anatomy of brainstem and its related syndromes

1. Dr. Shaitan Singh

2.  Introduction
 Enumerate its parts
Describe the external features of brainstem
Describe the internal features of brainstem at various levels
 Blood supply of brainstem
 The anatomical basis of clinical syndromes associated with brainstem
 Brainstem reflex

3. INTRODUCTION
 The Brainstem is formed by the myelencephalon, the metencephalon and
mesencephalon.
 The brainstem is the 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 and the medulla
oblongata is continuous below with spinal cord.
 Brainstem is located in the posterior cranial fossa.

4.  The brainstem, like spinal cord consists of nerve fibers and nerve cells. Most of the
nerve fibers in the brainstem are arranged longitudinally in the form of tracts as in
the spinal cord.
 The nerve cells of the brainstem do not form a single central mass of grey matter as
in the spinal cord; rather most of them are aggregated to form the well-defined
nuclei.
 In addition to the well defined tracts and nuclei, the brainstem consists of diffuse
system of nerve cells and nerve fibers called reticular formation.
 The central canal of spinal cord continues upwards in the lower part of the
brainstem and then it widens and moves dorsally to form the fourth ventricle.

5. EMBRYOLOGY OF THE BRAINSTEM

6. The sulcus limitans divides the neural tube into the dorsal alar plate,
which contains sensory neuroblasts, and the ventral basal plate, which
contains motor neuroblasts. In the brainstem, the sulcus limitans
separates the motor nuclei from the sensory nuclei.

7. The Rule of 4 of the Brainstem
the rule of 4 states that there are 4 midline or medial structures beginning with
M, 4 structures to the side beginning with S, 4 cranial nerves (CNs) in the
medulla, 4 in the pons, and 4 above the pons.
The 4 medial structures are the motor pathway (corticospinal tract), medial
lemniscus (ML), medial longitudinal fasciculus (MLF), and motor nuclei and
nerves.
The 4 lateral (side) structures are the spinocerebellar tracts, spinothalamic tract,
sensory nucleus of CN V, and sympathetic.
The 4 CNs in the medulla are IX, X, XI, and XII; the 4 in the pons are V, VI, VII, and
VIII; and the remainder are above the pons.

8. EXTERNAL ANATOMY OF BRAINSTEM:
(A) The ventral surface :
The rostral limit of the brainstem is demarcated by the optic tracts as they
sweep around to reach the lateral geniculate bodies.
Descending from beneath the optic tracts are the massive cerebral
peduncles.

9. At the upper margin of the
interpeduncular fossa are the mammillary
bodies.
CN III (oculomotor) emerges from the
fossa and runs forward between the
superior cerebellar and posterior
cerebral arteries
At the caudal limit of the interpeduncular
fossa is the junction between the
midbrain and pons
The root of CN V (trigeminal) is attached
laterally at the level of the midpons
The bulge of the anterior pons, due to
fibers of the middle cerebellar peduncle
(MCP)

10.  The furrow of the basilar artery, the basilar sulcus, grooves the pons from below to above.
 At the pontomedullary junction, from medial to lateral, CNs VI (abducens), VII (facial), and
VIII (acoustic) exit.
 The nervus intermedius lies just lateral to the main root of the facial nerve.
 The vestibular division of CN VIII lies medial and slightly rostral to the cochlear division.
 it extends from the pontomedullary junction and the striae medullares above to the
lowermost roots of the hypoglossal nerve and the lowest plane of the pyramidal
decussation—just rostral to the emergence of the highest rootlets of C1 at the level of the
foramen magnum.

11.  Running down the anterior aspect of the medulla are the twin columns of the medullary
pyramids, which contain the corticospinal tracts
 Interlacing bundles of crossing fibers at the caudal most extent of the medulla mark the
decussation of the pyramids.
 Caudal to the decussation is the spinal cord.
 Just lateral to the pyramids in the upper medulla is the oval bulge of the olive, beneath
which lies the inferior olivary nucleus.
 The CN XII (hypoglossal) filaments exit in the gutter between the pyramid and the olive. CNs
IX (glossopharyngeal), X (vagus), and the cranial root of XI (accessory) exit in the retro-
olivary sulcus, in sequence from rostral to caudal.

