Internal structure of pons
There are 3 levels of transverse
sections :
1-lower level : level of facial colliculus.
2-mid-level : level of trigeminal nerve.
3-upper level : level of 4 lemnisci.
The pons is divided into 2 main
divisions by the transversely running
fibres of trapezoid body :
1-Basilar part (Basis pontis) : it is the
ventral part of pons,
-its contents are constant in all levels.
2-Tegmentum : it is the dorsal part of
pons. Superiorly,tegmentum of pons is
continuous with tegmentum of midbrain.
-its contents vary in the 3 levels of pons.
.
Ventral (basilar) part of pons
in all levels:
It is marked by numerous transverse
pontocerebellar fibres which arise
from pontine nuclei.
Caudal pons at
level of facial
colliculus
Caudal
pons
Mid-pons at level of
trigminal nerve.
These Pontocerebellar transverse
fibres cross the midline to pass to
contralateral cerebellum, forming
middle cerebellar peduncle (brachium
pontis),where trigeminal nerve (V)
pierce it.
Ventral (Basilar)part of pons
in all levels:
Corticospinal & corticobulbar
fibres (pyramidal tract) appear as
small,separate bundles running
longitudinally between fascicles of
transverse pontine fibres.
T.S of caudal pons at
level of facial
colliculus
T.S of mid-pons at level of
trigeminal nerve
T.S of rostral pons at
level of 4 lemnisci
Dorsal( tegmental) part of caudal pons at
level of facial colliculus :
Trapezoid body consists of
acoustic fibres responsible for hearing,
T.S of caudal
pons
arising from cochlear nuclei (dorsal & ventral
cochlear nuclei lying dorsal & ventral to
inferior crebellar peduncle in the lower part of
pons).
-These acoustic fibres crossing opposite
side of pons forming decussation of trapezoid
body, then the axons ascend into rostral pons
& midbrain as lateral lemniscus and
terminate in inferior colliculus.
-it lies in the anterior part of tegmentum.
T.S of caudal
pons
T.S of rostral pons
Dorsal (tegmental)part of caudal pons at
level of facial colliculus :
Abducent nerve nucleus :
T.S through caudal part of pons at
the level of facial colliculus.
site : in posterior aspect of caudal
pons near floor of 4th ventricle.
-It is encircled by fibres of facial N.,
forming elevation in the the floor of
4th ventricle called facial colliculus.
-Its efferent fibres : pass
downwards traversing medial
lemniscus & pyramidal Tract bundles
to emerge anteriorly at junction
between pons & pyramid of
medulla, supplying lateral rectus
muscle
Dorsal (tegmental)part of caudal pons at
level of facial colliculus :
Facial motor nucleus :
site : in posterior part of caudal pons.
Its efferent fibres encircle abducent
nucleus,then pass anterolaterally to
emerge at the junction between pons &
olive of medulla, (supplying muscles of
facial expression).
Medial longitudinal fasciculus
or bundle :
-it is an association tract, lies close to
midline.
-found throughout the brain stem,and
descend into spinal cord.
-It links vestibular nuclei with motor ocular
nuclei in the brain stem, supplying
extraocular muscles (oculomotor, trochlear
& abducent nuclei) to serve corrdination of
head and eye movements.
Dorsal (tegmental) part of caudal pons at
level of facial colliculus :
Vestibular nuclei : they are 4 nuclei lie
near to vestibular area of floor of 4th
ventricle.
-they receive afferent fibres for
unconscious deep sensation (equilibrium),
from the vestibular nerve.
-they send efferent fibres as :
vestibulo-ocular fibres through
medial longitudinal fasciculus.
T.S through caudal part of
pons at the level of facial
colliculus.
Spinal tract & nucleus of trigeminal
nerve: lie on the anteromedial aspect of
inferior cerebellar peduncle.
-carrying pain & temperature sensations
from the face & scalp.
-The axons of the cells of this nucleus cross
to opposite side, ascending as trigeminal
lemniscus to end in thalamus.
Dorsal (tegmental)part of mid-pons at
level of trigeminal nerve :
Trigeminal motor nucleus : medial in
position. its axons form the motor root of
trigeminal N. which passes along
mandibular N. (supplying muscles of
mustication).
T.S of mid-pons a level of trigeminal N. .
Trigeminal sensory nucleus : lateral in
position. It receives afferent touch
sensation from face & scalp.
-It sends efferent fibres, cross to opposite
side to join axons of spinal nucleus of
trigeminal, forming trigeminal lemniscus in
rostral pons – midbrain - thalamus.
Superior cerebellar peduncle : lies
posterolateral to motor nucleus of trigeminal
T.S of mid-pon at level of
trigeminal nerve
Medial longitudinal fasciculus, spinal
lemniscus , medial lemniscus &
trapezoid body.
Dorsal (tegmental) part of Rostral Pons
at level of 4 lemnisci :
Superior cerebellar peduncle :
T.S of rostral pons
T.S of mid-pons at
level of trigeminal
nerve.
