Hearing 3: Internal Ear

Question 1

What are the components of the cochlea, and how is it organized?

Answer 1

Components:
Vestibular and auditory receptors (cochlea).
Organization:
Cochlea is a bony spiral tube with 2.5 turns around the modiolus.
Divided by the basilar membrane and Reissner’s membrane into:
Upper scala vestibuli.
Lower scala tympani.
Scala media.
Ends at the oval window (closed by the footplate of the stapes) and the round window (closed by a flexible 2nd tympanic membrane).
Both scala vestibuli and scala tympani contain perilymph and communicate at the apex of the cochlea.

Question 1

What is the composition of perilymph, and what is the endocochlear potential?

Answer 1

Perilymph Composition:
Similar to Extracellular Fluid (ECF), high in Na+ and low in K+.
Endocochlear (Endolymphatic) Potential:
Endolymph is +80 compared to perilymph.
Maintained by continuous transport of K+ into scala media by the stria vascularis.

Question 2

Describe the structure and function of the Organ of Corti.

Answer 2

Structure:
Located on top of the basilar membrane from the apex to the base of the cochlea.
Contains short hair cells (auditory receptors) surrounded by tall supporting cells.
Hair cells have stereocilia that pierce the reticular lamina.
Function:
Receptor for hearing.
Processes, amplifies, and transmits sound signals.

Question 3

Explain the types and functions of hair cells in the Organ of Corti.

Answer 3

Types:
Outer Hair Cells (OHCs): Improve hearing by affecting the vibration pattern of the basilar membrane.
Inner Hair Cells (IHCs): Primary sensory cells that generate action potentials in the auditory nerve.
Functions:
OHCs: Retrograde nervous mechanism for controlling ear sensitivity to different sound pitches.
Damage to OHCs results in marked hearing loss, while IHCs remain functional.

Question 4

What is the role of the Basilar Membrane in the auditory system?

Answer 4

Role:
Separates scala media from scala tympani.
Consists of 20,000 to 30,000 basilar fibers that vibrate like reeds.
Presbycusis results from a decrease in the ability to hear high frequencies due to changes in the basilar membrane.

Question 5

Describe the routes through which sound reaches the Organ of Corti.

Answer 5

Routes:
Ossicular Route (Ossicular Conduction): Main pathway for normal hearing.
Air Route (Air Conduction): Unimportant in normal hearing, becomes relevant after damage to the tympanic membrane and auditory ossicles.
Bone Route (Bone Conduction): Via vibration of the skull by loud sounds or vibrating objects.

Question 6

What is the mechanism of stimulation of the auditory nerve?

Answer 6

Propagation of sound wave to inner ear fluid.
Movement of the Organ of Corti:
Sound waves spread from the oval window.
Most sound energy is transferred directly from scala vestibuli to scala tympani, causing vibration of the basilar membrane and Organ of Corti.

Question 7

How is the receptor potential generated in hair cells, and what is the role of synaptic transmission in auditory signal processing?

Answer 7

Generation of Receptor Potential:

Hair cell depolarization occurs when stereocilia bend away from the limbus.
Potassium channels open, leading to hair cell depolarization.
Hair cell hyperpolarizes when stereocilia bend toward the limbus.

Release of Synaptic Transmitter:
Hair cell depolarization opens voltage-gated Ca2+ channels.
Entry of Ca2+ triggers the release of excitatory neurotransmitters (e.g., glutamate or aspartate).
Stimulates auditory nerve fibers.

Question 8

What is the importance of potassium recycling in hair cells?

Answer 8

Importance:
When stereocilia bend away from the limbus, K+ channels open, leading to hair cell depolarization.
When stereocilia bend toward the limbus, K+ channels close, resulting in hair cell hyperpolarization.
This mechanism sensitizes hair cells and enhances their ability to respond to even slight sounds.