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The Midbrain
Josephine M. Lumitao MD, MHPED, FPOGS
Section of Neuroanatomy
Department of AnatomyUSTFMS
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Functions of the Midbrain
Conduit for ascending and descending fibertracts
Contains nuclei of cranial nerves III and IV and
midline nuclei with diverse functions
Center for pupillary light reflex
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Developmental Features of the
Midbrain
Dorsoventral orientation of alar-basal plates Neural cavity- Cerebral aqueduct
Alar plate derivatives : Inferior colliculus
Superior colliculus Substantia nigra
Red nucleus
Basal plate derivatives : Trochlear nucleus
Oculomotor nucleus
Edinger-Westphal nucleus
Marginal zone invaded by fibers that form cruscerebri/cerebral peduncles
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Boundaries of the midbrain
Ventral surface: Caudal boundary- Superior pontine sulcus where
crus cerebri enters the pons
Rostral boundaryexit of crura cerebri from
cerebral hemispheres and mammillary bodies
Dorsal surface:
- Caudal boundaryExit of trochlear nerves
- Rostral boundary- midbrain-diencephalicboundary formed by posterior commissure
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External FeaturesDorsal Surface
Caudal boundary: Exit oftrochlear nerves
Rostral boundary:Posterior commissure
Four elevations- Corpora
quadrigemina Inferiorcolliculus(caudal)
Superior colliculus(rostral)
Pineal gland extends
posteriorly above andbetween the superiorcolliculus
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External FeaturesVentral Surface
Caudal boundary: Superiorpontine sulcus
Rostral boundary:
Mammillary bodies
Crura cerebripair oflarge axon bundles from
cerebral hemisphere
caudal to the optic tract
Oculomotor nerves exit atinterpeduncular fossa
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Lateral View of Midbrain
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Internal Anatomy of Midbrain
Tectumarea posterior tocerebral aqueduct; containsthe superior and inferiorcolliculus
Tegmentumextends from
base of tectum; does notinclude substantia nigra;contains ascending anddescending fiber tracts andcranial nerve nuclei
Basis pedunculiAnterolateral portion ofmidbrain on each side madeup of Substantia nigra andCrus cerebri
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Caudal MidbrainLevel of the Inferior Colliculus
-TECTUM:
Inferior colliculus made up of a
large central nucleus, dorsal
nucleus and lateral nucleus;
auditory relay nuclei receiving
fibers of the lateral lemniscus
Brachium of the inferior colliculus
projects to medial geniculate body
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Inferior Colliculus
Central nucleus-egg shaped core of the inferiorcolliculus at the base of afferent fibers of the laterallemniscus; consist of parallel shaped layers of cellswith disc shaped dendritic fields; receives andintegrates information from multiple hindbrainauditory sources and projects to medial geniculatenucleus; ascending projections converge in a point-to-plane order, low frequencies posterolaterally andhigh frequencies anteromedially
Paracentral nucleimade up of pericentral nucleus(posterior) and external (lateral) nucleus; receive input
from central nucleus, cerebral cortex and non-auditoryinput from spinal cord and superior colliculus;projects to MGN, superior colliculus and reticularformation; involved in functions related to attention,multisensory integration and auditory-motor reflex
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Inferior Colliculus receives auditory input from
Lateral lemniscus; projects to medial geniculate body
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Caudal MidbrainLevel of the Inferior Colliculus
TEGMENTUM:
- Periaqueductal graycollection of
nerve cell bodies that surround the
