The brain stem consists of the medulla, pons, and midbrain. It is situated in the posterior cranial fossa. The medulla is the lowest part and connects with the spinal cord. It contains nuclei for cranial nerves and tracts for sensory and motor functions. The pons is in the middle and connects the midbrain with the medulla. It contains pontine nuclei and transverse fibers. The midbrain connects the hindbrain and forebrain. It contains the cerebral peduncles and tectum including the superior and inferior colliculi.

1. The Brain Stem

2. • The brain stem
consists of
▫ The medulla
oblongata
▫ The pons
▫ The midbrain
• It is situated in the
posterior cranial
fossa.

3. The Medulla
• Lower part of the brain stem.
• Extents from the lower border
of the pons to just above the
attachment of first cervical
spinal nerve.
• Inferiorly medulla is
continuous with the spinal
cord
• It lies in the anterior part of
the posterior cranial fossa
extending down to foramen
magnum
• Anteriorly, medulla is related
to the clivus & meninges.
• Posteriorly medulla is related
to the vallecula of the
cerebellum.

4. External Features
The medulla is divided in to right
& left halves by the anterior &
posterior median fissure.
• Each half is further divided
in to anterior, lateral &
posterior regions by the
anterolateral &
posterolateral sulci.
• The anterior region is in the
form of longitudinal elevation
called as the pyramid.
▫ It is made of corticospinal
fibers
• In the lower part of the
medulla many fibers of the
right & left pyramids cross in
the midline forming
pyramidal decussation.

5.  The upper part of the
lateral region has an
oval elevation called the
olive.
 Which is produced by the
underlying mass of grey
matter (inferior Olivay
nucleus).
 The rootlets of XII nerve
emerge from the
anterolateral sulcus
between pyramid &
olive.
 The rootlets of the IX
and X nerves & cranial
part of the XI emerge
through the
posterolateral fissure
behind the olive.

6.  The posterolateral region lies
between the posterolateral sulcus
& the posterior median fissure,
 The upper part of this region has
a “V” shaped depression which
is the lower part of the floor of
the IV ventricle.
 Below the floor there is
longitudinal elevations,
 From medial to lateral side they
are
 The fasciculus Gracilis,
 The fasciculus Cuneatus &
 The inferior cerebellar
peduncle.
 The upper ends of the fasciculus
gracilis & cuneatus expand to form
the gracile & cuneate tubercles.
 These tubercles are formed by the
underlying masses of grey matter
called the nucleus Gracilis, &
nucleus Cuneatus.

7.  The medulla is
divide in to 2 parts.
 The lower closed
part & the upper
open part with a
central canal.
 The upper open
part is where the
central canal
opens out to form
the IV ventricle.

8. Internal Structure Of Medulla
• The internal structure of
medulla is studied by
examining TS through it at 3
places
TS through the Lower part of the
medulla passing through the
Pyramidal decussation
• It resembles the TS of the
spinal cord
White matter
• There is Pyramids anteriorly.
• The decussation of the
pyramidal tract forms the
important feature of the
medulla at this level, the
fibers run laterally & form
the lateral cortcospinal
tracts.

9. • The fasciculus gracilis &
the fasciculus cuneatus
occupy the broad posterior
white column
• The other features are
similar to spinal cord
Grey matter
• The decussating
Pyramidal fibers separate
the anterior horn from the
grey matter & forms the
spinal nucleus of the
spinal accessory nerve &
supra Nucleus for the 1st
cervical nerve.
• The nucleus gracilis, &
nucleus cuneatus are
continuous with the
central grey matter.