12. DORSAL SURFACE OF BRAINSTEM:
 The most rostral extent of the brainstem is marked by its junction with the pulvinar of the
thalamus.
 The prominent mounds of the superior and inferior colliculi form the quadrigeminal plate.
 The pineal body extends caudally between the superior colliculi.

 The superior colliculus is connected to the lateral geniculate body by the brachium of the
superior colliculus, and the inferior colliculus to the medial geniculate by its brachium.
 Just caudal to the inferior colliculus CN IV (trochlear) exits

13. The fourth ventricular floor is rhomboid
or diamond shaped and is called the
rhomboid fossa
The superior cerebellar peduncle (SCP)
forms the upper, lateral walls of the
fourth ventricular cavity, and the
inferior cerebellar peduncle (ICP) forms
the walls elsewhere
At the lateral recesses of the ventricle,
near the foramina of Luschka, the
vestibular and cochlear nerves enter
The striae medullares of the fourth
ventricle is a band of myelinated fibers
running across the ventricular floor. The
fibers arise from the external arcuate
nucleus, which lies anterior to the
medullary pyramids, and are bound for
the ICP

14.  Paired midline humps in the ventricular floor rostral to the stria medullares are the facial
colliculi, beneath which are the nuclei of CN VI and the internal genu of CN VII.
 Meridians caudal to the striae are the hypoglossal trigones, beneath which lie the nuclei of
CN XII.
 Lateral to the hypoglossal trigones are the vagal trigones (ala cinerea), beneath which are the
dorsal motor nuclei of the vagus nerves.
 The area postrema (chemoreceptor trigger zone) is a narrow strip along the caudal aspect of
the vagal trigone.
 Far laterally, near the entry zones of CN VIII, are the vestibular areas.

15.  At the caudal tip of the fourth ventricle is the obex, the point at which the fourth ventricle
communicates with the central canal of the spinal cord.
 The shape of the rhomboid fossa at the caudal end of the ventricle resembles a writing pen;
it is referred to as the calamus scriptorius.
 On the dorsal surface caudal to the ventricle are the gracile tubercles in the midline and the
cuneate tubercles just laterally; these merge into the gracile and cuneate fasciculi inferiorly.
 Lateral to the gracile and cuneate tubercles are the ICPs.

16. Brainstem Organization:
 The brainstem, throughout its length, is composed of three parts: tectum (roof),
tegmentum (midportion), and base.
 In the midbrain, the tectum consists of the quadrigeminal plate.
 In the pons and medulla, the tectum devolves into nonfunctional tissue forming the roof
plate of the fourth ventricle, the anterior (superior) medullary velum in the pons, and the
posterior (inferior) medullary velum in the medulla.
 The contents of the tegmentum are variable from level to level and include the CN motor
and sensory nuclei.
 Running throughout the length of the tegmentum is the RF.

18. Reticular Formation:-
 The core of the brainstem is the RF, a loose network of cells and fibers that has extensive
interconnections with other brainstem structures as well as complex, polysynaptic
projections rostrally and caudally.
The RF terminates as the intralaminar nuclei of the thalamus.
 There are three cell populations in the RF: the raphe nuclei, the medial reticular nucleus,
and the lateral reticular nucleus.
 The raphe nuclei are a detached series of individual nuclear groups that lie in the midline
from the rostral midbrain to the caudal medulla.
 All the raphe nuclei send serotonergic projections widely throughout the nervous system.
 the midbrain raphe nuclei project to the hemispheres, those in the pons to the brainstem
and cerebellum, and those in the medulla to the spinal cord.

21.  The lateral reticular nucleus contains small neurons and is primarily afferent. it receives
collateral projections from ascending and descending long tracts.
 The lateral reticular nucleus projects primarily to the medial reticular nucleus. The cells of
the medial reticular nucleus are larger, and these magnocellular neurons send projections
up and down the neuraxis.
 The medial reticular nucleus gives rise to two major descending tracts. The medial
reticulospinal (bulbospinal) tract arises from the medullary nucleus and the lateral
reticulospinal (pontospinal) tract from the pontine nucleus.

