Auditory Pathway

Ear receptors and Eighth cranial nerve (Organ of Corti)
  • Cochlear hair cells are located on basilar membrane (In Organ of Corti):
  1. At the base of cochlea (thinner basilar membrane): encodes high-frequency sounds
  2. At the apex of cochlea (thicker basilar membrane): encodes low-frequency sounds.
  3. Sound waves entering scala vestibuli and back to scala tympani causes vibration of basilar membrane.
  • Vibration of basilar membrane leads to movement of sterocilia of hair cells against the tectorial membrane that leads to generation of action potential and production of electrical impulses that travel down the CN VIII (cochlear division).
  • Axons of spiral ganglion bipolar axons form the cochlear nerve.
Cochlear Nuclei (Pontomedullary junction)
  • Cochlear nuclei is divided into 2 divisions: Ventral cochlear nuclei and Dorsal cochelar nuclei in relation to the inferior cerebellar peduncle in Ponto-medullary junction.
  • Anterior (ventral) cochlear nucleus is dedicated to low frequency (Apical) sounds.
  • Posterior (dorsal) cochlear nucleus is dedicated to high frequency (Basal) sounds.
  • Anterior cochlear nucleus → Anterior acoustic stria → Bilateral superior olivary nucleus (Binaural sound localization pathway)
  • Crossing over of anterior acoustic stria due to bilateral projection form a trapezoidal shape (trapezoid body).
  • Posterior cochlear nucleus → Posterior acoustic stria → Contralateral lateral lemniscus and inferior colliculus (Monoaural sound localization pathway)
Skips superior olivary nucleus.
  • Superior Olivary Nuclei (Pons)
  • Located in pons
  • 1st auditory nuclei to reveive binaural input and use the binaural input to localize sound forces.
Lateral Lemniscus (Midbrain)
  • Projection fibers from Ipsilateral Superior olivary nucleus.
Inferior Colliculus (Midbrain)
  • Receives fibers from ipsilateral superior olivary nuclei through lateral lemniscus.
Medial Geniculate Body (Thalamus)
  • Receives projections from ipsilateral inferior colliculus via brachium of inferior colliculus.
Auditory Cortex
  • Auditory radiations from MGB to respective transverse gyri of Heschl (Brodmann areas 41 and 42), deep to the superior
  • temporal gyrus in the Sylvian fissure via sublentiform part of internal capsule.
  • The auditory association cortex surrounds the primary auditory cortex.
Clinical Relevance

Since, the projection is bilateral to the superior olivary nucleus –
  • Lesions of cochlear part of CN VIII or cochlear nuclei at pontomedullary junction cause profound unilateral sensory hearing loss.
  • All other lesions to the auditory pathway above the cochlear nuclei cause bilateral suppression of hearing and decreased ability to localize a sound source.
The first relay of the primary auditory pathway occurs in the cochlear nuclei in the brain stem, which receive Type I spiral ganglion axons (auditory nerve); at this level an important decoding of the basic signal occurs: duration, intensity and frequency.
The second major relay in the brain stem is in the superior olivary complex: the majority of the auditory fibres synapse there having already crossed the midline.
Leaving this relay, a third neuron carries the mesage up to the level of the superior colliculus (mesencephalus ). These two relays play an essential rĂ´le in the localisation of sound.
A last relay, before the cortex, occurs in the medial geniculate body (thalamus ); it's here that an important integration occurs: preparation of a motor response (eg vocal response).
The final neuron of the primary auditory pathway links the thalamus to the auditory cortex, where the message, already largely decoded during its passage through the previous neurons in the pathway, is recognized, memorized and perhaps integrated into a voluntary response.
In human, the primary auditory cortex (3) is located in the temporal area (2) within the lateral sulcus (1).

Exam Question
  • Medial geniculate body is related to Hearing.
  • Damage to one of the auditory cortex causes no noticable deficit at all as there is overlap of cortex.
  • Higher auditory centre determines Sound localization.
  • Cranial nerve VIII lesion affects Position sense and equilibrium.
  • Auditory cortex is present in area Area 41.
  • Auditory transmission is via Lateral lemniscus.
  • Genu of internal capsule is not associated with the Auditory pathway.
  • Trapezoid body,Medial geniculate body ,Lateral leminiscus are concerned with the Auditory pathway.
  • Primary auditory area is Superior temporal gyrus.
  • Auditory processing of spoken language is done by Temporal lobe

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