10. TS passing through middle of the
medulla (through the sensory
decussation)
Grey matter
 The nucleus Gracilis & nucleus
Cuneatus are much larger & are
separated from the central grey
matter, the fasciculus Gracilis
& fasciculus Cuneatus will end
in these nuclei.
 The nucleus of spinal tract of
the trigeminal nerve is also
separate from the central grey
matter.
 The central grey matter
contains the following
 Hypoglossal nucleus
 Dorsal nucleus of the vagus
 Nucleus of Tractus solitarius

11. White matter
• The nucleus
Gracilis &
Cuneatus give
rise to internal
arcuate fibers,
these fibers
cross to
oppossite side
where they
form Para
median band
of fibers called
the medial
meniscus.
• The pyramidal
tracts lie
anteriorly

12. TS through the upper part of the medulla
passing through the IV ventricle
Grey matter
• The nuclei of several
cranial nerves are
seen in the floor of
the IV ventricle
▫ Hypoglossal nucleus
▫ The dorsal nucleus
of the vagus
▫ The nucleus of
tractus solitarius
▫ The inferior&
medial vestibular
nuclei
• The nucleus of the
spinal tract of the
trigeminal nerve lies in
the dorsolateral part.
• The inferior olivary
nucleus is the largest
mass of grey matter
seen at this level.

13. White matter
• The inferior
cerebellar peduncle
occupies the
posterolateral part,
lateral to IV
ventricle
• Other tracts seen in
the spinal cord can
also be seen.

14. Applied Anatomy
• The vital centers ( respiratory & vaso- motor ) are
situated in the lower part of the floor of the IV
ventricle formed by the medulla.
▫ An injury to the medulla is therefore usually fatal
• Common vascular lesions involving the medulla
are
▫ Thrombosis of the posteior inferior cerebellar artery &
Thrombosis of the vertebral artery. These two lesions
cause lateral & medial medullary syndromes,
respectively.
• Bulbar paralysis may be acute.
▫ It is characterized by the paralysis of the muscles
supplied by the last 4 cranial nerves which arise
from the medulla.
▫ Paralysis of respiratory & vasomotor would be
fatal

15. The Pons

16. The
pons is the middle part
of the brain stem
connecting the mid brain
with the medulla.
• External features
▫ The pons has 2
surfaces ventral &
dorsal surfaces
▫ The ventral or anterior
surface is convex in
both directions & is
transversely striated.
▫ In the median plane, it
shows a vertical
sulcus called the
basilar sulcus
 Which lodges the basilar
artery.

17. • Laterally the surface is
continuous with the middle
cerebellar peduncle.
• The trigeminal nerve is
attached to this surface at
the junction of the pons with
the peduncle.
• The nerve has 2 roots, a
small motor root which lies
medial to the much larger
sensory root.
• The Abducent, facial &
vestibulocochlear nerves are
attached to the lower border
of the ventral surface.
• The dorsal or posterior
surface is hidden by the
cerebellum, which forms the
upper ½ of the IV ventricle.

18. Internal structure of the pons
• In TS the pons shows two
parts a ventral & dorsal
parts.
• The ventral or basilar part
▫ It is continuous inferiorly
with pyramids of the
medulla & on each side
with the cerebellum through
the middle cerebellar
peduncle
• The dorsal or tegmental
part
▫ It is a direct upward
continuation of the medulla
(excluding the pyramids )

19. • The basillar part has same structure
through out the pons & has the
following features.
Grey matter
• It is represented by the pontine nuclei
which are scattered among the
longitudinal & transverse fibers.
• The pontine nuclei form an important
part of the corticoponto-cerebellar
pathway.
• Fibers from all these nuclei will go to
the opposite ½ of the cerebellum.
White matter
• It consists of the longitudinal &
transverse fibers
• The longitudinal fibers include
▫ The corticospinal &
corticonuclear(pyramidal)
▫ The corticopontine fibres ending in the
pontine nuclei
• The transverse fibers are
pontocerebellar fibers beginning from
the pontine nuclei & going to the
opposite ½ of the cerebellum through
the inferior cerebellar peduncle

20. Tegmentum in the upper part of the pons
Grey matter
▫ The special features are the
motor & superior nuclei of
the trigeminal nerve.
▫ The motor nucleus is
medial to the superior
sensory nucleus
White matter
▫ Immediately behind the
ventral part of the pons
there is transverse band of
fibers that is made up (
from medial to lateral side )
of
 The medial leminiscus
 The trigeminal leminiscus
 The spinal leminiscus
 The lateral leminiscus.
▫ The trigeminal leminiscus
contains fibers arising in
the spinal nucleus of the
trigeminal nerve &
traveling to the thalamus.
▫ The lateral leminiscus is the
part of auditory pathway.

