The document provides an overview of the midbrain, including its: 1. Anatomy, with descriptions of its major parts like the cerebral peduncles, substantia nigra, and corpora quadrigemina. 2. Internal structures seen on transverse sections at different levels, such as the cerebral aqueduct, oculomotor and trochlear nerve nuclei. 3. Connections of structures like the superior colliculus and red nucleus. 4. Functions including roles in visual, auditory, and motor pathways through the brainstem.

1. MID BRAIN
DR SANA YASEEN
ANATOMY DEPARTMENT
KIMS

2. BRAIN STEM REVIEW

3. BRAIN STEM
 It is a stalk-like part of the brain which connects the spinal cord with the forebrain.
 From below upwards it consists of three parts:
 medulla oblongata,
 pons, and
 midbrain.
 The midbrain is continuous above with the cerebral hemispheres.
 The medulla oblongata is continuous below with spinal cord.
 It is located in the posterior cranial fossa.
 Its ventral surface lies on the clivus.
 Posteriorly, the pons and medulla are separated from the cerebellum by the cavity of the fourth
ventricle.

4. Anterior view

5. FUNCTIONS OF
BRAIN STEM
four major functions:
 It provides passage to various
ascending and descending tracts
that connect the spinal cord to the
different parts of the forebrain.
 It contains important autonomic
reflex centres (vital centres)
associated with the control of
respiration heart rate and blood
pressure.
 It contains reticular activating
system which controls
consciousness.
 It contains important nuclei of the
last ten cranial nerves (i.e. IIIrd to
XIIth).
The impairment of reticular activating
system leads to progressive loss of
consciousness.

6. “
”
MID BRAIN
EXTERNAL FEATURES
 ANTERIOR VENTRAL VIEW
 POSTERIOR DORSAL VIEW
INTERNAL FEATURES
 AT SUPERIOR COLLICULLUS
 AT INFERIOR COLLICULUS

7. INTRODUCTION TO MID
BRAIN
 It is the upper and shortest part
of the brain-stem.
 It is about 0.8 inches , app 2cm
long and 2.5 cm wide.
 It connects the pons a cerebellum
to diencephalon
 Its cavity, the cerebral aqueduct
(aqueduct of Sylvius) connects the
third ventricle with the fourth
ventricle.
 It passes through the tentorial
notch.

9. It is related
on each side to the
 optic tract,
 parahippocampal gyrus,
 posterior cerebral artery, and
basal
Anteriorly to
 interpeduncular structures, such
as mammillary bodies, tuber
cinereum, etc.;
.
Posteriorly to
 splenium of corpus callosum,
 great cerebral vein,
 pineal body and
 posterior ends of right and left
thalami.

12. Cerebral
peduncle
OPTIC NERVE
BASILAR
ARTERY
ANTERIOR TO MIDBRAIN AT INTERPEDUNCULAR FOSSA

13. POSTERIOR VIEW
OF MIBRAIN
RELATION OF TENTORIUM CEREBELLI
WITH BASILAR ARTERY

14. ANTERIOR / VENTRAL VIEW

17.  It presents two crura cerebri which emerges from the cerebral hemispheres and
converge downwards to enter the pons forming the posterolateral boundaries of the
interpeduncular fossa.
 The superficial surface of the crus cerebri is finely corrugated by the underlying
longitudinal fibres.
 It is crossed transversely from above downwards by
 optic tract,
 posterior cerebral artery,
 superior cerebellar artery
 The oculomotor nerve emerges from a groove (occulomotor sulcus) on the medial side
of the crus cerebri ventrally.
 The trochlear nerve emerges on the dorsal aspect of the midbrain and curls around the
lateral aspect of the cerebral peduncle to appear on the ventral aspect of the midbrain
lateral to the oculomotor nerve.
 These two nerves run forward between the posterior cerebral and the superior cerebellar
arteries.

18.  Within interpeduncular
fossa is the depression
called posterior
perforating
 Its so called as it
posses small openings
for the central
branches of posterior
cerebral arteries to
runs through it.

19. POSTERIOR / DORSAL VIEW

20.  It presents four rounded elevations:
two superior and two inferior colliculi (or corpora quadrigemina).
 The colliculi are separated from each other by a cruciform sulcus.
 The vertical limb of sulcus when traced above forms a surface
depression which lodges the pineal body
 Traced below, it becomes continuous with the frenulum veli (a median
ridge on the dorsal surface of the superior medullary velum).
 The trochlear nerves emerges one on each side of the upper part of
frenulum veli after decussation in the superior medullary velum.

