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Anatomy of brain stem and associated lesions:
PONS and MIDBRAIN
Brain stem
• Underneath the limbic system is the brain stem.
• This structure is responsible for basic vital life functions such as
breathing, heartbeat, and blood pressure.
• The brain stem is made of
– Midbrain
– Pons
– Medulla.
Pons
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Forepart of the hind-brain
Situated in front of the cerebellum
Behind and below, the pons is continuous with the medulla
oblongata, but is separated from it in front by a furrow in which the
abducent, facial, and acoustic nerves appear.
Associated with cranial nerves:
– Trigeminal (V) chewing
– Abducens (VI) movement of eyeballs
– Facial (VII)- facial expression; secretion of saliva and tears
– Vestibulcochlear (VIII) –balance, equilibrium, hearing
Ventral or Anterior surface of pons
• Basilar sulcus
Midline sulcus occupied by basilar artery
• Transverse running fibres on the surface: pontocerebellar fibres
which continues as Middle cerebellar peduncle.
• Trigerminal nerve emerge at lateral part of pons
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Dorsal or Posterior surface of Pons
• Hidden by cerebellum
• Bounded laterally by superior cerebellar
peduncle
• Median sulcus in the midline
• Facial colliculus
– Paramedian elevation raised by
underlying abducent nerve covered by
winding fibres of facial nerve.
• Trochlear nerve
Internal structure of pons
• Basilar or ventral portion
Contains both longitudinal and transverse
fibers intermixed with pontine nuclei
• Dorsal tegmental portion Continuation of the
reticular formation of the medulla oblongata
• Structures present at the level of facial
nucleus.
• CN VI nucleus – Abducens nerve
• CN VII nucleus – Facial nerve
• CN VIII – Vestibular Nuclei
Pure sensory location lateral
• Structures present at the level of
facial nucleus.
• CN VI nucleus – Abducens nerve
• CN VII nucleus – Facial nerve
• CN VIII – Vestibular Nuclei
Pontine nuclei
Project to cerebellum
Continuation of
arcuate nuclei (over
pyramids)
•
Level of
facial nucleus
(CN VII)
– Pure sensory location lateral
CN VIII – Vestibular Nuclei
Balance
4th ventricle
Connection of pons to cerebellum
Restiform body (inf. cerebellar
peduncle)
Middle cerebellar peduncle
Medial lemniscus Ascending 2nd
order sensory neurons
Descending upper motor neurons
CN VI nucleus = Abducens nerve
“somatic efferent” nerve that controls the
movement of a single muscle, the lateral rectus
muscle of the eye.
Abduction of eye
Interna CN VII nucleus – Facial nerve
the muscles of facial expression, and
lcontrols
functions in the conveyance of taste sensations
structu
from the anterior two-thirds of the tongue and oral
cavity
re
of
pons
• Structures present at the level of middle
cerebellar peduncle.
•Level of middle cerebellar peduncle
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Medial lemniscus
Lateral lemniscus
Trapezoid body
Trigerminal nucleus (sensory and motor)
Pontine nuclei
At the level of mid pons
Lateral
lemniscus
4th
ventricle
Medial
lemniscus
fibers from
dorsal column
Middle cerebellar
peduncle
Trapezoid body – transverse
fibers in pontine tegmentum
Trigerminal
tract
The pontine nuclei are a part of the pons which
store the memory of intention during motor
activity
Corticospinal
tract,
corticobulbar
tract,
corticopontine
fibers
Cranial Nerve of the Mid Pons
CN V
Motor
trigeminal
nucleus
4th
ventricle
Trigerminal
fascicles
Principal
trigerminal
Sensory
nucleus
Trigerminal
nerve
•Level of locus ceruleus
Internal structure of pons
• Structures present at the level of upper
pons
• MLF
• Superior cerebellar peduncle
• Locus ceruleus
• Parabrachial nucleus
• Pediculopontine nucleus
Upper
Pons
Periaqueductal gray
matter
4th
ventricle
MLF
Locus
ceruleus
Decussation
Superior
cerebellar
peduncle
Transverse pontocerebellar fibers
Midbrain
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Landmarks
Cerebral peduncles
Optic chiasm
Interpeduncular fossa
(Superior colliculi)
(Inferior colliculi)
(Superior cerebellar peduncle)
Cranial Nerves
III, IV
Midbrain is divided at the level of
cerebral aqueduct into
Dorsal portion called tectum which
