolymph.

3. Tectorial Membrane

This gelatinous structure lies above the hair cells and is crucial for their activation. When the basilar membrane vibrates, the hair cells are deflected against the tectorial membrane, triggering nerve impulses.

4. Organ of Corti

This is the sensory organ of hearing, located on the basilar membrane. It contains:

• Inner Hair Cells (IHCs): Approximately 3,500 IHCs are responsible for transducing sound vibrations into electrical signals.
• Outer Hair Cells (OHCs): Approximately 12,000 OHCs amplify and refine the cochlear response, enhancing sensitivity and frequency discrimination.
• Supporting Cells: These cells provide structural support and maintain the ionic environment for hair cell function.
• Deiters’ Cells: These are supporting cells located within the Organ of Corti.
• Claudius’ Cells: These are supporting cells located at the edges of the Organ of Corti.

5. Stria Vascularis

This highly vascularized structure located on the lateral wall of the scala media is responsible for maintaining the unique ionic composition of the endolymph, particularly its high potassium concentration. It’s essential for hair cell function.

Detailed Breakdown of the Organ of Corti

Component	Function
Inner Hair Cells	Primary sensory receptors; transduce vibrations into neural signals.
Outer Hair Cells	Amplify and refine cochlear response; enhance sensitivity and frequency discrimination.
Tectorial Membrane	Deflects hair cells upon basilar membrane vibration.
Basilar Membrane	Supports the Organ of Corti; vibrates in response to sound.