Neuro Lec 19 Auditory

Human Hearing Range

20 Hz to 20,000 Hz (20 kHz).

What is ultrasound?

Sound waves above the human hearing range (>20,000 Hz), used in medical imaging.

What is infrasound?

Sound waves below the human hearing range (<20 Hz), often felt as vibrations.

Anatomy of the Middle Ear

Components: Contains the tympanic cavity, ossicles (malleus, incus, stapes), and middle ear muscles (tensor tympani and stapedius).

Ossicles' Function: Transmit sound vibrations from the eardrum to the inner ear fluid via the oval window, amplifying sound.

Muscles' Function: Protect the inner ear by dampening loud sounds (acoustic reflex).

Endolymph

Location: Found within the membranous labyrinth of the cochlea, especially the scala media, as well as in vestibular structures.

Ion Composition: High in potassium (K+), unlike other body fluids, which typically have higher sodium (Na+).

Tip Links and Potassium Channels

Tip Link: Thin, filament-like structures connecting stereocilia on hair cells.
Function: Regulates potassium channels by opening them when the hair cell cilia bend, leading to depolarization and initiation of nerve impulses.

Function: Regulates potassium channels by opening them when the hair cell cilia bend, leading to depolarization and initiation of nerve impulses.

Motor Proteins in Outer Hair Cells

Function: Use motor proteins (e.g., prestin) to change cell length in response to sound vibrations.

Purpose: Amplifies sound by increasing the sensitivity and frequency selectivity of the cochlear response.

Cranial Nerve VIII and Hair Cells

Focus: Auditory component (cochlear nerve) of cranial nerve VIII.

Connections: Primarily connects to inner hair cells, which are the main transmitters of sound information to the brain.

Binaural Processing in Auditory Pathway

Definition: When auditory information is processed from both ears.

Occurrence: Begins at the superior olivary complex, allowing sound localization and binaural integration as the signal travels to the cortex.

Tonotopic Map

Definition: Organization of auditory neurons based on sound frequency.

Explanation: Neurons in the cochlea and brain are arranged by frequency, with high frequencies detected at the base of the cochlea and low frequencies at the apex.

What causes Conduction Hearing Loss?

Problems in the outer or middle ear, such as earwax blockage or otosclerosis.

What is Sensorineural Hearing Loss?

Hearing loss that results from damage to inner ear structures or the auditory nerve.

What structures are affected in Sensorineural Hearing Loss?

• Cochlea – Damage to hair cells or cochlear nerve.

• Auditory Nerve – Damage to the nerve fibers transmitting sound.

• Auditory Pathways – Brainstem or cortical processing centers (rare cases).

Tinnitus and Otoacoustic Emissions for Hearing Loss

Tinnitus: Ringing or buzzing in the ears, often a sign of hair cell damage or dysfunction.

Otoacoustic Emissions: Sounds generated by the cochlea in response to stimulation; testing emissions can help detect hearing loss early.

Cochlear Implants and Tonotopic Map

Function: Cochlear implants use electrodes to stimulate different parts of the cochlea, directly engaging the tonotopic map.

Advantage: Provides a frequency-specific simulation of sound by stimulating appropriate regions, allowing for more natural sound perception in hearing-impaired individuals.