Auditory System | Anatomy2Medicine
auditory system

Auditory System

Auditory System

 

  • The Auditory System

 

      • is an exteroceptive special somatic afferent (SSA) system.
      • detects sound frequencies from 20 Hz to 20,000 Hz.
      • ordinary conversation ranges between 300 and 3000 Hz.
      • functions over an intensity range of 120 decibels (dB)
      • it can discriminate changes in intensity between 1 dB and 2 dB.
      • is characterized by tonotopic (pitch) localization at all levels of the neuraxis.
      • Presbyacusis – there is a loss of high-frequency tones with advanced age.

 

  • Auditory Pathway
  • Hair cells of the organ of Corti

 

        • are innervated by peripheral processes of bipolar cells of the spiral ganglion.
        • consists of two types

 

  • Inner hair cells
  • synapse with numerous afferent fibers, each of which makes contact with only one hair cell

 

            • the majority of fibers in the cochlear nerve come from the inner hair cells.

 

  • Outer hair cells

 

            • synapse with afferent fibers that contact numerous other outer hair cells
            • outnumber the inner hair cells three to one.

 

  • Bipolar cells of the spiral (cochlear) ganglion

 

        • project peripherally to hair cells of the organ of Corti.

 

  • project centrally as the cochlear nerve to the dorsal and ventral cochlear nuclei of the medullopontine junction
  • Cochlear nerve (CN VIII)

 

        • extends from the spiral ganglion to the cerebellopontine angle, where it enters the brainstem.

 

  • Cochlear nuclei

 

        • are the only auditory nuclei that do not receive binaural input
        • Damage results in unilateral deafness.

 

  • Dorsal cochlear nucleus
  • underlies the acoustic tubercle of the floor of the fourth ventricle

 

        • receives input from the cochlear nerve (CN VIII).
        • projects contralaterally to the lateral lemniscus.

 

  • Ventral cochlear nucleus
  • receives input from the cochlear nerve (CN VIII).

 

        • projects bilaterally to the superior olivary nuclei.
        • projects contralaterally to the lateral lemniscus.
        • gives rise to the trapezoid body (ventral acoustic striae).

 

  • Superior olivary nucleus
  • is located in the pons at the level of the facial nucleus.

 

        • receives input from ventral cochlear nuclei.
        • projects bilaterally to the lateral lemniscus.
        • plays a role in sound localization and binaural processing.

 

  • gives rise to the efferent olivocochlear bundle, a cochlear feedback pathway.
  • Trapezoid body

 

        • is located in the caudal pontine tegmentum at the level of the abducent nucleus
        • is transversed by intra-axial abducent fibers of CN VI.

 

  • contains decussating fibers from the ventral cochlear nucleus.
  • Lateral lemniscus

 

        • receives input from the contralateral cochlear nuclei.
        • receives input from the superior olivary nuclei
        • is connected to the contralateral lateral lemniscus via commissural fibers
        • projects to the nucleus of the inferior colliculus.

 

  • Nucleus of the inferior colliculus

 

        • receives input from the lateral lemniscus.
        • projects via the brachium of the inferior colliculus to the medial geniculate body projects to the superior colliculus to mediate audiovisual reflexes.

 

  • Medial geniculate body

 

        • receives input from the nucleus of the inferior colliculus.
        • projects via the auditory radiation to the primary auditory cortex, the transverse gyri of Heschl (areas 41 and 42).
        • projects to the amygdala.

 

  • Auditory radiation

 

        • extends from the medial geniculate body via the posterior limb of the internal capsule to the transverse gyri of Heschl.

 

  • Transverse temporal gyri of Heschl

 

        • contain the primary auditory cortex (areas 41 and 42)
        • receive auditory input via the auditory radiation.
        • project to the auditory association cortex (area 22).

 

  • Efferent Cochlear (Olivocochlear) Bundle

 

    • is a crossed and uncrossed tract
    • arises from the superior olivary nucleus
    • projects to the hair cells of the organ of Corti.
    • suppresses auditory nerve activity when stimulated.
    • plays a role, through inhibition, in “auditory sharpening.”