cerebral aqueduct; opiate receptors;
receives spinomesencephalic tract and
projects to nucleus raphe magnus; for
modulation of pain transmission-Trochlear nucleussupplies superior
oblique muscle
-Medial longitudinal fasciculus
-Mesencephalic nucleusand tract
-Decussation of superior cerebellar
peduncle-crossed fibers from deep
cerebellar nuclei to red nucleus and/or
thalamus
-Anterolateral system and medial
lemniscus; Central tegmental tract
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Trochlear Nerve
Only motor cranial nerve formed by axons thatcross the midline before their exit
Longest intracranial course
Trochlear nucleus provides GSE fibers tosuperior oblique which functions to direct the
eyeball down when turned inwards (intorsion,
depression)
Receives projections from neurons in frontaleye field via the rostral interstitial nucleus of
the MLF (medial longitudinal fasciculus)
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Caudal MidbrainLevel of the Inferior Colliculus
-BASIS PEDUNCULI:
-Substantia nigrapars compacta
(dopamine rich) and pars
reticulata (GABA)
-Crus cerebrimiddle 3/5
Corticospinal and corticobulbar;medial 1/5 frontopontine (Arnolds
bundle); lateral 1/5 temporo-
parieto-occipito pointine fibers
(Turcks bundle)
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Substantia Nigra
Functionally associated with the basal nuclei Made up of pars compacta and pars reticulata
Pars compactalarge number of neuromelanin
containing cells rich in dopamine; project to
neostriatum(caudate nucleus and putamen) as part ofnigrostriatal pathway
Pars reticulata- loose aggregations of medium to large
sized neurons rich in GABA; project to superior
colliculus, thalamus and pontine reticular formation Parkinson disease- associated with loss of dopamine ;
resting tremors, akinesia, bradykinesia , blank stare,
paucity of movement
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Parkinsons Disease
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http://images.search.yahoo.com/search/images/view?back=http%3A%2F%2Fimages.search.yahoo.com%2Fsearch%2Fimages%3Fp%3Dsuperior%2Bcolliculus%26ei%3DUTF-8%26fr%3Dslv8-msgr%26b%3D21&w=625&h=797&imgurl=www.neuroanatomy.wisc.edu%2Fvirtualbrain%2FImages%2F23B.jpg&rurl=http%3A%2F%2Fwww.neuroanatomy.wisc.edu%2Fvirtualbrain%2FBrainStem%2F23Colliculus.html&size=108.8kB&name=23B.jpg&p=superior+colliculus&type=jpeg&no=39&tt=184&oid=99f3b7df8a53fccc&ei=UTF-8http://images.search.yahoo.com/search/images/view?back=http%3A%2F%2Fimages.search.yahoo.com%2Fsearch%2Fimages%3Fp%3Dsuperior%2Bcolliculus%26ei%3DUTF-8%26fr%3Dslv8-msgr%26b%3D21&w=648&h=553&imgurl=www.neuroanatomy.wisc.edu%2Fvirtualbrain%2FImages%2F23A.jpg&rurl=http%3A%2F%2Fwww.neuroanatomy.wisc.edu%2Fvirtualbrain%2FBrainStem%2F23Colliculus.html&size=78.9kB&name=23A.jpg&p=superior+colliculus&type=jpeg&no=25&tt=184&oid=512cf4cf2e4a8c58&ei=UTF-8http://images.search.yahoo.com/search/images/view?back=http%3A%2F%2Fimages.search.yahoo.com%2Fsearch%2Fimages%3Fp%3Dsuperior%2Bcolliculus%26ei%3DUTF-8%26fr%3Dslv8-msgr%26b%3D21&w=625&h=797&imgurl=www.neuroanatomy.wisc.edu%2Fvirtualbrain%2FImages%2F23B.jpg&rurl=http%3A%2F%2Fwww.neuroanatomy.wisc.edu%2Fvirtualbrain%2FBrainStem%2F23Colliculus.html&size=108.8kB&name=23B.jpg&p=superior+colliculus&type=jpeg&no=39&tt=184&oid=99f3b7df8a53fccc&ei=UTF-8http://images.search.yahoo.com/search/images/view?back=http%3A%2F%2Fimages.search.yahoo.com%2Fsearch%2Fimages%3Fp%3Dsuperior%2Bcolliculus%26ei%3DUTF-8%26fr%3Dslv8-msgr%26b%3D21&w=648&h=553&imgurl=www.neuroanatomy.wisc.edu%2Fvirtualbrain%2FImages%2F23A.jpg&rurl=http%3A%2F%2Fwww.neuroanatomy.wisc.edu%2Fvirtualbrain%2FBrainStem%2F23Colliculus.html&size=78.9kB&name=23A.jpg&p=superior+colliculus&type=jpeg&no=25&tt=184&oid=512cf4cf2e4a8c58&ei=UTF-8 -
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TECTUM:
Superior Colliculus-made up ofalternating gray matter and white matter;
