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AL-Qassim University- College of Medicine
Nervous system & special sense block
(2014-1435)
Anatomy of The Brain Stem
Part 1:
Prepared by
Dr / Amani
Almallah
Brain stem
Brain stem is continuous below with the spinal cord & above with diencephalon
(thalamus & structures related).
It consists of three parts from below upwards;
1- Medulla Oblongata
2- Pons
3- Midbrain: its
cavity is the cerebral
Aqueduct.
Rhombencephalon
Medulla Oblongata
It is the lowest part of the brain stem. It continuous with the pons superiorly &
with the spinal cord inferiorly opposite the upper border of the posterior arch of
atlas (at the level of the foramen magnum). Just above the attachment of the
first cervical nerve root.
Function of medulla:Nuclei in the medulla are associated with autonomic
control, cranial nerves, and motor/sensory relay.
I- Autonomic nuclei: 1- Respiratory rhythmicity centers.
2- Cardiovascular centers: Cardio-inhibitory/cardio-acceleratory centers.
3- Centers concerning digestive Activities as (Vasomotor center alters the tone
of vascular smooth muscle).
4- It contains relay stations and reflex centers.
Additional Centers: Emesis, deglutition, coughing, hiccupping, and sneezing.
External features of Medulla
The lower 1/2 of the medulla contains an upward extension from the central
canal so called (closed medulla) & its upper ½ related posterioly to the 4th
ventricle so called (open medulla).
A- Anterolateral surface
1- Anterior median fissure: it is an upwards extension of the anterior median
fissure of the spinal cord. In the lower medulla; this fissure is obliterated by
Pyramidal (motor) decussation.
2- Pyramid: an elongated elevation on each side of the anterior median fissure.
Pyramid is formed by pyramidal tract (corticospinal) fibers before decussation.
fibers decussate to form (motor decussation).
N.B: pyramid is related to two cranial nerve (hypoglossal lateral to it &
abducent above it) .
3- Anterolateral fissure: A groove lateral to the pyramid, between it & the olive. It
gives off (exit) of rootlets of hypoglossal N.
4- Olive: an elevation formed by bulging of inferior olivary nucleus (which is one
of extra-pyramidal center).
5- Posterolateral fissure: a groove between the olive & inferior cerebellar
peduncle. Gives exit to the glossopharyngeal, vagus & cranial accessory 9th,
10th, & cranial part of 11th ) from above downwards.
4- Inferior cerebellar peduncle: it is the ridge lateral to cuneate tract
posteriorly & also lateral to the olive anteriorly. It passes upwards & laterally
deep to the middle cerebellar peduncle connecting the medulla with cerebellum.
ICP forms the infrolateral boundary of the 4th ventricle.
B- Posterior surface
I- posterior surface of lower part (closed medulla):
Continuous with the posterior surface of the spinal cord & presents the
following features from medial to lateral:
1- Posterior median fissure: it is an upwards extension of the posterior median
sulcus of the spinal cord. It extends upwards to the lower angle of the 4th ventricle.
2- Gracile tract & nucleus: forming a longitudinal lateral to the sulcus (it
overlies gracile tracts) . It ended above by an elevation called Gracil tubercle
(overlies the gracil nucleus).
3- Cuneate tract & nucleus: a longitudinal elevation lateral to the Gracile ( as
it overlies cuneate tracts) ending above by oval expansion called cuneate
tubercle which overlies the cuneate nucleus.
Lower 4 cranial nerves are attached to the medulla:
1- Rootlets of Hypoglossal nerve (XII) emerge in groove between
Pyramide & olive.
2- Rootlets of the glossopharyngeal (IX), vagus (X), cranial part of
accessory (XI) arranged from above downwards appear in the groove
between olive & ICP.
I- posterior surface of Upper part (opened medulla):
Forms the lower part of the floor of the 4th ventricle, separated from the
posterior surface of the pons by transverse fibers called Stria medullares fibers
OR (medullary stria).