largely consists of inferior and superior
colliculi
Ventral portion, known as tegmentum
Descending
upper motor
neurons
External Structure of Midbrain
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Optic chiasm 1
Interpeduncular fossa 2
Oculomotor nerve (CN III) 3
Trochlear nerve (CN IV) 4
Pons 5
Cerebral peduncles (crus cerebri) 6
Cranial Nerves of the Midbrain
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Anterior exit
CN III (1)
CN VI (5)
Posterior exit
CN IV (2)
MLF - Medial longitudinal fasciculus (7)
Vestibular nuclei (6)
Pons (3)
Internal divisions of
midbrain
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Tegmentum is bounded ventrally by the
massive fibre system of the crus cerebri
The term cerebral peduncle is sometimes
used as a synonym for crus cerebri
Cerebral peduncle refers to the whole
midbrain on either side excluding the tectum
Ventral to the colliculi the cerebral aqueduct
runs the length of the midbrain
Surrounding the aqueduct is a pear shaped
arrangement of grey matter called periaqueductal grey
Substantia nigra
• These neurones project to the caudate nucleus and
putamen of the basal ganglia in the forebrain
• Degeneration of the pars compacta of the substantia nigra
is associated with Parkinson’s disease
• Other non pigmented subdivision of the substantia nigra is
called the pars reticulata
• Pars reticulata is considered to be a functional homologue
of the medial segment of the globus pallidus which is also
part of the basal ganglia
Internal Structure of Midbrain
• Cross section at the level of inferior
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colliculus
CN IV Trochlear nerve
MLF
Dorsal raphe nucleus
Substantia nigra
Mesencephalic nucleus of V
•Level of inferior colliculus
Lower Midbrain
Mesencephalic nucleus of V
analogous to dorsal root
ganglion
but within CNS
Cerebral
aqueduct
Inferior
colliculus
CN IV
Trochlear nerve
MLF
Substantia
nigra
Basis
peduncularis
Crus
cerebri
(cerebral
peduncle)
Caudal part of
midbrain
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In the caudal part of the midbrain
the inferior colliculus constitutes
part of the ascending acoustic
projection
Ascending auditory fibres run in
the lateral lemniscus which
terminates in the inferior colliculus
Efferent fibres from the colliculus
terminate in the medial geniculate
nucleus of the thalamus
This nucleus projects to the
auditory cortex of the temporal lobe
Internal Structure of
Midbrain
• Cross section at the level of superior
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colliculus:
Medial geniculate body
Superior colliculus
Crus cerebri (cerebral peduncle)
Red nucleus –Controls arm movement
Substantia nigra
•Level of superior colliculus
Upper midbrain
Medial geniculate
body
hearing
Red nucleus – relay from cortex
and cerebellum to spinal cord,
inferior olive, reticular
formation, cerebellum
Controls arm movement
Superior
colliculus
Substantia
nigra
Crus
cerebri
(cerebral
peduncle)
Cranial Nerves of Upper Midbrain
CN III Oculomotor
nucleus
Edinger Westfal nucleus
Parasympathetic to ciliary
ganglion
Pupillary sphincter ciliary
muscles
MLF
Rostral part of midbrain
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The superior colliculus of the
rostral area of the midbrain is
part of the visual system
Its main afferents are
corticotectal fibres originating
from the visual cortex of the
occipital lobe and from the
frontal eye field of the frontal
lobe
These inputs are concerned with
controlling movements of the
eyes
Pontine hemorrhage
• Pons supplied by basilar artery, anterior,
inferior and superior cerebellar arteries.
• If hemorrhage is unilateral
– Facial paralysis on the side of lesion
(facial nerve nucleus)
– Paralysis of limbs on opposite side
(corticospinal fibres)
– Paralysis of conjugate ocular deviation
(abducent nerve nucleus)
• If bilateral
– Pinpoint pupils (ocular sympathetic fibres)
– Bilateral paralysis of face and limbs
– poikilothermia
Tumors of pons
• Astrocytoma of pons, most common tumor of brainstem
Blockage of cerebral aqueduct
• Cerebral aqueduct is one of the narrowest part of
ventricular system.
• In congenital hydrocephalus, or tumors of midbrain,
aqueduct may be blocked which blocks the flow of CSF.