22. Brainstem Nuclei and long tracts
.

29. MIDBRAIN ANATOMY
The midbrain is composed of tectum, tegmentum, and base.
The tectum is the quadrigeminal plate and the base the crus cerebri.
There are two segmental levels with different characteristics
(1) Superior Colliculus Level
(2) Inferior Colliculus Level

30. Superior Colliculus Level

31. Superior Colliculus Level
.
 In the tegmentum at this level, the most prominent structure is the red nucleus, which gives
rise to a major descending motor pathway, the rubrospinal tract.
 After decussating, the rubrospinal tract descends in the brainstem and then in the lateral
funiculus of the spinal cord, lying just beside the pyramidal tract; it functions to facilitate
flexor tone.
 The third nerve nuclei lie in the midline anterior to the aqueduct.
 The medial geniculate bodies are located just lateral to the colliculi and important relay
nuclei in the auditory system

32.  At this level, the long ascending sensory tracts lie far laterally.
 The ML, so named because it was in the midline in the medulla, has by now in its ascent
drifted laterally, and been joined by ascending fibers of the anterolateral (spinothalamic)
system and trigeminothalamic tract.
 Lying in the area adjacent to the aqueduct is the SCP, the major efferent pathway from the
cerebellum.
 The gray matter immediately surrounding the aqueduct is one of the characteristic sites for
lesions in Wernicke’s encephalopathy.

33.  Anteriorly, at this level, the base of the midbrain is composed of the cerebral peduncle
which consists of the substantia nigra and crus cerebri.
 The crus cerebri is a direct continuation of the internal capsule and conveys mostly
descending corticospinal and corticobulbar fibers.
 It can be approximately divided into fifths. The lateral fifth contains the
parietotemperopontine tract
 the corticospinal and corticobulbar tracts occupy the middle three-fifths.
 the medial fifth consists of the frontopontine tract.

34. Inferior Colliculus Level:
 The inferior colliculus is a relay station in the auditory pathway; it receives fibers from the
lateral lemniscus and sends fibers to the medial geniculate body through the brachium of
the inferior colliculus.
 The medial geniculate body in turn sends fibers to the auditory cortex.
 In the tegmentum at this level, the most prominent morphologic feature is the
decussation of the SCP .
 The major component of the SCP is the dentatothalamic (dentatorubrothalamic) tract,
which is crossing in the midline, coming from the cerebellum, primarily the dentate
nucleus, en route to the contralateral ventral lateral nucleus of the thalamus, with
collaterals to the red nucleus.

36.  The fourth nerve nuclei lie posteriorly just beneath the aqueduct.
 The fourth nerve takes a highly aberrant course out of the brainstem, curving posteriorly to
decussate in the tectum and exit through the dorsal surface.
 The fourth is the only CN to cross and the only one to exit dorsally.
 The remainder of the tegmentum and base are essentially the same as at the superior
collicular level

37. PONS:-
 At the level of the pons, the tectum consists of the nonfunctional anterior medullary velum.
 The base is rounded and protuberant (the “belly” of the pons) and consists of descending
corticospinal and corticobulbar fibers admixed with crossing pontocerebellar fibers entering
the MCP.

 The tegmentum of the pons contains numerous important structures. The major long tracts
include the medial and lateral lemnisci, the spinothalamic tracts, and the MLF.
 Near the midline in the gray matter lies the nucleus of CN VI, encircled by the fibers of CN
VII. Just within or adjacent to the CN VI nucleus in the pontine paramedian RF lies the
pontine lateral gaze center.

39.  Fibers of CN VI exit anteriorly, in the same manner as fibers of CN III exit the midbrain
into the interpeduncular fossa.
 After looping around the CN VI nucleus, CN VII fibers exit the pons laterally, cross the
cerebellopontine angle (CPA) in company with CN VIII.
 The trigeminal ganglion lies just beside the pons in a depression in the petrous ridge,
called Meckel’s cave.
 A large sensory and a smaller motor root join the ganglion to the pons.