Posterior surface of the opened medulla
1- A median longitudinal fissure: upwards extension of the posterior median
sulcus of the spinal cord.
2- An inverted V-shaped depression called Inferior Fovea. The area between
the two limbs of V is called vagal trigone (overlies the dorsal vagal nucleus).
3- Hypoglossal trigone: medial to the inferior fovea (overlies hypoglossal
nucleus).
4- Vestibular trigone: lies lateral to the inferior fovea (overlies inferior
vestibular nucleus).
Blood Supply to the Medulla: The Medulla is supplied by :
1- Posterior inferior cerebellar artery:supplies lateral medullary areas. It
supply spinothalamic tract, spinal trigeminal nucleus and tract, fibers from the
nucleus ambiguous, dorsal motor nucleus of the Vagus (cranial nerve X) &
inferior cerebellar peduncle. So Occlusion of the posterior inferior cerebellar
artery will produce a lateral medullary syndrome or Wallenberg’s syndrome
2- Anterior spinal artery: sends blood to the paramedian region of the closed
medulla.
3- Posterior spinal artery: supplies rostral areas, including the gracile and
cuneate fasiculi and nuclei, along with dorsal areas of the inferior cerebellar
peduncle.
4- Vertebral artery: supply areas of both upper & lower part of medulla.
Pons
It is a bridge wedged between the midbrain & medulla.
Contains: 1- Sensory and motor nuclei for 4 cranial nerves
Trigeminal (5), Abducent (6), Facial (7), and Vestibulocochlear (8)
2- Respiratory nuclei:Apneustic & pneumotaxic centers work with the medulla
to maintain respiratory rhythm.
3- Nuclei & tracts that process and relay informations to & from the
cerebellum
4- Contains many tracts carrying signals:
from cerebrum to cerebellum & medulla up to thalamus
between right and left hemispheres of cerebellum from brainstem to cerebellum
5- Cranial attachments of CN V, VI, VII, VIII
Features of the external surface of pons
I- Anterior surface
1- Basilar groove: longitudinal groove in the middle line lodge the basilar
artery.
2- Transverse Streaks: on each side of the basilar groove, produced by
transverse ponto-cerebellar fibers that collected on either side to form middle
cerebellar peduncle.
3- Trigeminal nerve: arise by large sensory & small motor root from the lateral
part of the anterior surface.
4- Middle cerebellar peduncle (MCP):
A thick bundle formed of ponto-cerebbellar fibers which collect at the lateral
part of the pons. It turns backwards to sink into the corresponding cerebellar
hemisphere.
Pons
5- Abducent nerve: emerge from the lower border of the pons near the middle
line (between the pons & pyramid of medulla).
6- Facial (7) & vestibulo-cochlear (8):
emerges from the ponto-cerebellar angle which is
a triangular space between:
Lower border of the MCP……….. Above
Upper border of medulla …………. Below
The cerebellum ……………………. Behind.
II- Posterior surface of the pons
Forms the upper part of the floor of the 4th ventricle &
present the following features:
1- Median longitudinal sulcus: in the middle line.
2- Medial eminence: a longitudinal elevation on either side of the median sulcus.
3-Facial colliculus: it is a Knob-like elevation in the lower part of medial
eminance . It is produced by abducent nucleus and fibers of facial nerve looping
around the abducent nucleus.
Medial eminence is bounded laterally by sulcus limitans. The sulcus limitans is
markedly depressed at the level of facial collicullus to form superior fovea.
4- Superior fovea separating the medial eminence from a the vestibular area.
5-The vestibular area indicates the position of vestibular nuclei.
6- Middle cerebellar peduncle:
Thick bundle is formed by a
collection of Transverse pontine
fibers.It emerges from the lateral
aspect of the pons, then curves
backwards to enter the corresponding
cerebellar hemisphere.
