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Chapter 13: Auditory System: Pathways and Reflexes

Lincoln Gray, Ph.D., Department of Communication Sciences and Disorders, James Madison University


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13.1 Connections in the Central Auditory System

Cochlear Nucleus, Superior Olive, Lateral Lemniscus, Inferior Colliculus, Medical Geniculate, Superior Temporal Gyrus

Connections in the central auditory system are complex, but a simple summary is that information proceeds from the Organ of Corti to spiral ganglion cells and the VIIIth nerve afferents in the ear, to the cochlear nuclei, many crossing in the trapezoid body to the superior olive in the brain stem. Then all ascending fibers stop in the inferior colliculus in the midbrain and the medial geniculate body in the thalamus, before reaching the cortex in the superior temporal gyrus. All auditory afferents synapse in the cochlear nuclei and in the thalamus. Beyond that simplification, second order fibers from the cochlear nuclei proceed rostrally in several different pathways. Afferents are generally distributed bilaterally so unilateral damage at any level does not usually result in deafness in either ear.

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Figure 13.1
Fast-acting auditory afferents through dorsal cochlear nucleus. Press PLAY to activate the animation. Then, click on the cochlea and text to gain further information.

Figure 13.1 shows a fiber traveling somewhat directly from the cochlea to the cortex. This is a fast acting system. These fibers synapse in the dorsal cochlear nucleus, and may function as a general warning (as when you might jump from a loud sound). These fibers decussate and ascend in the lateral lemniscus to the inferior colliculus.

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Figure 13.2
Ascending pathway for most auditory afferents. Press PLAY to activate the animation. Then, click on the cochlea and text to gain further information.

Figure 13.2 shows the more numerous connections that work their way rostrally through a more detailed pathway. This slow acting system involves much more processing and may provide more detailed information about the sound, such as its location. These fibers synapse in the ventral cochlear nucleus. Fibers from the ventral cochlear nucleus synapse in the ipsilateral and contralateral superior olivary nucleus. Some fibers from the ventral cochlear nucleus cross the midline in the trapezoid body. Thus, cells in the superior olive receive inputs from both ears and are the first place in the central auditory system where binaural processing (stereo hearing) is possible. The output of the superior olive travels in the lateral lemniscus. Some nuclei within the lateral lemniscus further process the sound. Most of these afferents synapse in the inferior colliculus. All afferents then synapse in the medial geniculate body of the thalamus. Thalamic afferents reach the superior temporal gyrus through the sub-lenticular portion of the internal capsule.

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Figure 13.3
Binaural auditory afferents. Press PLAY to activate the animation. Then, click on the cochlea and text to gain further information.

Figure 13.3 shows the same detail processing system as in Figure 13.2, only now with the more realistic situation of input from both ears. The two different patterns of dashed lines combine to form a solid line above the superior olive, meant to indicate the combination of monaural inputs into bilateral and binaural activation.

Primary auditory cortex, or Herschel’s gyrus in insular cortex, is tonotopically organized. Afferents from this longitudinal strip on the superior temporal gyrus diverge to a wide variety of other cortical processing areas, including Wernicke’s area in the parietal lobe where speech is processed.

Auditory afferents are tonotopically organized from the ear to the cortex. This starts with high frequencies transduced at the base of the cochlea, and low frequencies transduced at the apex (see Figure 12.7). Click on the cochlea in Figure 13.3 to see the color code of pitch, as if the cochlea were a piano. Low frequency fibers then pass in the central core of the VIIth nerve surrounded by high frequency fibers (see Auditory System: Structure and Function). This segregation of high and low frequencies persists throughout the CNS. As seen in Figure 13.3, low frequencies are more lateral in primary auditory cortex.

The video below by Sarah Baum, Heather Turner, Nadeeka Dias, Deepna Thakkar, Natalie Sirisaengtaksin and Jonathan Flynn further explains the structures, functions and pathways of the auditory system in "The Journey of Sound".

