Ch8.
Brainstem
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Dr. EMAD ABU ALRUB MD PhD
AAUP
Dr. Emad Abu Alrub MD PhD
BY WAJIHA NASEER
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Textbook of clinical neuroanatomy
Brainstem
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
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oblongata, pons, and midbrain.
The midbrain is continuous above with the cerebral hemispheres
and the medulla oblongata is continuous below with spinal cord.
Dr. Emad Abu Alrub MD PhD
Brainstem 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.
The brainstem consists of diffuse system of nerve cells and
nerve fibres called reticular formation. 2
The brainstem serves the following four major functions:
1. It provides passage to various ascending and descending
tracts that connect the spinal cord to the different parts of the
forebrain.
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2. It contains important autonomic reflex centres (vital
centres) associated with the control of respiration heart rate
Dr. Emad Abu Alrub MD PhD
and blood pressure.
3. It contains reticular activating system which controls
consciousness.
4. It contains important nuclei of the last ten cranial nerves (i.e.
IIIrd to XIIth).
N.B. The bilateral destructive lesions of the brainstem are fatal because
of damage to ‘vital centres’ which control respiration, heart rate, and blood 3
pressure. The impairment of reticular activating system leads to progressive
loss of consciousness, followed by stupor and coma.
Medulla Oblongata
The medulla oblongata is the direct upward continuation of
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the spinal cord, extending from foramen magnum to the
lower border of the pons.
Dr. Emad Abu Alrub MD PhD
It forms the lowest part of the brainstem and lies almost
vertically in the anterior part of the posterior cranial fossa
between the clivus (superior surface of the basi-occiput) in
front and the vallecula of the cerebellum behind.
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The medulla is shaped like a truncated cone (bulb-like)
hence its alternative name ‘bulb’ (cf/confer/compare. bulbar
paralysis).
Medulla oblongata measures about 3 cm in length, 2 cm in
breadth (at the widest part) and 1.25 cm in thickness).
The medulla contains vital centres which are essential for
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life. These are:
a. cardiac centre,
Dr. Emad Abu Alrub MD PhD
b. vasomotor centre,
c. respiratory centre.
Medulla provides attachment to last four cranial nerves.
The lower part of the medulla like the spinal cord contains
the central canal.
In the upper part of the medulla this canal widens and moves
dorsally to form the lower part of the fourth ventricle. Thus,
the medulla is divided into a lower closed part and an upper 5
open part.
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External Features
The medulla is divided into right and left symmetrical halves by
anterior median fissure and posterior median sulcus (Fig. 8.1).
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Dr. Emad Abu Alrub MD PhD
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FIG. 8.1 External features on the anterior (ventral) aspect of the
brainstem. Note the attachment of last cranial nerves.
Medulla Oblongata External Features:
The anterior median fissure is continuous below with the
corresponding fissure on the spinal cord, and above it ends into
a small triangular depression called foramen caecum, at the
lower border of the pons.
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It is interrupted in its lower part by the bundles of fibres
crossing obliquely from one side to the other, the decussation
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of pyramids.
The posterior median sulcus continues below with the
corresponding sulcus of the cord and is present only in the
lower half of the medulla. Above its lips diverge to form the
boundaries of a triangular area, the lower part of the floor of
fourth ventricle.
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Continued..
Medulla Oblongata External Features:
Each half of the medulla is marked by two sulci—
anterolateral and posterolateral, which are direct upward
continuations of the corresponding sulci of the cord.
The anterolateral sulcus extends along the lateral border of the
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pyramid and along it emerges the rootlets of the hypoglossal
(XII cranial) nerve.
Dr. Emad Abu Alrub MD PhD
The posterolateral sulcus lies between the olive and the inferior
cerebellar peduncle and along it emerges the rootlets of
glossopharyngeal (IX), vagus (X) and accessory (XI)
cranial nerves.
The anterolateral and posterolateral sulci with nerve roots
divide the surface of each half of the medulla oblongata into
anterior, posterior and lateral regions like that in the spinal
cord. 9
FIG. 8.2 Showing
decussation of
pyramidal
(corticospinal) tracts
in the lower part of
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medulla oblongata
and location of
lateral and anterior
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corticospinal tracts
in the spinal cord. (P
= pyramid.)
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Features on the anterior (ventral) aspect of medulla
(Fig. 8.1)
The ventral aspect of medulla presents following features:
1. Pyramids. These are two elongated elevations, one on either
side of anterior median fissure and are produced by the
corticospinal (pyramidal) fibres.
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2. Olives. These are oval elevations, posterolateral to the
pyramids and are produced by an underlying mass of grey
matter called inferior olivary nucleus.
Dr. Emad Abu Alrub MD PhD
3. Rootlets of the hypoglossal nerve. These emerge from the
anterolateral sulcus between the pyramid and the olive.
4. Inferior cerebellar peduncles. These are thick bundles of
fibres lying posterolateral to the olive, and attach the
medulla with the cerebellum.
5. Rootlets of the IXth, Xth, and XIth (cranial part) cranial
nerves. These emerge through the posterolateral sulcus 11
separating the olive from the inferior cerebellar peduncle.