Mid brain
It is the shortest part of the brain stem, extends between the pons inferiorly & the
diencephalon superiorly. It occupies the tentorial notch of the tentorium cerebelli.
The mid brain has a narrow canal cerebral Aqueduct of Sylvius. This canal connects the
3rd ventricle above with the 4th ventricle below. An imaginary line passing across the
duct divides the midbrain into ventral & dorsal parts.
In transverse section the midbrain is divided into:
1- Basis peduncle (called crus cerebri) two thick pillar forms the ventral part.
2- Tectum: the posterior part of the midbrain has marked by 4 elevations (Corpora
quadrigemina): 1- Two superior colliculi: concerned with visual reflexes
1.Two inferior colliculi: they are reflex center of hearing.
Oculomotor nerve emerges from the medial side of the cerebral peduncle while the
Trochlear nerve emerges posteriorly, winding laterally around the lateral side of the
cerebral peduncle on each side.
Brain Stem
•Blood supply
•Arterial :
- Medial part of crus cerebri and tegmentum by branches of basilar and post.
cerebral artery.
-Lateral part of crus and tegmentum by post. And superior Cerebellar arteries.
-The colliculi by post. Cerebral and sup cerebellar arteries.
•Venous: veins pass to basal vein or great cerebral vein of Gallen.
•Applied anatomy:
•Weber ‘ s syndrome: lesion affecting the cerebral peduncle and oculomotor
fibers that results in ipsilat. LMNL of oculomotor and contralateral hemiplegia.
•Benedikt s syndrome: oculomotor + red nulcleus .
Fourth ventricle
It is the cavity of the hind d brain.
It is a diamond (rhomboid) shaped space when seen from behind & tent-shaped when
seen from the side between:
The posterior surface of pons superiorly & upper part of medulla inferiorly
The cerebellum posteriorly..
It has : -- floor anteriorly
-- Roof posteriorly
-- 4 lateral boundaries (where the roof & floor meet).
-- 4 angles (superior, Inferior & 2 lateral angles).
Roof: is formed by the superior medullary velum superiorly & inferior medullary velum
inferiorly and inferior vermis of the cerebellum in the middle.
(The velum is a thin layer of pia & ependyma bridging the space between the 2
peduncles).The lower part of the roof is invaginated by Tela choroidae (which is a
double layer of pia mater encloses the choroid plexus of 4th ventricle).
The upper part of both superior cerebllar peduncles encroach medially & share
in the roof.
Floor: is formed by posterior surface of pons & upper part of medulla separated
from each other by medullary stria.
I- Upper boundary is formed by superior cerebellar peduncle (SCP) & superior
medullary velum.
II- Inferior boundary is formed by inferior cerebellar peduncle (ICP), gracile
& cuneat tubercles on each side & inferior medullary velum.
Laterally: upper part is formed by superior cerebellar peduncle & lower part by
gracile & cuneate tubercles and inferior cerebellar Peduncle.
The 4 Angles of the 4th ventricle:
1- Superior angle: where the 4th ventricle is continuous above with the cerebral
aqueduct.
2- Inferior angle: where the 4th ventricle is continuous above with the central
canal of the closed medulla
3- Two lateral angles :which extends laterally to form lateral recess.
Apertures:
The roof contains three apertures which transmit the CSF from the 4th ventricle
to subarachnoid space
1.One Median aperture (Foramen of Magendi): in the lower most part of the
roof of 4th ventricle in the median plane. It opens into cerebellomedullary
cistern of subarachnoid space.
2.Two Lateral apertures (Foramena of Luschka): they are one on each side in
the lateral. Choroid plexus partly protrudes out through the lateral aperature.
Locus ceoruleus
A collection of pigmented neurons (contain melanin) lies subjected to the upper
part of the floor of the 4th ventricle closely ventral to the mesencephalic
nucleus of V (close to lateral side of uppermost part of medial eminence).
Their axon are widely distributed in diencephalon, telencephalon, cerebellum
Their cells produce adrenaline.