The Auditory Projection Pathway

A spiral ganglion, wound around the modiolus, is composed of bipolar sensory neurons. Their dendrites originate at the hair cells and their axons form the cochlear nerve. The cochlear nerve joins the vestibular nerve, discussed later, and the two together become the vestibulo-cochlear nerve (cranial nerve VIII).

The cochlear nerve fibers from each ear lead to cochlear nuclei on both sides of the pons. There, they synapse with second-order neurons that ascend to the nearby superior olivary nucleus of the pons (fig. 16.17). By way of cranial nerve VIII, the superior olivary nucleus

Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition

16. Sense Organs

Text

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Primary auditory cortex

Temporal lobe of cerebrum

Medial^ geniculate nucleus of thalamus

Inferior colliculus of midbrain

Cochlea

Cranial nerve VIII

Cochlear tuning

- Superior olivary nucleus of pons

Chapter 16 Sense Organs 605

Auditory ^reflex (head turning)

- Neck muscles

Cranial nerves V3 and VII

Tensor tympani and stapedius muscles

Tympanic reflex

Vestibulocochlear nerve

Cochlear nuclei of pons

Tensor tympani and stapedius muscles

Cochlea (b)

Cochlear Nucleus

Cochlea (b)

Primary auditory cortex

Thalamus

Inferior colliculus

Superior olivary nucleus

Cochlear nucleus

Medulla oblongata

Figure 16.17 Auditory Pathways in the Brain. (a) Schematic. (b) Brainstem and frontal section of the cerebrum, showing the locations of auditory processing centers.

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Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

606 Part Three Integration and Control issues the efferent fibers back to the cochlea that are involved in cochlear tuning. By way of cranial nerves V3 and VII, it issues motor fibers to the tensor tympani and stapedius muscles, respectively. The superior olivary nucleus also functions in binaural31 hearing—comparing signals from the right and left ears to identify the direction from which a sound is coming.

Other fibers from the cochlear nuclei ascend to the inferior colliculi of the midbrain. The inferior colliculi help to locate the origin of a sound in space, process fluctuations in pitch that are important for such purposes as understanding another person's speech, and mediate the startle response and rapid head turning that occur in reaction to loud or sudden noises.

Third-order neurons begin in the inferior colliculi and lead to the thalamus. Fourth-order neurons begin here and complete the pathway to the primary auditory cortex, which is in the superior margin of the temporal lobe deep within the lateral sulcus (see photo on p. 585). The temporal lobe is the site of conscious perception of sound, and it completes the information processing essential to bin-aural hearing. Because of extensive decussation in the auditory pathway, damage to the right or left auditory cortex does not cause a unilateral loss of hearing.

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Responses

  • liliana
    What are the Projection Pathway layers for hearing and equilibrium?
    5 months ago
  • Tesmi
    Where does decussation occur in auditory pathway?
    3 months ago

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