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Anatomy of the pons
1. Anatomy of the Pons
Brief overview of Pontine syndromes
Daniel Vela-Duarte, MD
Department of Neurology
Loyola University Medical Center
June 2012
2. Functional Neuroanatomy
The pons is located between the medulla (caudally) and the midbrain
(rostrally).
The corticospinal tracts are more diffuse in the pons
The medial lemniscus is still situated near the midline
The Spinothalamic tract and the descending hypothalamic fibers continue to
course together in the lateral pons
The lateral lemniscus (An ascending auditory pathway), is lateral and dorsal
to the medial lemniscus.
It carries the bulk of ascending auditory fibers from both cochlear nuclei to the
inferior colliculus of the midbrain.
The medial longitudinal fasciculus (MLF) is located near the midline,
beneath the fourth ventricle.
3.
4.
5. The cerebellum overlies the pons, It is connected by three pairs of cerebellar peduncles.
The fourth ventricle is found between the dorsal surface of the pons and the cerebellum.
The ventral surface of the pons is
dominated by fibers, which form a
large ventral enlargement that
carries fibers from pontine nuclei
to the cerebellum in the middle
cerebellar peduncle.
6. Vascular supply to the Pons
The Pons is supplied by the;
Basilar artery, contributions of this main artery can be
further subdivided;
Paramedian branches, to medial pontine region
Short circumferential branches, supply anterolateral pons
Long circumferential branches, run laterally over the anterior surface of
the Pons to anastomose with branches of the anterior inferior cerebellar
artery (AICA).
Some reinforcing contributions by the anterior inferior
cerebellar and superior cerebellar arteries
7. Blood supply
Additional branches from the
Basilar artery:
Anterior Inferior cerebellar Artery
(AICA), first branch of the basilar
artery
It supplies anterior inferior surface
(Inferior pons)
Superior cerebellar artery
Emerges from the basilar artery,
rostrally.
It supplies cerebellar
cortex, white matter and central
nuclei
8. Blood supply
Labyrynthine artery
Variable in origin, supplies the
inner ear.
Divides into two branches;
a. anterior vestibular
b. common cochlear
•It could emerge from:
Wende et. al., 1975, (sample size of 238)
1. Basilar (16%)
2. AICA (45%)
3. Superior cerebellar (25%)
4. PICA (5%)
5. Remaining 9% were of duplicate
origin
9.
10.
11. Blood Supply
The paramedian branches of the Basilar artery supplies the paramedian
regions of the Pons, including:
corticospinal fibers
the medial leminiscus,
abducens nerve and nucleus (CN VI)
pontine reticular area,
periaquaductal gray areas
12. Blood Supply to the Pons
The paramedian branches
of the Basilar artery
supply
corticospinal fibers,
the medial
leminiscus, abducens
nerve and nucleus (cranial
nerve VI) ,
pontine reticular area,
periaquaductal gray areas
13. Medial Pontine Syndrome/ Middle Alternating
Hemiplegia
Paramedian branches of basilar artery occlusion
Clinical picture Where’s the lesion ?
contralateral hemiplegia of arm (corticospinal fibers in
& leg basilar pons)
contralateral loss/decrease of (medial lemniscus)
proprioception, vibration,
discriminative touch
ipsilateral lateral rectus muscle (abducens nerve fibers or n
paralysis
(paramedian pontine
paralysis of conjugate gaze reticular formation/pontine
toward side of lesion gaze center)ucleus—CN 6)
14. Blood Supply to the Pons
Obstruction of the paramedian pontine arteries will produce a
middle alternating hemiplegia (also termed medial pontine
syndrome)
which is characterized by;
1. Hemiplegia of the contralateral arm and leg, due to damage to
the corticospinal tracts
2. Contralateral loss of tactile discrimination, vibratory and position
sense, due to damage to the medial lemniscus
3. Ipsilateral lateral rectus muscle paralysis, due to damage to the
abducens nerve or tract (can cause diplopia “double vision”)
15. Blood Supply to the Pons
Occlusions of long branches circumferential branches of the basilar
artery produce a lateral pontine syndrome, characterized by;
1. Ataxia, due to damage to the cerebral peduncles (middle and superior)
2. Vertigo, nausea, nystagmus, deafness, tinitus, vomiting, due to
damage to vestibular and cochlear nuclei and nerves
3. Ipsilateral pain and temperature deficits from face, due to damage to
the spinal trigeminal nucleus and tract
4. Contralateral loss of pain and temperature sense from the body,
due to damage to the anterolateral system (spinothalamic)
5. Ipsilateral paralysis of facial muscles and masticatory muscles, due
to damage to the facial and trigeminal motor nuclei (cranial nerves
VII and V)
16. Case # 1.
A 48 year old man, right handed, suffered a sudden weakness of his left arm and leg
which caused him to fall while shaving. He was helped to his feet but his left arm and
leg felt stiff. In addition, he complained of seeing "double".
On exam
normal mental status. There was no evidence of increased intracranial pressure
though his blood pressure was 200/95. There was a spastic paresis with extensor
plantar response in the left extremities and loss of vibratory and positional sense on
the left. The patient walked with an ataxic gait. Pain and temperature sensations were
normal.
There was diplopia when the patient looked toward the right side.
At rest , the right eye deviated toward the nose (internal strabismus or squint) while
the left eye looked straight ahead.
There was a paralysis of conjugate gaze toward the right (i.e, the right eye did not
move laterally toward the right though the left eye did)
Ocular convergence was normal.
Temple University School of Medicine's Department of Anatomy and Cell Biology
17. Case # 2
A 55 year old man was brought to the hospital after suddenly
falling to the ground unable to move his right arm and leg.
The neurologic exam revealed that the limbs on the right side
had markedly diminished strength, heightened deep tendon
reflexes, ankle clonus, Babinski and increased resistance to
passive stretch. The left arm and leg had near normal
strength but performed in an uncoordinated manner on the
finger-to-nose test and the heel-to-shin test.
Cranial nerve examination was significant in that the upon
smiling the man did not elevate his mouth on the right side
and could not blow out his right cheek; he could tightly close
his eyelids on both sides.
Temple University School of Medicine's Department of Anatomy and Cell Biology
The cerebellum overlies the pons, It is connected by three pairs of cerebellar peduncles. The fourth ventricle is found between the dorsal surface of the pons and the cerebellum. The ventral surface of the pons is dominated by fibers, which form a large ventral enlargement that carries fibers from pontine nuclei to the cerebellum in the middle cerebellar peduncle.
Though spastic paresis indicates involvement of the pyramidal tracts from the cerebrum on down, in this case, because of the 6th nerve injuryat the level ofthe pons is indicated. In the pons the pyramidal tracts are in the basis pontis, and in this case the side opposite the weakness, i.e., the right side. Ataxic gait, vibratory and positional deficits on the left suggest injury to the medial lemniscus, which lies near the midline in the ventral tegmentum, on the right. The ataxia could also have a cerebellar component due to injury of the basis pontis and the pontine nuclei. Normal pain and temperature perception indicate that the lesion was more limited to the midline rather than lateral where the spinothalamic and 5th nerve components lie. Gaze paralysis to the right and internal strabismus of the right eye indicate weakness of the right lateral rectus and injury to the fibers of the right abducens nerve. If the 6th nucleus had been involved the medial rectus of the left eye would have shown signs as well, due to involvement of the nearby PPRF. Since convergence was preserved and only the lateral rectus of the right eye was paralyzed this was a lesion involving only the fibers of the 6th nerve.This constellation of symptoms is consistent with the midline distribution of the paramedian branches of the basilar artery and occlusion of its branches in the caudal pons.