 

 

Test Your Knowledge

  • Question 1
  • A
  • B
  • C
  • D
  • E

Afferent activity in the central auditory system generally proceeds through the following areas. Which is THIRD in the sequence?

A. cochlear nucleus

B. inferior colliculus

C. medial geniculate

D. superior olivary nucleus

E. superior temporal gyrus

Afferent activity in the central auditory system generally proceeds through the following areas. Which is THIRD in the sequence?

A. cochlear nucleus This answer is INCORRECT.

This is first in the sequence. VIIIth nerve afferents synapse in either the dorsal or ventral cochlear nuclei.

B. inferior colliculus

C. medial geniculate

D. superior olivary nucleus

E. superior temporal gyrus

Afferent activity in the central auditory system generally proceeds through the following areas. Which is THIRD in the sequence?

A. cochlear nucleus

B. inferior colliculus This answer is CORRECT!

All auditory afferents synapse in this midbrain structure.

C. medial geniculate

D. superior olivary nucleus

E. superior temporal gyrus

Afferent activity in the central auditory system generally proceeds through the following areas. Which is THIRD in the sequence?

A. cochlear nucleus

B. inferior colliculus

C. medial geniculate This answer is INCORRECT.

This is fourth in the sequence. All sensory afferents (except olfactory) synapse in the thalamus.

D. superior olivary nucleus

E. superior temporal gyrus

Afferent activity in the central auditory system generally proceeds through the following areas. Which is THIRD in the sequence?

A. cochlear nucleus

B. inferior colliculus

C. medial geniculate

D. superior olivary nucleus This answer is INCORRECT.

This is second in the sequence. The superior olive receives bilateral inputs from the cochlear nuclei.

E. superior temporal gyrus

Afferent activity in the central auditory system generally proceeds through the following areas. Which is THIRD in the sequence?

A. cochlear nucleus

B. inferior colliculus

C. medial geniculate

D. superior olivary nucleus

E. superior temporal gyrus This answer is INCORRECT.

This is fifth in the sequence. This is primary auditory cortex.

 

 

 

 

 

 

 

 

  • Question 2
  • A
  • B
  • C
  • D
  • E

Auditory afferents travel through the midbrain in the

A. medial lemniscus

B. lateral lemniscus

C. medial longitudinal fasciculus

D. trapezoid body

E. sublenticular portion of the internal capsule

Auditory afferents travel through the midbrain in the

A. medial lemniscus This answer is INCORRECT.

The medial lemniscus carries somatosensory afferents.

B. lateral lemniscus

C. medial longitudinal fasciculus

D. trapezoid body

E. sublenticular portion of the internal capsule

Auditory afferents travel through the midbrain in the

A. medial lemniscus

B. lateral lemniscus This answer is CORRECT!

C. medial longitudinal fasciculus

D. trapezoid body

E. sublenticular portion of the internal capsule

Auditory afferents travel through the midbrain in the

A. medial lemniscus

B. lateral lemniscus

C. medial longitudinal fasciculus This answer is INCORRECT.

The medial longitudinal fasciculus carries vestibular interconnections (among other connections).

D. trapezoid body

E. sublenticular portion of the internal capsule

Auditory afferents travel through the midbrain in the

A. medial lemniscus

B. lateral lemniscus

C. medial longitudinal fasciculus

D. trapezoid body This answer is INCORRECT.

Close, but incorrect. Axons from the cochlear nucleus decussate to innervate the contralateral superior olive in the trapezoid body, but this structure is in the pons.

E. sublenticular portion of the internal capsule

Auditory afferents travel through the midbrain in the

A. medial lemniscus

B. lateral lemniscus

C. medial longitudinal fasciculus

D. trapezoid body

E. sublenticular portion of the internal capsule This answer is INCORRECT.

Close, but incorrect. Auditory afferents from the medial geniculate body travel to primary auditory cortex in the sublenticular portion of the internal capsule, but this structure is in the forebrain.

 

 

 

 

 

 

 

 

 

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