Brainstem 3
Midbrain
Dr Rania Gabr
Objectives
• Identify the gross features of the brainstem.
• Briefly describe the internal structure of the brainstems
(ascending and descending pathways, sensory and motor
cranial nuclei, substantia nigra, red nucleus, olivary nucleus
and reticular formation).
• Describe the main connections of the sensory cranial
nuclei.
• Describe the main connections of the motor cranial nuclei.
• Review the blood supply of the brainstem.
• Describe lesions in the brainstem such as medial medullary
syndrome and lateral medullary syndrome.
• Describe the main connections of the substantia nigra and
the red nucleus.
Midbrain
Shortest part of the brain stem,
not more than 2cm in length, lies
in the posterior cranial Fossa.
Crus cerebri
For descriptive purposes, divided
into:
Dorsal tectum and right and left
Cerebral Peduncles.
-Cerebral peduncles contains:
1-Descending fibers that go to
the cerebellum via the pons
2-Descending pyramidal tracts
-Running through the midbrain is
the hollow cerebral aqueduct
which connects the 3rd and 4th
ventricles of the brain.
Each cerebral peduncle
divides further into ventral
crus cerebri (massive fibrous
mass) and a dorsal
Tegmentum by a pigmented
lamina
“ Substantia nigra”
MID BRAIN – VENTRAL
SURFACE
 Large column of
descending fibers (crus
cerebri or basis pedunculi),
on either side, separated by
a depression called the
interpeduncular fossa with
posterior perforated
substance.
 Nerve emerging from
Midbrain (one):
• Occulomotor (3rd): from
medial aspect of crus
cerebri.
Dorsal surface-External Features
• On the posterior surface, we find:
• Superior and inferior colliculi
• Trochlear nerve emerges below the
inferior colliculus
• Superior and inferior brachii
• Superior medullary velum
• Superior brachium: a
ridge of white matter
carrying visual
information from the
superior colliculus to
the optic tract
• Inferior brachium: a
ridge of white matter
passing anteriorly
from each inferior
colliculus to the
medial geniculate
body of the thalamus
MID BRAIN – DORSAL SURFACE
 Marked by 4 elevations:
1. Two superior colliculi:
concerned with visual
reflexes.
2. Two inferior colliculi: forms
part of auditory pathway.
 Nerve emerging from
Midbrain (one):
• Trochlear (4th): just caudal
to inferior colliculus (The
only cranial nerve emerging
from dorsal surface of the
whole brain stem).
INFERIOR COLLICULUS Level
• Inferior colliculus is a
large nucleus of gray
matter that lies beneath a
corresponding surface
elevation.
• It is part of the auditory
pathway.
• It receives fibers from the
lateral lemniscus.
• Its efferent fibers pass to
the thalamus
1. Trochlear nucleus:
lies in the central gray
matter close to the
median plane just
posterior to the medial
longitudinal bundle.
The fibers of the
trochlear nerve
decussate in the
superior medullary
velum.
2. Decussation of the
superior cerebellar
peduncles in the mid line.
3. Substantia nigra:
• Occupies the most
ventral part of the
tegmentum.
• It consists of pigmented,
melanin containing
neurones.
• It projects to the basal
ganglia. Its degeneration
is associated with
Parkinson’s disease.
Mask
Face
Flexion of
the Trunk
Pill-Rolling Tremors
Slow Shuffling Feet
movement
ASCENDING LEMINISCI
• Composed Of:
– Spinal (Lateral &
anterior spinothalamic
tracts)
– Trigeminal (Lateral &
medial).
– Lateral lemniscus.
– Medial lemniscus.
• Position:
– Deeply placed lateral
to the medial
longitudinal fasciculus.
CRUS CEREBRI
 It is a massive mass ventral
to the substantia nigra.
 It consists entirely of
descending cortical efferent
fibers (Frontopontine,
Corticospinal &
corticobulbar and
Temporopontine Fibres) to
the motor cranial nerve
nuclei and to anterior horn
cells.
 Involved in the
coordination of movement.
SUPERIOR COLLICULUS Level
 A large nucleus of gray
matter that lies beneath
corresponding elevation.
 It forms part of the visual
reflexes.
 Its efferent fibers go to the
anterior horn cells & to
cranial nuclei 3, 4, 6, 7 & 11).
 It is responsible for the reflex
movements of the eyes, head
and neck in response to visual
stimuli, as in following a
moving object or altering the
direction of the gaze.
SUPERIOR COLLICULUS Level
1. Occulomotor
nucleus:
• Situated in the
central gray matter
close to the median
plane.
• The fibers of the
occulomotor nerve
passes anteriorly
through the red
nucleus to emerge
on the medial side
of the crus cerebri.