40.  The cochlear component consists of fibers from the organ of Corti and the spiral ganglion of
the cochlea, which synapse in the cochlear nuclei. From the cochlear nuclei a complex,
crossed and uncrossed, ascending pathway with multiple nuclear relays arises.
 auditory fibers eventually ascend in the lateral lemniscus en route to the inferior colliculus,
then to the medial geniculate, and on to the auditory cortex in the temporal lobe.
 The vestibular component consists of fibers from the vestibular ganglion, which synapse in
one of the four vestibular nuclei. Fibers from these nuclei ascend and descend the brainstem
and spinal cord as vestibulospinal tracts and as part of the MLF.

41. Medulla Oblongata:-
 In the medulla, the tectum consists of the posterior medullary velum.
 The velum is continuous inferiorly with the tela choroidea, to which the choroid plexus is
attached, which makes up the caudal part of the ventricular roof.
 The base consists of the medullary pyramids, which are made up of fibers of the
corticospinal tract.
 About 90% of the corticospinal tract crosses to the other side at this level, forming the
decussation of the pyramids, and continues as the lateral corticospinal tract.

42.  The remainder of the corticospinal fibers descend ipsilaterally in the anterior corticospinal
tract and then decussate at the local spinal level.
 At the level of the decussation, the arm fibers lie medial and rostral to the leg fibers; the
arm fibers decussate first and then assume a position medially in the lateral corticospinal
tract in the spinal cord.
 tegmentum of the medulla is conveniently divided into medial and lateral portions,
especially because of differences in their blood supply.
 The medial medulla contains the ML in a vertical midline position (homunculus erect) with
the MLF capping it posteriorly

43.  The hypoglossal nerve nucleus lies in the midline and projects axons that exit anteriorly in
the groove between the pyramid and the olive.
 The olive is a prominent, wrinkled structure lying just posterior to the pyramids. Neurons
in the olive project axons that cross the midline to enter the contralateral ICP.
 The ICP is a prominent structure arising from the lateral aspect of the medulla; it receives
fibers from the ascending spinocerebellar tracts as well as from the olive.
 The lateral medulla contains the spinal tract and nucleus of CN V; nearby run ascending
spinothalamic (anterolateral system) fibers.

44.  The nucleus ambiguus, because of its branchial arch origin, lies deep in the tegmentum
anterolaterally, in a position analogous to that of the CN VII nucleus in the pons.
 It extends from the level of entry of CN VIII at the upper border of the medulla to the level of
the decussation of the ML or even to the beginning of the corticospinal decussation.
 From the nucleus ambiguus, motor fibers exit laterally to enter both the ninth and tenth
nerves.
 The dorsal motor nucleus of the vagus, the autonomic component of CN X, sends fibers
laterally to join the exiting ambiguus fibers.

47. Vascular Supply of brainstem
The large regional arteries of the brainstem have the following three types of branches:
1. The paramedian arteries:
which penetrate the ventral brainstem surface and supply the midline structures.
2. The short circumferential arteries:
which traverse laterally on the brainstem and penetrate its ventrolateral and lateral surfaces.
3. The long circumferential arteries:
which course around the brainstem and supply its posterior structures and cerebellum.

48. Vertebro-basilar System
The vertebrobasilar system consists of the two vertebral arteries (VAs), the basilar artery
(BA), and their branches.
Four VA segments are identified. Only one—the V4 segment—is intracranial.
Normal Anatomy
V1 (Extraosseous) Segment.
VA arises from the isubclavian artery and to enter the C6 transverse foramen.
V2 (Foraminal) Segment:
The V2 segment from C6 to C1 transverse foramen

50. V3 (Extraspinal) Segment :
The V3 segment begins after the VA exits the C1 transverse foramen. to pierce the dura at the
foramen magnum.
The only major V3 branch is the posterior meningeal artery.
V4 (Intradural) Segment :
it courses in front of the medulla.
It gives off small anterior and posterior spinal arteries and medullary perforating branches.
The posterior inferior cerebellar artery (PICA) arises from the distal VA, gives off the
perforating medullary, choroid, tonsillar, and inferior cerebellar branches basilar artery

51. The two VAs unite at or near the pontomedullary junction to form the BA.
The BA courses superiorly in the prepontine cistern, lying between the clivus in front and the
pons behind.
It terminates in the interpeduncular fossa by dividing into the two posterior cerebral arteries.
Numerous small but critical basilar perforating arteries arise from the entire dorsal surface of
the BA to supply the pons and midbrain