layers I-III receive input from retina and
visual cortices and project to thalamus;
layers IV-VII subserve gaze changesincluding eye movement and project to
thalamus, brain stem and spinal cord
-Detection of movements in visual fields
for visual orientation, searching and
tracking
-For audiovisual reflexes as it receives
input from inferior colliculus
-Connects with the lateral geniculate
nucleus via brachium of superior
colliculus
Features of Rostral MidbrainLevel of
Superior Colliculus
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TEGMENTUM:
Red nucleusMagnocellular divisionsend axons which cross the midline at
ventral tegmental decussation of Forel to
form the rubrospinal tract that innervates
the proximal limb flexorsParvocellular division (rostral)
axons form the rubroolivary tract that
descends in the central tegmental tract
and terminates in the ipsilateral inferior
olives
Receives afferents from the
contralateral deep cerebellar nuclei and
ipsilateral cerebral cortex
Features of Rostral MidbrainLevel of
Superior Colliculus
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TEGMENTUM:
Dorsal tegmental decussation of
Meynertcrossed projections of
superior colliculus to brain stem areas
(tectoreticular, tectoolivary and
tectospinalcervical spinal cord)Tectobulbospinal system of brain stem
MLF
Medial lemniscus
Anterolateral system
Central tegmental tract
Ventral tegmental decussation of
Forel-crossed axons from red nucleus
that form ruprospinal tract
Features of Rostral MidbrainLevel of
Superior Colliculus
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Oculomotor Nucleus
Composed of:
- central nucleus of Perlia that supplies the
ciliary muscle for convergence and accommodation
- lateral dorsal somatic cell column that innervates all extraocular musclesexcept the lateral rectus and superior oblique
- Midline dorsal subnucleus(caudal central subdivision) supply the levatorpalpebrae superioris
- midline dorsal visceral cell column (Edinger-Westphal nucleus) , origin ofpreganglionic parasympathetic fibers to the ciliary ganglion, postganglionicfibers of which innervate sphincter pupillae and ciliary muscles
Does not receive direct cortical projections via corticobulbar pathway; cortex offrontal eye field exerts its control over oculomotor neurons via projections torostral interstitial nucleus of the MLF (riMLF, the vertical gaze center) and thesuperior colliculus
Accessory nuclei: interstitial nucleus of Cajal, nucleus of Darkschewtisch,
nucleus of posterior commissure and riMLF Oculomotor nerve emerge at interpeduncular fossa medial to crus cerebri
Ipsilateral lesion will result in an abducted and depressed eye, diplopia (doublevision), mydriasis (dilated pupil) non reactive to light and cannot accommodateand drooping of eyelids (ptosis)
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R ti l d R h N l i f
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Reticular and Raphe Nuclei of
Midbrain
Reticular Nuclei arecuneiform and
subcuneiform nuclei
which forms part of
ARAS for maintaining
alertness and
wakefulness
Dorsal raphe nucleus
serotonergic neurons
that modulate neuronalactivity involved in
sleep/dream cycles
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Midbrain-Diencephalic junction
Pretectal Areaconsist of groups of cells related to visualsystem; brainstem center for pupillary light reflex; receives inputfrom superior colliculus and project bilaterally to Edinger-Westphal nucleus mediating direct and consensual pupillary lightreflex
Posterior commissurerostral boundary of midbrain;functionally, contains fibers from the pretectal nuclei that projectto contralateral Edinger Westphal nucleus and upgaze fibersfrom riMLF; important in consensual pupillary light reflex andupward vertical gaze
Ventral tegmental area of Tsai-diffuse cell group
