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 4 th

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 4 th 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.