21. • Applied Anatomy
▫ Unilateral lesion in the lower part of the pons
results in paralysis of the facial nerve on the side
of the lesion, & paralysis of the limbs (
hemiplegia ) on the opposite side (crossed
hemiplegia or Millard Gubler Syndrome)

22. The Mid
Brain

23. • The mid brain is also called the
mesencephalon.
• It connects the hind brain with the
fore brain.
• Its cavity is known as the cerebral
aqueduct,
▫ It connects the 3rd ventricle with 4th
ventricle
• The mid brain passes through the
tentorial notch
• The medial & lateral geniculate
bodies ( meta thalamus ) are
situated on the posterolateral aspect
of the mid brain.
• The superior colliculus is connected
to the lateral geniculate body by the
superior brachium.
• The inferior colliculus is connected
to the medial geniculate body by the
inferior brachium.

25. In TS the following are
the major
subdivisions
• The Tectum
▫ It is the part posterior
to the aqueduct.
▫ It is made up of right &
left, superior & inferior
colliculi
• The cerebral peduncle.
▫ Each ½ of the midbrain
anterior to the aqueduct
is called the cerebral
peduncle.
▫ Each cerebral peduncle
is subdivided in to
 The crus cerebri
anteriorly
 The substasia nigra
in the middle
 The tegmentum
posteriorly

26. Internal structure of the mid brain
This can be studied at the level of inferior &
superior colliculi
T S of mid brain at the level of inferior
colliculus
Grey matter
• The inferior colliculus receives
the afferents of the lateral
meniscus, & gives efferents to
the medial geniculate body,
which helps in localizing the
source of sounds.
• The substantia nigra is a lamina
of grey matter made up of
deeply pigmented nerve cells.
• The central ( peri aqueductal )grey
matter contains
▫ The nucleus of the trochlear nerve in
the ventromedial part.
▫ The mesencephalic nucleus of the
trigeminal nerve in the
lateral part.

27. White matter
• The crus cerebri contains
▫ The pyramidal tract in the middle
▫ Frontopontine fibres,
parietopontine, & occipitopontine
fibres in the lateral 1/6th .
The tegmentum contains
ascending tracts as follows
• The leminisci ( medial,
spinal, trigeminal, & lateral
) are arranged in the form of
a band in which they lie in
order mentioned ( from med
to lateral )
• The decussating of the
superior cerebellar
peduncles is seen in the
median plane.
• The tectospinal &
rubrospinal tracts are
present.

28. TS of mid brain at the level of superior colliculus
Grey matter
• The central grey matter
contains
▫ The nucleus of the occulomotor
nerve in the ventromedial part
▫ The mesencephalic nucleus of
the trigeminal nerve in the
lateral part
• The superior colliculus receives
afferents from the retina (
visual ) & various other
centres.
▫ It gives afferents to the spinal
cord (tectospinal ).
▫ It controls the reflex of the
movements of the eyes, & of the
head & neck in response to the
visual stimuli.
• The pretectal nucleus lies deep
to the superolateral part of the
superior colliculus.
▫ It receives the afferents from
lateral root of the optic nerve.
▫ It gives efferents to the Edinger
Westphal nuclei of both sides.

29. • The red nucleus is about
0.5 cm in diameter
▫ It receives the afferent
fibres from the superior
cerebellar peduncle, the
globus pallidus, the
subtahalamic nucleus &
the cerebral cortex.
▫ It gives efferents to the
spinal cord (rubrospinal
tract), reticular
formation, the thalamus,
the olivary nucleus, the
subthalamic nucleus.
▫ It has an inhibitory
influence on muscle tone.
White matter
• The crus cerebri has same
tracts as described above

30. Applied Anatomy
• Lesion in the upper part of the midbrain can
produce a paralysis of muscles supplied by the
oculomotor nerve on the side of lesion, along
with a hemiplegia on the opposite side (Weber’s
syndrome).
• A similar lesion in the pons, results in a
paralysis of the lateral rectus muscle (abducent
nerve) on the side of lesion with hemiplegia on
the opposite side (Raymond’s syndrome).
• Alternatively, facial paralysis of one side can be
combined with contralateral hemiplegia
(Millard Gubler syndrome).