21. POSTERIOR/ DORSAL VIEW
SUPERIOR COLLICULUS

22. POSTERIOR / DORSAL VIEW
INFERIOR COLLICULUS

23.  Thick ridges of white matter extending from lateral side of
each colliculus constitute their brachia.
 superior brachium connect the superior colliculus to the lateral
geniculate body and the optic tract, and contributes in visual reflex.
 inferior brachium connect the inferior colliculus to the medial
geniculate body, and contributes in auditory reflex
 Trochlear nerve (IV CN) emerges lateral to frenulum orsal to
inferior colliculus runs in the posterior aspect of midbrain than
curve to run over lateral aspect of cerebral peduncles and
traverses interpeduncular cistern to petrosal end of cavernous
sinus.

26. INTERNAL STRUCTURE

27.  Each cerebral
peduncle is further
subdivided into three
parts,
 from dorsal to
ventral these are:
(a) tegmentum,
(b) substantia nigra, and
(c) crus cerebri.

28. Internal Structure
 The transverse section of midbrain shows a tiny canal, called cerebral aqueduct.
 A coronal plane passing through the aqueduct divides the midbrain into two
parts;
 small posterior part and
 large anterior part
 The small posterior part is called tectum and consists of four colliculi.
 The large anterior part is divided into two equal right and left halves by a vertical
plane, the cerebral peduncle.

29. Crus cerebri (basis pedunculi)
 It is the part of cerebral peduncle
 It is situated anterolateral to the substantia nigra.
 It contains important descending tracts which connect the cerebral cortex
to the anterior horn cells of the spinal cord, cranial nerve nuclei, and
pontine nuclei.
• corticospinal and corticonuclear fibres (pyramidal tract) occupy the
middle 2/3 of crus
• frontopontine fibres occupy the medial 1/6 of crus.
• temporopontine, parietopontine, and occipitopontine fibres occupy the
lateral 1/6 of the crus.

31. Substantia nigra
 It is a curved (crescent-shaped) pigmented band of grey matter.
 It is situated between tegmentum and crus cerebri. The concavity is smooth and
directed towards the tegmentum.
 From its convex margin spiky processes project into the substance of the crus
cerebri.
 It is a large motor nucleus that extends throughout the length of midbrain.
 It is divided into two parts:
(a) the dorsal part (pars compacta) containing medium sized cells and
(b) a ventral part (pars reticularis) containing fewer cells.
 The pars reticularis is intermingled with the fibres of crus cerebri.

32.  The substantia nigra is made up of deeply pigmented nerve cells which
contain melanin & iron.
 These cells synthesize dopamine which is carried through their axons
(nigrostriatal fibres) to the corpus striatum.
Clinical Correlation
 The degeneration or destruction of substantia nigra causes deficiency of
dopamine in the corpus striatum leading to a clinical condition called
Parkinsonism.

33. Transverse
Section of the
Midbrain at the
Level of the
Inferior Collicul

34. Grey matter
 It has a central grey matter (grey matter around the cerebral aqueduct)
 It contains two nuclei:
(a) nucleus of trochlear nerve
(b) mesencephalic nucleus of trigeminal nerve.
1. Trochlear nerve nucleus is situated close to the median plane just posterior to the medial longitudinal
fasciculus (MLF).
The emerging fibres of the trochlear nerve pass laterally and posteriorly around the central grey matter
and leave the midbrain just below the inferior colliculi.
The fibres of trochlear nerve now decussate in the superior medullary vellum and
wind round the lateral aspect of the midbrain to enter the lateral wall of cavernous sinus

36. 2. The mesencephalic nucleus of trigeminal nerve lies in the lateral edge
of the central grey matter.
It receives proprioceptive impulses from muscles of mastication, teeth,
ocular and facial muscles.
3. An ovoid mass of grey matter underneath the inferior colliculus forms
the nucleus of inferior colliculus.
 It receives the afferent fibres of lateral lemniscus and gives the efferent
fibres to the medial geniculate body through the inferior brachium.
 4. Substantia nigra.
 5. The reticular formation is smaller than that in the pons and is
situated ventrolaterally between the medial lemniscus and the central
grey matter.

