Auditory Pathways of The Central Nervous System

Ascending Auditory Pathway

Transmits sound from the ear —> Auditory Cortex

Cochlear Nucleus

First synapse for auditory nerve fibers

Starts binaural processing

Superior Olivary Complex

Located above the cochlear nucleus in the medulla

Key for sound localization

Input: Ipsilateral from Cochlear Nucleus and Contralateral from Trapezoid Body Nucleus

Output: Lateral Lemniscus -—> IC —> Midbrain

Medial Superior Olive

Processes low frequencies and interaural time differences

Localizes sound based on timing

Lateral Superior Olive

Processes high frequencies and interaural intensity differences

Localizes sound based on loudness

Medial NTB

Sends inhibitory signals to the LSO

Helps to sharpen intensity differences between sounds in two ears

Lateral Lemniscus

Tract of nerve fibers in the brainstem

Signals from the SOC and cochlear nucleus travel through here

Inferior Colliculus

Located in the midbrain

Integrates binaural information, sound localization, auditory reflexes

Sends information to the Medial Geniculate Body

Input: Bilateral from lateral lemniscus

Output: Medial Geniculate body of the Thalamus

Medial Geniculate Body

Thalamic relay station

Integrates sound information (pitch, loudness, timing) for perception

Input: Ipsilateral and Contralateral IC

Output: Ipsilateral Auditory Cortex

Auditory Cortex

Located in the superior temporal gyrus

Responsible for sound perception, processing, recognition, and integration

Input: Ipsilateral MGB

Output: Higher order cortical areas

Descending Auditory Pathway

Carries efferent (brain —> ear structures) signals to:

Functions:

• Regulates auditory sensitivity

• Support selective attention and noise suppression

• Protects the auditory system from loud noises

Corticofugal System

Origin: Primary auditory cortex

Modulates auditory input at brainstem and cochlear levels

Olivocochlear System

Origin: Superior olivary complex

Protects the auditory system from excessive sound exposure

Reduces perception of background noise

Maintains dynamic range of hearing

MOC Pathway

Sends efferent fibers to outer hair cells

Modulates cochlear amplification and reduces cochlear sensitivity (Provides inhibitory feedback)

Reduces OHC activity to protect against acoustic overstimulation

LOC Pathway

Fibers sent to inner hair cells and the auditory nerve

Modulates auditory nerve firing and enhances sound coding (Fine tunes signals)

Cochlear Implant

Bypasses hair cells to directly stimulate the auditory nerve

Electrode array placed into the scala tympani

Brainstem Implant

For those with auditory nerve damage

Stimulates brainstem pathways directly