Download 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
 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.
 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).
 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 headturning reflexes after sudden sounds or visual
images.
 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.
 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.
 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
Site of lesion
Ventral part of cerebral
peduncle, including
axons of oculomotor
nerve and descending
motor fibers in the basis
pedunculi
Clinical features
Ipsilateral oculomotor
nerve palsy with
contralateral hemiplegia
or hemiparesis
 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.
Site of lesion
Cerebral 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
Clinical features
Ipsilateral oculomotor nerve
palsy with contralateral
hemiparesis and tremor