SUPERIOR COLLICULUS Level
2. Red nucleus :
• A rounded mass of gray
matter that lies between
the substantia nigra and the
cerebral aqueduct in the
central portion of the
tegmentum.
• Its red coloration is due to
its vascularity and the
presence of an iron
containing pigment in the
cytoplasm of its neurons.
• It is involved in motor
control.
RETICULAR FORMATION
 It is a complex
matrix of nerve
fibers & small
groups of nerve
cells that extends
throughout the brain
stem and projects to
thalamic nuclei that
influence large
areas of the cerebral
cortex.
RETICULAR FORMATION
 It has a number of
important functions i.e.
Respiratory and Cardiovascular centers are
located in the medullary
and caudal pontine
reticular formation.
 Some reticular neurons
have long ascending
and descending axons
that allow profuse
interaction with other
neuronal systems.
RETICULAR TRACTS
 Reticulo spinal tracts:
 Influence a muscle tone
& posture
 Reticular Activating
system: RAS
Midbrain portion of RAS most
likely is its center
 Formed of some of the
ascending fibers of the
reticular formation.
 They activate the
cerebral cortex through
the thalamus.
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RAS
Functions as a net or filter for sensory input.
Filters out repetitive stimuli.
Allows passage of infrequent or important
stimuli to reach the cerebral cortex.
• Unless inhibited by other brain regions, it
activates the cerebral cortex – keeping it alert
and awake.
RETICULAR NEURONES
 Raphe Nuclei:
 Midline reticular nuclei.
– Its ascending fibers to the
cerebral cortex are
involved in the mechanisms
of sleep.
– Its descending fibers to the
spinal cord are involved in
the modulation of Pain.
 Locus Coeruleus:
 Pigmented neurons that lie
in the tegmentum of the
caudal mid brain & rostral
pons
 It is the main
noradrenergic cell group of
the brain. The locus
coeruleus is the principal site
for brain synthesis
of norepinephrine
Internal Features
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Midbrain at level of inferior colliculus
shows the following:
Corticospinal fibers
Frontopontine fibers
Temporopontine fibers
Substantia nigra
Decussation of superior cerebellar
peduncle
Medial longitudinal fasciculus
Medial lemniscus
Spinal lemniscus
Lateral lemniscus
Nucleus of trochlear nerve
Trigeminal lemniscus
Internal Features
Midbrain at level of superior
colliculus shows the following:
• Corticospinal fibers
• Frontopontine fibers
• Temporopontine fibers
• Substantia nigra
• Red nucleus
• Decussation of rubrospinal tracts
• Medial longitudinal fasciculus
• Medial lemniscus
• Spinal lemniscus
• Trigeminal lemniscus
• Nucleus of oculomotor nerve
Clinical Notes
Raised intracranial pressure and Arnold –Chiari
malformation leads to:
• Herniation of medulla and tonsils of cerebellum
• Traction of the lower 4 cranial nerves
• Paralysis of the above mentioned nerves
Lateral Medullary Syndrome (Wallenberg)
• Occlusion of posterior inferior
cerebellar artery
• All structures supplied by this artery
will be affected:
1-Nucleus ambiguous
2-Nucleus of spinal tract of trigeminal N
3-Vestibular nuclei
4-Descending sympathetic fibers
5-Inferior cerebellar peduncle
Medial Medullary Syndrome
• Occlusion of medullary branch of
vertebral artery
• All structures supplied by this artery
will be affected
1-Pyramidal tract
2-Medial leminiscus
3-Hypoglossal nerve
Pontine Hemorrhage
• Pons is supplied by basilar,
anterior inferior and superior
cerebellar arteries
• Unilateral occlusion of one of the
above vessels will affect the
1-Facial nerve
2-Abducent nerve
3-Vestibular nuclei
4-Corticospinal tract
5-Trigeminal nerve nuclei
6-Cochlear nuclei
7-Medial and spinal leminisci
8-Middle cerebellar peduncle
Vascular Lesions in Midbrain
• Weber’s syndrome
• Occlusion of a branch of
posterior cerebral artery
• Affects oculomotor nerve and
crus cerebri
• Benedikt’s syndrome
• Affects red nucleus and medial
leminiscus
SUMMARY
The brain stem is composed (from above
downwards) of: midbrain, pons & medulla
oblongata which are continuous with each other,
with diencephalon above & with spinal cord
below.
The brain stem is connected with cerebellum
through three pair of cerebellar peduncles.
The brain stem is the site of cranial nuclei, the
pathway of important ascending & descending
tracts & the site of emergence of cranial nerves
(from 3rd to 12th).
Cranial nerves (with the exception of 4th) emerge
from ventral surface of brain stem.