Download 07. Pons Internal Features 0102010-10-01 05:141.9

• Pons is divided into
 Basal (ventral, basilar) part
 Tegmental (dorsal) part
4th V
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B
B
Ventral (Basal) Pons
• The basal part of
the pons consists of:
 Longitudinal fiber
bundles
 Transverse fiber
bundles
 Pontine nuclei,
collection of
neurons that lie
among the
bundles.
TF
LF
TF
PN
LF
• The longitudinal fibers are
descending fibers that enter the
pons from the midbrain.
• They are composed of:
LF
• corticospinal (pyramidal tract)
• corticobulbar &
• corticopontine fibres
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LF
• Corticospinal fibers
 originate in cerebral cortex and make synapses with motor
neurons of contralateral ventral horns of spinal cord at all
levels,
 are numerous, form small separate bundles, at rostral levels
but many coalesce at caudal levels and reassemble as the
pyramids of the medulla.
• Corticobulbar fibers
 originate in cerebral cortex and make synapses with cells of
motor nuclei of the cranial nerves (3,4,5,6,7,12) in brain stem.
• Corticopontine fibers
 originate in cerebral cortex and make synaptic contacts with
cells of the pontine nuclei of the same side (ipsilateral).
TF
• The transverse fibers,
composed of pontocerebellar
fibers, originate in pontine
nuclei, cross the midline, run
laterally, converge to form
the middle cerebellar
peduncle (brachium pontis)
and enter into the
contralateral cerebellar
hemispheres
MCP
TF
• The pontine nuclei are
small groups of cells
scattered among the
longitudinal and
transverse fasciculi.
• The axons of the neurons
of the pontine nuclei
cross the midline, form
the transverse bundles of
pontocerebellar fibers,
and enter the cerebellum
through the middle
cerebellar peduncle.
PN
• The activities of the cerebral cortex are thus connected
to the cerebellar cortex through relay in the pontine
nuclei. (Corticopontine-pontocerebellarcerebellodentate-dentatothalamic-thalamocortical)
• This circuit contributes to the precision and efficiency
of voluntary movements.
Dorsal Pons (Tegmentum)
• The pontine tegmentum is
structurally similar to the medulla
& midbrain and contains:
• Ascending & descending fibers:
medial, spinal, trigeminal and
lateral lemnisci, ventral
spinocerebellar tract, medial
longitudinal fasciculus, and
superior & inferior cerebellar
peduncles
• Cranial nerves nuclei:
vestibulocochlear, facial, abducent
and trigeminal nerve nuclei
Medial lemniscus: twists as it leaves the
medulla, and lies horizontally in the ventral
pontine tegmentum. The fibers from the
cuneate nucleus are medial to those from
the gracile nucleus. It carries
proprioceptive & fine touch sensation from
opposite side of body to the thalamus.
Trigeminal lemniscus: located lateral to
the medial lemniscus, a band of ascending
fibers carrying pain, temperature, touch &
proprioception from opposite side of face &
scalp to the thalamus.
Spinal lemniscus: located just medial to
lateral lemniscus, it is a band of ascending
fibers carrying pain, tempreture & crude
touch from opposite side of body to the
thalamus.
LL
SL
TL
ML
Lateral lemniscus: most
laterally located, is a band of
ascending fibres carrying
hearing sensation from both
ears (mainly from opposite
side) to the auditory cortex in
temporal lobe.
• Ventral spinocerebellar tract
traverses the most lateral part of
the tegmentum and then curves
dorsally and enters the cerebellum
through the superior peduncle
• Medial longitudinal fasciculus:
located near the midline in the
pontine tegmentum
• Inferior cerebellar peduncles
enter the cerebellum from the
caudal part of the pons. At this
level, they lie medial to the middle
cerebellar peduncles and form the
lateral walls of the fourth ventricle
MLF
SCP
MCP
4th V
ICP
ICP
• Superior cerebellar peduncles
consist:
• mainly of dentatoruberal &
dentatothalamic fibers that
originate in cerebellar nuclei and
enter the brain stem immediately
caudal to the inferior colliculi of the
midbrain. The fibers cross the
midline at the level of the inferior
colliculi in the decussation of the
superior cerebellar peduncles
• also contains afferent fibers that
enter the cerebellum: the ventral
spinocerebellar tract and
tectocerebellar fibers.
4th V
SCP
• Vestibulocochlear Nerve
• Fibers from the cdorsal and ventral
cochlear nuclei ascend in the pons
• Most of the fibers cross in the midline.
