MIDBRAIN

1. MIDBRAIN

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

3. 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

4. substantianigra • 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

5. The external aspect of the basis of the midbrain is called the cerebral peduncle.

6. Caudal midbrain- level of inferior colliculus

7. 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).

8. 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

12. 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.

13. 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).

15. Rostral midbrain- level of superior colliculus

16. 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.

18. 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.

19. 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.

20. 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.

21. 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.

22. 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

23. Weber syndrome

25. 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.

26. Benedikt's syndrome

27. Blockage of cerebral aqueduct