Hearing 4: Auditory pathway

Question 1

What is the pathway of the 1st order neuron in the auditory system?

Answer 1

1st Order Neuron:
Cochlear afferent fibers relay in the cochlear nuclei in the medulla.

Question 2

Describe the pathway of the 2nd order neuron in the auditory system.

Answer 2

2nd Order Neuron:
Cochlear nuclei → Superior olivary nucleus on both sides (mainly opposite side).
Further relay in the lateral lemniscus.
Ascends to the inferior colliculus (center for auditory reflexes).
Projects to the medial geniculate body in the thalamus.

Question 3

Explain the course of the 3rd order neuron in the auditory pathway.

Answer 3

3rd Order Neuron:
From the medial geniculate body, fibers form the auditory radiation.
Projects to the auditory cortex, mainly located in the superior temporal gyrus.
Signals from both ears are transmitted to both sides, mainly the opposite side.
Collaterals pass into the RAS (reticular activating system), activating the nervous system in response to loud sounds.
Maintains spatial orientation from cochlea to cortex.

Question 4

Define the primary and secondary auditory cortex.

Answer 4

A. Primary Auditory Cortex:
Directly excited from the medial geniculate body.
B. Auditory Association Cortex (2nd Auditory Cortex):
Excited secondarily by impulses from the primary auditory cortex.

Question 5

What functions are associated with the auditory cortex?

Answer 5

Auditory cortex is concerned with:
Perception of sound and the psychic sensation of sound pitch.
Detection of the direction of sound.
Recognition of sequential patterns of sound during speaking or music listening.
Centrifugal fibers from the superior olivary nucleus and auditory cortex to the organ of Corti are inhibitory, allowing direct attention to specific sounds and rejecting other sound qualities.

Question 6

Compare the effects of bilateral and unilateral destruction of the auditory cortex.

Answer 6

Bilateral Destruction:
Greatly decreases sensitivity for hearing.
Does not cause complete deafness.
Inability to recognize tonal patterns of sound and localization.
Unilateral Destruction:
Slightly decreases hearing in the opposite ear.
No deafness due to many crossover connections in the auditory pathway.
Lesions in Auditory Association Areas:
Inability to understand or interpret the meaning of sound.