The decussating fibers constitute the
trapezoid body which intersects the
medial lemnisci and then turn rostrally in
the lateral part of the tegmentum to
form the lateral lemniscus
• Some fibers ascend ipsilaterally to join
the ipsilateral lateral lemniscus.
• Lateral lemniscus lies lateral to the
medial lemniscus in the first part of its
course and then moves dorsally to end in
the inferior colliculus of the midbrain
TB
Trapezoid
body
• The superior vestibular
nucleus, extends into the
pons.
 Fibers from the vestibular
nuclei, some crossed and
some uncrossed, ascend in
the medial longitudinal
fasciculus.
 The fibers terminate mainly
in the abducens, trochlear,
and oculomotor nuclei,
establishing connections
that coordinate
movements of the eyes
with movements of the
head.
• Facial Nerve
• The facial motor nucleus lies in the
ventrolateral part of the
tegmentum
• Axons arising from the nucleus
course dorsomedially and then
form a compact bundle, which
loops over the abducens nucleus
beneath the facial colliculus
forming the internal genu .
• After leaving the genu, the fibers
pass between the nucleus of origin
and the spinal trigeminal nucleus,
emerging as the motor root of the
facial nerve at the junction of the
pons and medulla.
VI
V-Spinal
nucleus
• The abducens nucleus is located
beneath the facial colliculus.
• The efferent motor fibers of the nucleus
run ventrally and leave the brain stem as
the abducens nerve between the pons
and the pyramid of the medulla
• The internuclear neurons have axons
that travel in the contralateral medial
longitudinal fasciculus to the division of
the oculomotor nucleus that supplies the
medial rectus muscle.
• This arrangement provides for
simultaneous contraction of the lateral
rectus and contralateral medial rectus
when the eyes move in the horizontal
plane.
Trigeminal Nerve Nuclei
• Trigeminal Nerve
• Sensory nuclei:
• Spinal tract and nucleus:
• Located in the lateral part
of the tegmentum of the
caudal half of the pons
lateral to the fibers of the
facial nerve.
• Chief or Principal nucleus:
• Located at the rostral end
of the spinal trigeminal
nucleus
• Receives fibers for touch,
especially discriminative
touch.
V-Spinal
Tract
&
Nucleus
V-Chief
Nucleus
• Mesencephalic nucleus:
• A slender column of cells
located beneath the lateral
edge of the rostral part of the
fourth ventricle, extending into
the midbrain
• Contains pseudo-unipolar cells
(cell bodies of primary sensory
neurons and the only such cells
in the central nervous system).
• The peripheral fibers of the
unipolar neurons is distributed
through the mandibular
division of the nerve to
proprioceptive endings in the
muscles of mastication
• Motor nucleus:
• Located medial to the
chief sensory nucleus
• Contains motor
neurons that supply
the muscles of
mastication, anterior
belly of digastric and
mylohyoid muscles
Motor
nucleus
• Trigeminothalamic tracts:
• Fibers from the trigeminal sensory
nuclei project to the thalamus via
the ventral & dorsal
trigeminothalamic tracts
collectively forming the trigeminal
lemniscus.
• The ventral trigeminothalamic
tract contains crossed fibers from
both chief sensory and spinal tract
nuclei
• The dorsal trigeminothalamic tract,
contains crossed and uncrossed
fibers, originating exclusively in the
chief sensory
Caudal pons
(Level of abducent & facial
nerve nuclei)
Dentate
nucleus
Facial
colliculus
MLF
4th ventricle
Abducent nerve
nucleus
Spinal
trigeminal tract
& nucleus
Facial nerve
fibers
Facial nerve
nucleus
Abducent nerve
fibers
Trapezoid body
MCP
Basilar
Pons
Medial
lemniscus
MCP
Middle pons
(Level of trigeminal nerve
nuclei)
SCP
MLF
4th ventricle
Motor nucleus
MCP
Trigeminal
nerve fibers
Trapezoid body
Chief sensory
nucleus
MCP
Basilar
Pons
Medial
lemniscus
Rostral pons
(Level of lemnisci)
Sup. Medullary
velum
4th ventricle
V -Mesencephalic
nucleus
SCP
Central tegmental
tract
Lateral lemniscus
Spinal lemniscus
MLF
trigeminal lemniscus
Basilar
Pons
Medial lemniscus
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Mainly by the paramedian and circumferential branches of the
basilar artery.
The anterior inferior cerebellar artery and the superior
cerebellar artery contribute branches to the middle and
superior cerebellar peduncles and to dorsal and lateral portions
of the pontine tegmentum