ventromedial to red nucleus and continuous with lateralhypothalamic area; contains dopaminergic neurons that projectto cortical and subcortical structures of limbic system (emotionalbehavior); fibers directed to nucleus accumbens that constitutethe mesolimbic or reward pathway
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Pupillary Light Reflex
Four neuron reflex arc:
Ganglion cells of retina
forms the OPTIC
NERVE
Pretectal area
Edinger-Westphal
nucleus (bilateral) of
OCULOMOTOR
NERVE
Ciliary Ganglion to
sphincter pupillae to
produce pupillary
constriction
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BLOOD SUPPLY OF
MIDBRAIN:
Superior cerebellar
artery supply the
tectum
Quadrigeminal artery
supply the posteriormidbrain and
ventrolateral regions
Posteromedial group
of branches from circleof Willis (P1 and P2)
from posterior
cerebral artery supply
ventromedial region of
midbrain
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Vascular Syndromes of the Midbrain
Webers syndromeipsilateralparalysis of extraocularmuscles except superioroblique and lateral rectus,dilated pupil ipsilaterally,contralateral paralysis ofextremities
Claude syndromeIpsilateralparalysis of all extraocularmuscles except LR and SO;ipsilateral dilated pupil,contralateral ataxia, tremorand incoordination
Benedikt syndromeipsilateral paralysis ofextraocular muscles exceptSO and LR; ipsilateral dilatedpupil; contralateral paralysisof extremities andcontralateral tremor and ataxia
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1 Which of the ff serves as caudal
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1.Which of the ff. serves as caudal
boundary of dorsal midbrain?
A. Superior pontine sulcus B. Exit of trochlear nerve
C. Mammillary body
D. Posterior commissure
2 Which of the ff forms the tectum in
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2.Which of the ff. forms the tectum in
rostral midbrain?
A. Superior colliculus B. Inferior colliculus
C. Cerebral peduncles
D. Cerebral aqueduct
3 Which of the ff is center for
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3.Which of the ff. is center for
pupillary light reflex?
A. Dorsal raphe nucleus B. Pretectal area
C. Posterior commissure
D. Ventral tegmental area of Tsai
Which of the ff. nuclei of the inferior colliculus
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.Which of the ff. nuclei of the inferior colliculusshows plane-to-order convergence of
projections?
A. Central nucleus B. Pericentral nucleus
C. Lateral nucleus
D. Paracentral nuclei
5 The pars compacta of the
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5.The pars compacta of the
substantia nigra is rich in_____.
A. GABA B. Serotonin
C. Dopamine
D. Norepinephrine
6 What is the functional significance
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6. What is the functional significance
of posterior commissure?
A. Direct pupillary light reflex B. Consensual pupillary light reflex
C. Upward vertical gaze
D. Both B and C
7 Which of the ff is a manifestation of
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7. Which of the ff. is a manifestation of
a lesion of R oculomotor nerve?
A. Ptosis of L eye B. Medially deviated R eye
C. Dilated R pupil
D. Dilated L pupil
8 Where do the superior colliculus
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8. Where do the superior colliculus
fibers project?
A. Medial geniculate body B. Lateral geniculate body
C. Red nucleus
D. Both A and B
9 Which of the ff structures is found
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9. Which of the ff. structures is found
in caudal midbrain?
A. Superior colliculus B. Red nucleus
C. Oculomotor nucleus
D. Decussation of superior cerebellarpeduncle
10 Which of the ff occupies the
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10. Which of the. ff. occupies the
middle 3/5 of the crus cerebri?
A. Frontopontine fibers B. Corticospinal fibers
C. Occipitopontine fibers
D. Temporopontine fibers