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MIDBRAIN. forms a transition (and fiber conduit) to the cerebrum also contains a number of important cell groups, including several cranial nerve nuclei.

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forms a transition (and fiber conduit) to the cerebrum

  • also contains a number of important cell groups, including several cranial nerve nuclei.

The base of the midbrain contains the cruscerebri, a massive fiber bundle that includes corticospinal, corticobulbar, and corticopontine pathways

  • The base also contains the substantianigra
substantia nigra
  • Its cells contain neuromelanin and receives afferent fibers from the cerebral cortex and the striatum
  • it sends dopaminergic efferent fibers to the striatum
  • The substantianigra plays a key role in motor control. Degeneration of the substantianigra occurs in Parkinson's disease
the corticobulbar fibers
The corticobulbar fibers
  • from the motor cortex to interneurons of the efferent nuclei of cranial nerves
  • are homologous with the corticospinal fibers.
  • fibers to the lower portion of the facial nucleus and the hypoglossal nucleus are crossed (from the opposite cerebral cortex).
  • All other corticobulbar projections are bilaterally crossed (from both cortices).

The fibers of the oculomotor (III) nerve exit between the cerebral peduncles in the interpeduncular fossa.

  • The fibers of the trochlear (IV) nerve exit on the other side of the midbrain, the tegmentum
  • contains all the ascending tracts from the spinal cord or lower brain stem and many of the descending systems.
  • A large red nucleus receives crossed efferent fibers from the cerebellum and sends fibers to the thalamus and the contralateral spinal cord via the rubrospinal tract.
  • The red nucleus is an important component of motor coordination.

Two contiguous somatic efferent nuclear groups lie in the upper tegmentum

  • the trochlear nucleus (which forms contralateral nerve IV)
  • the oculomotor nuclei (which have efferent fibers in nerve III).
tectum roof
Tectum [‘roof’]
  • formed by two pairs of colliculi
  • The superior colliculi contain neurons that receive visual as well as other input and serve ocular reflexes
  • the inferior colliculi are involved in auditory reflexes and in determining the side on which a sound originates.

The inferior colliculi receive input from both ears, and they project to the medial geniculate nucleus of the thalamus by way of the inferiorbrachium.

  • The superior brachium links the lateral geniculate nucleus and the superior colliculus.

The colliculi contribute to the formation of the crossed tectospinal tracts

  • These are involved in blinking and head-turning reflexes after sudden sounds or visual images.
periaqueductal gray matter
Periaqueductal Gray Matter
  • Contains descending autonomic tracts as well as endorphin-producing cells that suppress pain.
  • This region has been used as the target for brain-stimulating implants in patients with chronic pain.
superior cerebellar peduncle
Superior Cerebellar Peduncle
  • Contains efferent fibers from the dentate nucleus of the cerebellum to the opposite red nucleus (the dentatorubrothalamic system) and the ventral spinocerebellar tracts.
  • The cerebellar fibers decussate just below the red nuclei.
weber s syndrome
Weber's syndrome
  • in the basal midbrain, involves nerve III and portions of the cerebral peduncle
  • There is a nerve III palsy on the side of the lesion and a contralateralhemiparesis (because the lesion is above the pyramidal decussation).
  • The arterial supply is by the posterior perforators and branches of the posterior cerebral artery
benedikt s syndrome
Benedikt's syndrome
  • situated in the tegmentum of the midbrain
  • may damage the medial lemniscus, the red nucleus, and nerve III and its nucleus and associated tracts
  • This area is supplied by perforators and branches of circumferential arteries.