MIDBRAIN

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 crus cerebri, a massive fiber bundle that includes corticospinal, corticobulbar, and corticopontine pathways

The base also contains the substantia nigra

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 substantia nigra plays a key role in motor control. Degeneration of the substantia nigra occurs in Parkinson's disease

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

CAUDAL MIDBRAIN- LEVEL OF INFERIOR COLLICULUS

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

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).

ROSTRAL MIDBRAIN- LEVEL OF SUPERIOR COLLICULUS

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

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 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 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 contralateral hemiparesis (because the lesion is above the pyramidal decussation).

The arterial supply is by the posterior perforators and branches of the posterior cerebral artery

WEBER SYNDROMESite of lesion Clinical featuresVentral part of cerebral peduncle, including axons of oculomotor nerve and descending motor fibers in the basis pedunculi

Ipsilateral oculomotor nerve palsy with contralateral hemiplegia or hemiparesis

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.

BENEDIKT'S SYNDROME Site of lesion Clinical featuresCerebral peduncle, with oculomotor axons and descending motor fibers and extending dorsally to include the red nucleus and fibers from the contralateral side of the cerebellum

Ipsilateral oculomotor nerve palsy with contralateral hemiparesis and tremor

BLOCKAGE OF CEREBRAL AQUEDUCT