Anatomy lecture 7: pons and midbrain
The pons is the largest part of the brainstem, located above the medulla oblongata and below
the midbrain. It is a group of nerves that function as a connection between
the cerebrum and cerebellum (pons is Latin for bridge).
The pons develops from the embryonic metencephalon (part of the hindbrain, developed from
the rhombencephalon), alongside the cerebellum.
Figure 1 – The three major parts of the brainstem.
Anatomical Position
The pons is a horseshoe-shaped collection of nerve fibers
located in the anterior part of the posterior cranial fossa.
Its anatomical relations are as follows:
Posteriorly – the cerebellum, separated by the fourth ventricle.
Inferiorly – the medulla oblongata.
Superiorly – the midbrain lies immediately above the pons.
External Anatomy
Anterior Surface
The anterior or ventral surface of the pons is marked by a bulging (swelling outward) formed by
the transverse pontocerebellar fibers. These fibers wrap around the vertically oriented
brainstem. It measures around 2.5 cm in adults.
The basilar sulcus marks the midline of the anterior surface of the pons & is where the basilar
artery is located.
The basilar artery arises from the union of the two vertebral arteries at the junction between
the medulla oblongata and the pons between the abducens nerves (CN VI 6). It ascends
superiorly in the basilar sulcus of the ventral pons and divides at the junction of the midbrain
and pons into the posterior cerebral arteries.
Superior surface of pons is related to superior cerebellar artery and lower surface is related
to anterior inferior cerebellar artery.
The superior pontine sulcus separates the pons from the midbrain.
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, The pons can be broadly divided into 2 parts: the basilar part of the pons (ventral pons), and
the pontine tegmentum (dorsal pons).
The pontomedullary junction is an important anatomical landmark defined by the angle
between the lower border of the pons and the superior border of the medulla.
Several cranial nerves originate from the anterior\ ventral surface of the pons:
Cranial nerve V 5: trigeminal – originates from the lateral aspect of mid pons
Cranial nerve VI 6: abducens – originates from the pontomedullary junction, close to the
midline
Cranial nerve VII 7: facial – originates from the cerebellopontine angle, the more lateral
aspect of the pontomedullary junction.
Cranial nerve VIII 8: vestibulocochlear – originates laterally to the facial nerve.
Fig 2 – Ventral (anterior) surface of the pons.
Posterior Surface
The pons is intimately related to the cerebellum
and is connected to it by the middle cerebellar
peduncles. Removal of the cerebellum will reveal
the underlying fourth ventricle.
The floor of the fourth ventricle is composed of
the dorsal surface of the pons and the medulla.
There are some important anatomical landmarks
here:
The medial eminence of the floor of the
4th ventricle marks the midline of the
floor.
The facial colliculus is a bulging formed
by the fibers of the facial nerve looping
around the abducens nucleus.
The stria medullaris of the fourth ventricle is a bundle of nerve fibers crossing
transversely from the lateral aspect into the midline. They mark the posterior border
between the pons and the medulla.
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The pons is the largest part of the brainstem, located above the medulla oblongata and below
the midbrain. It is a group of nerves that function as a connection between
the cerebrum and cerebellum (pons is Latin for bridge).
The pons develops from the embryonic metencephalon (part of the hindbrain, developed from
the rhombencephalon), alongside the cerebellum.
Figure 1 – The three major parts of the brainstem.
Anatomical Position
The pons is a horseshoe-shaped collection of nerve fibers
located in the anterior part of the posterior cranial fossa.
Its anatomical relations are as follows:
Posteriorly – the cerebellum, separated by the fourth ventricle.
Inferiorly – the medulla oblongata.
Superiorly – the midbrain lies immediately above the pons.
External Anatomy
Anterior Surface
The anterior or ventral surface of the pons is marked by a bulging (swelling outward) formed by
the transverse pontocerebellar fibers. These fibers wrap around the vertically oriented
brainstem. It measures around 2.5 cm in adults.
The basilar sulcus marks the midline of the anterior surface of the pons & is where the basilar
artery is located.
The basilar artery arises from the union of the two vertebral arteries at the junction between
the medulla oblongata and the pons between the abducens nerves (CN VI 6). It ascends
superiorly in the basilar sulcus of the ventral pons and divides at the junction of the midbrain
and pons into the posterior cerebral arteries.
Superior surface of pons is related to superior cerebellar artery and lower surface is related
to anterior inferior cerebellar artery.
The superior pontine sulcus separates the pons from the midbrain.
1
, The pons can be broadly divided into 2 parts: the basilar part of the pons (ventral pons), and
the pontine tegmentum (dorsal pons).
The pontomedullary junction is an important anatomical landmark defined by the angle
between the lower border of the pons and the superior border of the medulla.
Several cranial nerves originate from the anterior\ ventral surface of the pons:
Cranial nerve V 5: trigeminal – originates from the lateral aspect of mid pons
Cranial nerve VI 6: abducens – originates from the pontomedullary junction, close to the
midline
Cranial nerve VII 7: facial – originates from the cerebellopontine angle, the more lateral
aspect of the pontomedullary junction.
Cranial nerve VIII 8: vestibulocochlear – originates laterally to the facial nerve.
Fig 2 – Ventral (anterior) surface of the pons.
Posterior Surface
The pons is intimately related to the cerebellum
and is connected to it by the middle cerebellar
peduncles. Removal of the cerebellum will reveal
the underlying fourth ventricle.
The floor of the fourth ventricle is composed of
the dorsal surface of the pons and the medulla.
There are some important anatomical landmarks
here:
The medial eminence of the floor of the
4th ventricle marks the midline of the
floor.
The facial colliculus is a bulging formed
by the fibers of the facial nerve looping
around the abducens nucleus.
The stria medullaris of the fourth ventricle is a bundle of nerve fibers crossing
transversely from the lateral aspect into the midline. They mark the posterior border
between the pons and the medulla.
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