38. • The decussation of the superior cerebellar peduncles occupies the central part of the
tegmentum.
 This forms the most important feature in the lower part of the midbrain.
• The lemnisci are arranged in the form of a curved compact band of white fibres in the ventrolateral
part of the tegmentum, lateral to cerebellar decussation and dorsal to the substantia nigra.
 From medial to lateral side these are:
 medial lemniscus, trigeminal lemniscus, spinal lemniscus, and lateral lemniscus.
• The medial longitudinal fasciculus lies on the side of median plane ventral to the trochlear nerve
nucleus.
• The tectospinal tracts lie ventral to the medial longitudinal fasciculi.
• The rubrospinal tracts lie ventral to the decussation of the superior cerebellar peduncles.
White matter

39. Transverse Section of the Midbrain at the
Level of the Superior Colliculi

40. Grey matter
 The central grey matter in each half contains two nuclei:
 oculomotor nerve nucleus
 mesencephalic nucleus.
1. Oculomotor nucleus , lies in the ventromedial part.
 The nuclei of two sides fuse together forming a single complex having a triangular outline.
 The oculomotor nuclei are bounded laterally by the medial longitudinal fasciculus.
 The Edinger-Westphal nucleus which supplies the sphincter pupillae and ciliary muscle, forms part of
the oculomotor nucleus and is located dorsal to the rostral two-thirds of the main oculomotor
nucleus
 The emerging fibres of oculomotor nerve pass ventrally through the tegmentum intersecting red
nucleus and medial part of the substantia nigra, and emerge in the posterior part of interpeduncular
fossa through the sulcus on the medial aspect of crus cerebri.

41. 2. Mesencephalic nucleus occupies the same position as in the lower part of the
midbrain.
3. Superior colliculus
 It is is a flattened mass formed of seven concentric alternating laminae of white
matter and grey matter.
 Unilateral lesion of the superior colliculus results in inability to track moving objects
in the contralateral field of vision, although the eye movements are normal.
4. Pretectal nucleus
 It is a small group of neurons and lies deep to the superolateral part of the superior
colliculus.
 It receives afferents from the lateral root of the optic tract and
 It gives efferents to the Edinger-Westphal nucleus (the parasympathetic component
of the oculomotor nucleus) of the same as well as of the opposite side.
 The pretectal nucleus is an important part of the pathway for pupillary light reflex
and consensual light reflex.
 Its lesion causes Argyll Robertson pupil in which light reflex is lost but
accommodation reflex remains intact.

42. Connections of the superior colliculus
The superior colliculus receives
afferent fibres from:
 1. The retinae (mainly the contralateral)
through the lateral geniculate body and
superior brachium,
 2. The spinal cord (pain and tactile fibres)
through spinotectal tract,
 3. The frontal and occipital visual cortex
(conjugate eye movements), and
 4. The inferior colliculus.
The efferent fibres from superior
colliculus form
 tectospinal and tectobulbar tracts, which
are
 probably responsible for the reflex
movements of the eyes, head, and neck in
response to visual stimuli.

44.  5. Red nucleus
 It is a cigar-shaped mass of grey matter which appears ovoid in cross-section.
 It is about 0.5 cm in diameter
 It is situated dorsomedial to the substantia nigra.
 In the fresh specimen it is red/pink in colour due to its high vascular supply and an iron containing pigment
present in the cytoplasm of its cells.

45. Connections of red nucleus
Efferents:
 (a) Rubrospinal, rubrobulbar and rubroreticular tracts.
 The fibres from red nucleus before forming these tracts
decussate forming ‘ventral tegmental decussation of Forel’
The fibres of rubrospinal tract end in the anterior horn cells
of the opposite side.
 The rubrobulbar tract ends in the motor nuclei of Vth and
VIIth cranial nerves (also in the nuclei of IIIrd, IVth and VIth
cranial nerves).
 (b) rubroolivary fibres, (c) rubrothalamic fibres, (d)
rubrocerebellar fibres, (e) rubronigral fibres.
 The red nucleus is considered as an integrating and relay
centre on the following pathways:
 (a) cortico-rubro-spinal, (b) cortico-rubro-nuclear, and (c)
cerebello-rubro-spinal
Afferents:
 (a) Cerebellorubral fibres from contralateral
dentate nucleus of the cerebellum through
superior cerebellar peduncle,
 (b) corticorubral fibres, mostly from the
ipsilateral motor area (area 4 & 6 of frontal
cortex),
 (c) pallidorubral fibres from globus pallidus
of the same side, (d) red nucleus also
receives fibres from: subthalamic nucleus
(corpus luysi), hypothalamus, substantia
nigra and tectum.

