Lecture Notes: Hearing and Sound Transmission

Overview

• Hearing is the physiological process of detecting, transmitting, and interpreting sound waves by the brain.
• Begins with sound waves entering the outer ear and traveling through the external auditory canal to the tympanic membrane, causing vibrations.
• Vibrations are transmitted through the middle ear via the ossicles to the inner ear.
• Inner ear: Vibrations cause cochlear fluid to move, stimulating hair cells along the cochlear membrane.
• Hair cells convert mechanical energy to electrical signals, transmitted via auditory nerve to brainstem and auditory cortex.

Sound Wave Characteristics

• Frequency: Determines pitch; higher frequency = higher pitch.
• Amplitude: Determines loudness; measured in decibels (dB).

Pathway of Sound Waves

1. Outer Ear
   • Sound directed by auricle into external auditory canal.
   • Sound waves hit the tympanic membrane, causing it to vibrate.
2. Middle Ear
   • Vibrations transmitted to the ossicles (malleus, incus, stapes).
   • Ossicles amplify vibrations, making them 20x more vigorous at oval window.
3. Inner Ear
   • Stapes at oval window creates fluid pressure in cochlear perilymph.
   • Pressure waves in perilymph cause movement of vestibular membrane.
   • Endolymph movement within cochlear duct causes basilar membrane to move.
   • Hair cells on basilar membrane bend against tectorial membrane, generating receptor potential.

Cochlea Structure

• Scala Vestibuli: Top chamber filled with perilymph.
• Scala Tympani: Bottom chamber filled with perilymph; continuous with Scala Vestibuli.
• Cochlear Duct: Middle channel containing endolymph.

Organ of Corti

• Located on basilar membrane, serves as the receptor for hearing.
• Hair cells (stereocilia) move against tectorial membrane, generating receptor potentials.

Signal Transmission

• Generated potentials from hair cells synapse with first-order sensory neurons.
• Impulses travel via the cochlear and vestibular branches of cranial nerve VIII (Vestibulocochlear nerve).
• Vestibular branch: Carries equilibrium information from semicircular canals and vestibule.
• Cochlear branch: Carries auditory information from the Organ of Corti.
• Combined nerve signals are sent to various brain regions for processing.

Summary

• Sound waves enter through the auricle and cause vibrations in the tympanic membrane.
• Ossicles amplify these vibrations; stapes transfers them to cochlear fluid.
• Movement of fluid leads to membrane movements, bending hair cells in the Organ of Corti.
• Bending generates receptor potentials which get transmitted to the brain via nerve VIII.
• Vestibular branch carries equilibrium signals; cochlear branch handles auditory signals.