52. The first major named BA branch is the anterior inferior cerebellar artery (AICA)
The AICA arises from the proximal BA and courses ventromedially to CNs VII and VIII,
frequently looping into the internal auditory meatus.
It supplies both nerves as well as a relatively thin strip of the cerebellar hemisphere that lies
directly behind the petrous temporal bone.
One or more (usually two to four) superior cerebellar arteries (SCAs) originate from each
side of the distal BA, course laterally below CN III, then curve posterolaterally around the
midbrain just below the tentorium.

53. Blood supply of medulla oblongata
The medulla oblongata receives its blood supply from the anterior and posterior spinal arteries,
the posterior inferior cerebellar artery, and branches of the vertebral arteries.
The blood supply to the medulla may be subdivided into two groups:
( 1) the paramedian bulbar branches
(2) the lateral bulbar branches

56. PARAMEDIAN BULBAR BRANCHES:
The paramedian portion of the medulla (the hypoglossal nucleus and emergent nerve fibers,
the medial longitudinal fasciculus, the medial lemniscus, the pyramids, and the medial part of
the inferior olivary nucleus) are supplied by the vertebral artery.
At lower medullary levels, the anterior spinal artery also contributes to the paramedian zone.
LATERAL BULBAR BRANCHES:
The lateral portion of the medulla is supplied by the intracranial vertebral artery (fourth
segment) or the posterior inferior cerebellar artery.
the basilar artery or the anterior inferior cerebellar artery also contributes

63. Vascular Supply of the Pons
The blood supply to the pons may be divided into three groups:
1) PARAMEDIAN VESSELS
The paramedian vessels (four to six in number) arise from the basilar artery and penetrate
perpendicularly into the pontine parenchyma.
They supply the medial basal pons, including the pontine nuclei, the corticospinal fibers, and
the medial lemniscus.
2) SHORT CIRCUMFERENTIAL ARTERIES
The short circumferential arteries also arise from the basilar artery and enter the brachium
pontis. These vessels supply the ventrolateral basis pontis.

65. 3) LONG CIRCUMFERENTIAL ARTERIES
The long circumferential arteries supply most of the pontine tegmentum and part of the
middle cerebellar peduncles and include the following:
1. The superior cerebellar artery:
arises from the basilar artery near its bifurcation, supplies the dorsolateral pons and
brachium pontis, the dorsal reticular formation, and the periaqueductal region.
2. The anterior inferior cerebellar artery:
most often arises from the basilar artery and supplies the lateral tegmentum of the lower
two-thirds of the pons and the ventrolateral cerebellum.
3. The internal auditory artery:
arises from the anterior inferior cerebellar artery (occasionally from the basilar artery) and
supplies the auditory, vestibular, and facial cranial nerves.

71. PONTINE SYNDROME

79. Vascular Supply of the Midbrain
PARAMEDIAN VESSELS
arise from the origins of the posterior cerebral arteries: supplying the medial peduncles
and the midbrain tegmentum, including the oculomotor nucleus, the red nucleus, and the
substantia nigra

81. CIRCUMFERENTIAL ARTERIES
The circumferential arteries include the following:
 The quadrigeminal arteries (arising from the posterior cerebral arteries): supply the
superior and inferior colliculi.
 The posterior choroidal arteries: supply the cerebral peduncles, the lateral superior colliculi,
the thalamus, and the choroid plexus of the third ventricle
 The posterior cerebral arteries, which also give rise to some mesencephalic branches.
 The superior cerebellar arteries

88. Examination of brainstem
reflexes in coma:
Midbrain and third nerve function are tested
by pupillary reaction to light
pontine function by spontaneous and reflex
eye movements and corneal responses, and
medullary function by respiratory and
pharyngeal responses
Reflex conjugate, horizontal eye movements
are dependent on the medial longitudinal
fasciculus (MLF) interconnecting the sixth
and contralateral third nerve nuclei
Head rotation (oculocephalic reflex) or
caloric stimulation of the labyrinths
(oculovestibular reflex) elicits contraversive
eye movements

89. THANKS