46. White matter
 Decussation of fibres (tectospinal and tectobulbar tracts) arising from superior colliculi forming
dorsal tegmental decussation (of Meynert).
 Decussation of fibres (rubrospinal tracts) arising from red nuclei forming ventral tegmental
decussation (of Forel).
 Medial longitudinal fasciculus (MLF) lies ventrolateral to the oculomotor nucleus.
 Tegmentum at this level also contains the same lemnisci (i.e. medial, trigeminal and spinal) as
those at the level of inferior colliculus except for the lateral lemniscus.
 The lateral lemniscus is not seen at this level because it terminates in the nucleus of inferior
colliculus.
 Emerging fibres of oculomotor nerve.

47. Medial Longitudinal Fasciculus
 It is a heavily myelinated composite tract found in the paramedian plane of the brainstem.
 The MLF extends cranially to the interstitial nucleus of Cajal (accessory oculomotor nucleus)
located at the junction of midbrain and diencephalon near the rostral end of the cerebral
aqueduct,
 caudally it becomes continuous with anterior intersegmental fasciculus of the spinal cord.
 The MLF consists of fibres arising mainly from vestibular nuclei
 some fibres also arise from nucleus of lateral lemniscus and interstitial nucleus of Cajal
 The fibres of MLF interconnect the nuclei of IIIrd, IVth, Vth and VIth cranial nerves and spinal
nucleus of accessory nerve.
 Chief function of MLF is to coordinate the movements of eyes, head and neck in response to
stimulation of the vestibulocochlear nerve.

48. Chief function of MLF is
to coordinate the
movements of eyes,
head and neck in
response to stimulation
of the vestibulocochlear
nerve.

49. MLF syndrome (internuclear ophthalmoplegia):
 It occurs due to lesion of in the upper part of the pons in the region between
abducent and oculomotor nuclei.
 The MLF syndrome is mostly seen in multiple sclerosis and presents following
clinical features:
 1. Isolates paralysis of medial rectus muscle of eyeball on the side of lesion on
attempted lateral gaze.
 2. Mononuclear horizontal nystagmus in the adducting eye contralateral to the
side of lesion.
 The convergence and vertical remain unaffected.

50. MLF SYNDROME
(INTERNUCLEAR
OPHTHALMOPLEGIA):

51. Blood Supply of the Midbrain
Arterial supply
 The midbrain is supplied by following arteries:
• Basilar artery through its posterior cerebral and superior cerebellar arteries.
Basilar artery also
 supplies mid-brain through direct branches.
• Branches of posterior communicating and anterior choroidal arteries.
Venous drainage
• The veins of midbrain drains into the great cerebral and the basal veins.

52. Weber’s syndrome
 Weber’s syndrome is produced by a vascular lesion in the basal region of the cerebral peduncle due to
occlusion of a branch of the posterior cerebral artery. This lesion involves the oculomotor nerve and the crus
cerebri
 It produces following important signs and symptoms
 — Ipsilateral lateral squint, due to involvement of third cranial nerve.
 – Contralateral hemiplegia, due to involvement of corticospinal tract in the crus cerebri.
 – Contralateral paralysis of the lower part of the face and tongue, due to involvement of the corticobulbar tract
in the crus cerebri.
 – Drooping of the upper lid (ptosis), due to paralysis of levator palpebrae superioris supplied by oculomotor
nerve.
 – Pupil is dilated and fixed to light and accommodation is lost on the side of lesion due to involvement of
parasympathetic component of oculomotor nerve (Edinger-Westphal nucleus)

53. Benedikt’s syndrome
 Benedikt’s syndrome occurs due to the vascular ischaemia of the tegmentum of midbrain involving
the medial lemniscus, spinal lemniscus, red nucleus, superior cerebellar peduncle and fibres of
oculomotor nerve
 It is characterised by following signs and symptoms:
 – Ipsilateral lateral squint and ptosis, due to involvement of oculomotor nerve fibres.
 – Contralateral loss of pain and temperature sensation, due to involvement of trigeminal and spinal
lemnisci.
 – Contralateral loss of tactile, muscle, joint and vibration sense, due to involvement of medial
lemniscus.
 – Contralateral tremors and involuntary movements in the limbs, due to involvement of red nucleus
and fibres of superior cerebellar peduncle entering into it

54. Parinaud’s syndrome
 Parinaud’s syndrome results from a lesion of the superior colliculi as occurs when
this area becomes compressed by the tumours of the pineal gland.
 It is characterised by the loss of upward gaze without affecting the other eye
movements.

55. Argyll Robertson pupil
 The Argyll Robertson’s pupil is a clinical condition in which light reflex is lost but
the accommodation reflex remains intact.
 occurs because of lesion in the vicinity of pretectal nucleus.