Features on the Posterior (Dorsal) Aspect of Medulla
(Fig. 8.3)
The dorsal aspect of the medulla is well demarcated into lower
closed and upper open parts.
Features of the closed part
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The closed part, on either side of posterior median sulcus,
presents three longitudinal elevations.
Dr. Emad Abu Alrub MD PhD
From medial to lateral these are: fasciculus gracilis, fasciculus
cuneatus, and inferior cerebellar peduncle.
Features of the open part
The open part of the medulla forms the lower part of the floor of
fourth ventricle.
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FIG. 8.3 External features on the posterior (dorsal) aspect of the brainstem.
Note the features in the floor of the fourthventricle.
Internal Structure of medulla oblongata
As in the spinal cord, in the medulla also, the white matter
surrounds the central mass of grey matter but in the latter,
the grey matter is broken up into nuclei that are separated
by nerve fibres.
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The internal structure of medulla is well appreciated by
examining its transverse sections at the following four
levels:
Dr. Emad Abu Alrub MD PhD
• At the level of decussation of pyramids.
• At the level of decussation of medial lemnisci.
• At the level of the olives.
• At the level just inferior to the pons.
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Dr. Emad Abu Alrub MD PhD
FIG. 8.4 Transverse section through the lower closed part of the
medulla oblongata at the level of pyramidal decussation.
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Dr. Emad Abu Alrub MD PhD
FIG. 8.5 Transverse section of medulla oblongata at the level of sensory
decussation. 16
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Dr. Emad Abu Alrub MD PhD
FIG. 8.6 Transverse section of medulla at the level of olives: 1. medial
longitudinal fasciculus, 2. tectospinal tract, 3. medial lemniscus. (NA =
nucleus ambiguus). 17
Blood Supply of the Medulla
The medulla is supplied by the following arteries:
1. Two vertebral arteries.
2. Anterior and posterior spinal arteries.
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3. Anterior and posterior inferior cerebellar arteries.
4. Basilar artery.
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Pons
The pons is the middle part of brainstem and situated between
the midbrain and the medulla.
It lies in the posterior cranial fossa on the clivus (formed by
the fusion of basi-sphenoid and basi-occiput) anterior to the
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cerebellum.
On either side, the pons is continuous as the middle cerebellar
peduncle, thus forming a bridge between the two cerebellar
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hemispheres, hence its name, pons (L. pons = bridge).
Pons is about 2.5 cm long and forms the upper half of the
floor of the fourth ventricle.
Structurally the pons consists of grey and white matter
containing number of nuclei and fibre tracts respectively.
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Internal Structure
A cross-section at any level of pons shows two distinct
regions:
1. A large ventral or basilar part, and
2. A small dorsal or tegmental part.
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The ventral or basilar part is continuous inferiorly with the
pyramids of the medulla and on each side with the middle
Dr. Emad Abu Alrub MD PhD
cerebellar peduncle.
The dorsal or tegmental part is a direct upward
continuation of the medulla excluding the pyramids.
The structure of basilar part is basically the same at all levels
of pons, but the structure of tegmental part vary considerably
in the upper and lower parts of the pons. 21
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Dr. Emad Abu Alrub MD PhD
Blood Supply of the Pons
The pons is supplied by the following arteries:
• Numerous (pontine) branches from the basilar artery.
• Anterior inferior cerebellar artery.
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Midbrain
The midbrain is the upper and shortest part of the brain-stem. It
is about 2.5 cm long and 2.5 cm wide.
It connects the hindbrain with the forebrain. Its cavity, the
cerebral aqueduct (aqueduct of Sylvius) connects the third
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ventricle with the fourth ventricle.
The midbrain passes through the tentorial notch.
Dr. Emad Abu Alrub MD PhD
It is related on each side to the optic tract, parahippocampal
gyrus, posterior cerebral artery, and basal vein;
anteriorly to the interpeduncular structures, viz. mammillary
bodies, tuber cinereum, etc.;
posteriorly to the splenium of corpus callosum, great cerebral
vein, pineal body and posterior ends of right and left thalami. 23
The midbrain like the other parts of the brainstem consists of
grey and white matter.
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Internal Structure
The aqueduct divides the midbrain into two parts:
A small posterior part and,
A Large anterior part (Fig. 8.11).
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The small posterior part is called tectum and consists of four
colliculi.
Dr. Emad Abu Alrub MD PhD
The large anterior part is divided into two equal right and left
halves by a vertical plane, the cerebral peduncle.
Each cerebral peduncle is further subdivided into three parts,
from dorsal to ventral these are:
1. Teg-mentum,
2. Substantia nigra, and
3. Crus cerebri.
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FIG. 8.11 Transverse section of the midbrain showing its main subdivisions.
Blood Supply of the Midbrain
Arterial supply
The midbrain is supplied by the following arteries:
Basilar artery through its posterior cerebral and
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superior cerebellar arteries. Basilar artery also
supplies mid-brain through direct branches.
Dr. Emad Abu Alrub MD PhD
Branches of posterior communicating and anterior
choroidal arteries.
Venous drainage
The veins of midbrain drains into the great
cerebral and the basal veins.
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