Vestibular system | Anatomy2Medicine
vestibular system anatomy,

Vestibular system

 

  • Vestibular Pathways
  • Hair cells
  • Bipolar neurons of the vestibular ganglion
  • are located in the fundus of the internal auditory meatus.
  • project, via their peripheral processes, to hair cells.

 

        • project their central processes, as the vestibular nerve, to the
          • via the vestibular nuclei of the medulla and pons
          • via the juxtarestiform body to the flocculonodular lobe of the cerebellum (vestibulocerebellum).

 

  • Vestibular nuclei

 

        • include the inferior, medial, superior, and lateral nuclei.
        • Receive input from the following structures:
          • Bipolar neurons of the vestibular ganglion
          • Flocculonodular lobe and uvula of the cerebellum
          • Vermis of the anterior lobe of the cerebellum
          • Vestibular nuclei of the contralateral side
          • Fastigial nuclei of the cerebellum
        • Project fibers to the following structures:
          • Flocculonodular lobe and uvula of the cerebellum
          • Vestibular nuclei of the contralateral side

 

  • Inferior olivary nucleus

 

            • receives input via the vestibulo-olivary tract.
            • mediates vestibular influence to the caudal vermis of the cerebellum.

 

  • Abducent, trochlear, and oculomotor nuclei

 

            • receive input via the medial longitudinal fasciculus (MLF).

 

  • Ventral horn motor neurons

 

            • receive vestibular input from two descending pathways

 

  • MLF
  • contains fibers from the medial vestibular nucleus that terminate in cervical and upper thoracic levels.
  • coordinates head, neck, and eye movements.
  • Vestibulospinal tract

 

              • contains fibers from the ipsilateral lateral vestibular nucleus and is found at all spinal cord levels.
              • facilitates extensor muscle tone in the antigravity muscles, thus maintaining upright posture.
          • Ventral posteroinferior (VPI) and ventral posterolateral (VPL) nuclei of the thalamus

 

  • receive bilateral input from the vestibular nuclei.
  • VPI and VPL nuclei of the thalamus

 

        • project to the primary vestibular cortex of the parietal lobe

 

  • VPI to area 2v
  • VPL to area 3a
  • Efferent Vestibular Connections
  • arise from neurons found in the vestibular nuclei.

 

      • exit the brainstem with the vestibular nerve

 

  • innervate hair cells in the cristae ampullares and maculae of the utricle and saccule.
  • are thought to modulate the spontaneous firing rate of vestibular nerve fibers.
  • Medial Longitudinal Fasciculus

 

      • extends from the spinal cord to the rostral midbrain.

 

  • contains ascending vestibulo-ocular fibers to the ocular motor nuclei of cranial nerve (CN) III, CN IV, and CN VI.
  • contains a descending medial vestibulospinal tract that coordinates head and eye movements.

 

      • mediates adduction of the eyeball in lateral conjugate gaze on command.
      • mediates vestibular nystagmus.

 

  • Transection results in medial rectus palsy on attempted lateral gaze; convergence is unaffected.
  • Vestibulo-ocular Reflexes

 

    • may be tested in conscious or unconscious subjects by stimulating the kinetic labyrinth.

 

  • Doll’s head eye phenomenon (oculocephalic reflex)

 

        • is not present in normal alert people unless they voluntarily fix vision.
        • Test method
          • consists of rapid movement of the head in horizontal or vertical planes.
        • Test results

 

  • With intact proprioception and brainstem (vestibular nuclei), the eyes move conjugately in the opposite direction.
  • Doll’s head eye movements are absent or abnormal when lesions of the vestibular nuclei and MLFs are present.
  • Vestibular nystagmus

 

        • consists of involuntary to-and-fro, up-and-down, or rotary movements of one or both eyes.
        • consists of a slow component, opposite the direction of rotation

 

  • consists of a fast compensatory component, in the direction of rotation.

 

        • is named after the fast component.
        • results from the stimulation of hair cells within the semicircular ducts on
          • rotation or
          • after irrigation of the external auditory meatus with hot or cold water.

 

  • Postrotational nystagmus

 

        • Test method
          • The subject sits in a Bárány chair with head erect and inclined 30° forward (to place horizontal canals in the plane of rotation).

 

  • subject is rotated to the right 10 turns within 20 seconds and then is suddenly stopped.

 

        • Test results in normal subjects

 

  • The subject with normal labyrinths will have a horizontal nystagmus to the left (fast phase).

 

          • The subject will past-point (show dysmetria), and tend to fall to the right, and experience a sensation of turning (vertigo) to the left.
          • The induced nystagmus usually lasts 15 to 40 seconds.

 

  • Caloric nystagmus

 

        • may be induced with cold- or hot-water irrigation of the external auditory meatusmay be used to stimulate each labyrinth separately.

 

  • may be used to evaluate unconscious patients.

 

        • may be used to stimulate individual semicircular canals.
        • Test method
          • Princple- stimulate the horizontal semicircular canal

 

  • While sitting erect, the subject tilts the head back 60°, or the recumbent subject elevates the head 30° from a horizontal position.

 

          • Cold or hot water is syringed into the external ear canal.

 

  • Test results

 

          • in normal subjects – COWS Cold, Opposite; Warm, Same.
            • Cold-water irrigation results in nystagmus to the opposite side and past-pointing and falling to the same side.
            • Hot water irrigation results in the reverse reactions.
          • Test results in comatose subjects
            • No nystagmus is seen.
            • With the brainstem intact, the eyes deviate to the side of cold irrigation.

 

  • With bilateral MLF transection, the abducting eye deviates to the side of cold irrigation.

 

            • With lower brainstem damage to vestibular nuclei, the eyes do not deviate.

 

  • Decerebrate and Decorticate Rigidity

 

      • Descending vestibulospinal and pontoreticulospinal pathways play an important role in the control of extensor muscle tone.

 

  • Transection of the brainstem or decortication results in a tremendous increase in antigravity tone.

 

      • Decerebrate rigidity (posturing)

 

  • results from a lesion that transects the brainstem between the red nucleus and the vestibular nuclei.
  • results from the tonic activity of the pontine reticular formation and the lateral vestibular nucleus, which activate alpha and gamma motor neurons that innervate extensor muscles.

 

        • is characterized by opisthotonos, which is extension, adduction, and hyperpronation of the arms and extension of the feet with plantarflexion.
        • is also known as gamma rigidity in its classic form.
        • can be abolished by

 

  • section of the vestibular nerve

 

          • destruction of vestibular nuclei or the vestibulospinal tract
          • dorsal or ventral rhizotomy.
      • Decorticate rigidity (posturing)

 

  • usually results from lesions of the internal capsule or the cerebral hemisphere.
  • results in posture that consists of flexion of the arm, wrist, and fingers with adduction in the upper extremity; and with extension, internal rotation, and plantarflexion in the lower extremity.

 

      • is characterized by a motor pattern that is typical of chronic spastic hemiplegia.
      • is known as bilateral spastic hemiplegia, in the form of bilateral decorticate rigidity.