The auditory pathway of the brain begins with the external auditory canal and includes the middle/inner ear and eventually the brainstem nuclei before sending final signals to the primary auditory cortex in the temporal lobe.
- Peripheral components:
- Outer ear:
- Auditory canal and tympanic membrane
- Transfers sound waves
- Middle ear:
- Airspace with 3 ossicles (malleus, incus, stapes)
- Conducts and amplifies sound
- Inner ear:
- Fluid-filled cochlea
- Tonotopy of basilar membrane: low frequency heard at apex and high frequency heard at base
- Vibration transduced via hair cells → auditory nerve signaling (CN VIII) → brainstem
- Outer ear:
- Central components:
- Consist of the organ of Corti and brainstem auditory nuclei
- Afferent fibers convey information from the organ of Corti → auditory nuclei in the brainstem:
- Cochlear nuclei: located at the dorsolateral side of the brainstem at the pontomedullary junction
- Superior olivary nuclei: located in the pons
- Lateral lemniscal nuclei: tract of axons in the brainstem carrying auditory information to the inferior colliculus of the midbrain
- Inferior colliculus: located in the midbrain
- Medial geniculate body of the thalamus: part of thalamic relay system
- Auditory cortex: located in the anterior and posterior transverse temporal areas
Circuitry of pathway
Outer ear → inner ear → depolarization of cochlear hair cells in the cochlea → both the ipsilateral and contralateral superior olivary nucleus → lateral lemniscus → inferior colliculus → medial geniculate bodies of the thalamus → auditory cortex of temporal lobe
The vestibular pathway of the brain begins with the utricle and saccule, with additional input from the semicircular canals. Information eventually reaches the brainstem nuclei before sending final signals to the thalamus and cerebellum.
- Monitors the orientation of the body with respect to gravity and stimulates the vestibulospinal tracts to elicit compensatory movements
- Head position relative to gravity is sensed by the 2 otolith organs, the utricle and saccule:
- Utricle: oriented horizontally
- Saccule: oriented vertically
- Angular acceleration of the head is sensed by the 3 semicircular canals.
- Hair cells in the utricle, saccule, and semicircular canals are displaced based on their position relative to gravity, leading to depolarization and stimulation of the vestibular portion of CN VIII.
- Afferent fibers of the vestibular apparatus convey information to 4 pairs of vestibular nuclei (superior, lateral, inferior, and medial) in the medulla and pons.
- Vestibular nuclei inputs:
- 1st-order afferents in vestibular ganglion
- Cerebellum: flocculonodular lobe
- Vestibular nuclei outputs:
- Oculomotor, trochlear, and abducens nuclei via medial longitudinal fasciculus (MLF)
- Cerebellum: flocculonodular lobe via inferior cerebellar peduncle
- Descending motor tract of lateral vestibulospinal tract
- Descending motor tract of medial vestibulospinal tract
- Vestibular nuclei inputs:
Vestibular pathway outputs
|Cranial nerve nuclei||Control over eye movements|
|Thalamus||Conscious perception of movement and gravity through connections to cortex|
|Cerebellum (flocculonodular lobe)||Coordination of postural adjustments|
|Lateral vestibulospinal tract||Walking upright|
|Medial vestibulospinal tract||Assisting in integration of head and eye movements|
- Acoustic neuroma: an acoustic neuroma is a benign tumor of Schwann cells that involves the cranial nerves within the cranium. Acoustic neuromas most frequently affect CV VIII but can also impact CN VII due to the location in the cerebellopontine angle. Acoustic neuromas often present with hearing loss and tinnitus. Treatment is with surgical removal.
- Hearing loss: hearing impairments are classified into conductive hearing loss and sensorineural hearing loss. Conductive hearing loss comes about when there is a problem transferring sound waves anywhere along the pathway from the outer ear, the tympanic membrane, or middle ear. In cases of sensorineural hearing loss, there is an error in the transmission of auditory stimuli from the cochlea to the auditory nuclei.
- Vertigo: the sensation of movement between oneself and the surroundings when no movement is actually occurring. Vertigo is not limited to a feeling of rotation (spinning); other forms include upward lifting, swaying, rocking, and unsystematic movement. Vertigo most often occurs due to problems within the semicircular canals.
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