T.S of rostral pons
-lies in the rostral part of pons,
forming lateral walls of 4th ventricle,
right & left.
-They are connected together by
sup.medullary velum which forms roof
of 4th ventricle.
-At the level of trigeminal nerve, it lies
posterolateral to motor nucleus of
trigeminal nerve.
Dorsal (tegmental) part of Rostral Pons at
level of 4 lemnisci :
Types of fibres in the S.C.P. :
(A) Afferent fibres :
1-ventral spino-cerebellar tract : it
carries proprioceptive impulses from
the limbs to cerebellum.
2-tecto-cerebellar tract : it carries
auditory & visual impulses from tectum
of midbrain to cerebellum.
(B)Efferent fibres :
1-Dendato-rubral tract : it is concerned
with coordination of movement. it ends
in red nucleus of midbrain – thalamusmotor & premotor area of cortex.
2-Dentato-thalamic tract : to carry
proprioceptive deep sensation from
dentate nucleus of cerebellum to end in
V.L.N. of thalamus.
T.S of rostral pons
Dorsal (tegmental) part of Rostral Pons at
level of 4 lemnisci :
Lateral lemniscus :
the most lateral lemniscus. it is a
band of ascending fibres carrying
hearing sensation from both ears
( mainly from opposite side). Ends
in auditory area in temporal lobe.
Spinal lemniscus : just
medial to lateral lemniscus. it is a
band of ascending Fs. Carrying
pain, tempreture & light touch from
opposite side of body. Ends in
P.L.V.N.of thalamus.
Dorsal (tegmental) part of Rostral Pons at
level of 4 lemnisci :
Trigeminal lemniscus: just
medial to spinal lemniscus.it is a
band of ascending Fs.carrying
superficial sensation, from opposite
side of face & scalp. ends in
P.M.V.N.of thalamus.
 Medial lemniscus :
-(the most medial lemniscus) is
marking the boundary between
ventral & tegmental portions of pons.
- it is a band of ascending fibres
carrying proprioceptive sensation
from opposite side of body. ends in
P.L.V.N. of thalamus.
Internal structure of Midbrain
The midbrain is divided into dorsal & ventral
portions at the level of cerebral aqueduct.
(A)Tectum : the smaller dorsal part behind
aqueduct. It is composed of 4 rounded swellings
(colliculi) :
1-2 superior colliculi : reflex centers of vision.
2-2 inferior colliculi : reflex centers of hearing.
(B)2 Cerebral peduncles : the larger ventral
part in front of aqueduct. It consists of 3 parts :
1-Crus cerebri (Basis pedunculi) : the most
anterior part which consists entirely of pyramidal
& cortico-pontine fibres
2-Substantia nigra : a thick lamina of grey matter
formed of deeply pigmented nerve cells lying
behind crus cerebri. It is an Extrapyramidal motor
centre.
3-Tegmentum : the post. part of cerebral
peduncle. It contains ascending tract, certain
nuclei, decussations & reticular formation.
Caudal midbrain at level of
Inferior colliculus The inferior colliculus is a
mass of grey matter which receives
ascending auditory pathway ,which
run in lateral lemniscus.
-Its Efferent Fs. end in medial
geniculate nucleus of thalamus,
which projects to auditory cortex of
temporal lobe - the centre of hearing.
T.S.of caudal midbrain at level of inferior colliculus.
The cerebral aqueduct runs
ventral to colliculi, and surrounding
by area of grey matter, the
peri-aqueductal (or central grey ).
Trochlear nucleus : lies ventral to
peri-aqueductal grey, its efferent Fs.
cross to opposite side to emerge
from back of midbrain, to supply
extraocular ms.(sup.oblique).
Auditory pathway & inferior colliculus
of midbrain (reflex center of hearing)
:
Caudal midbrain at level of
Inferior colliculus
Medial longitudinal fasciculus : is a
well defined bundle of fibres lies on
each side of median plane in midbrain
tegmentum.
-It extends throughout the brain stem,
and descends into spinal cord.
-It lies ventral and close to oculomotor,
trochlear & abducent nuclei.
-it receives fibres from vestibular
nuclei. -it sends efferents to ocular
motor nuclei -Its function is
coordination of eye,and head & neck
movements.
Medial longitudinal fasiculus in
midbrain :
Caudal midbrain at level of
Inferior colliculus
Decussation of superior
cerebellar peduncles (brachium
conjunctivum) : the fibres of each
peduncle cross to opposite side, forming
decussation in the central part of
tegmentum.
Medial lemniscus : it is a band of
.
ascending Fs. carrying proprioceptive
sensation from opposite side of body.
-It is the upward continuation of gracile &
cuneate tracts of opposite side.
-It lies in tegmentum, posterior to
substantia nigra. It ends in PLVN of
thalamus.
Caudal midbrain at level of Inferior
colliculus
Substantia Nigra : It is a large motor
T.S.of caudal midbrain at level of inferior colliculus.
nucleus of grey matter, lies at the most
ventral part of midbrain tegmentum, or just
behind the crus cerebri.
-It contains subdivision part, the pars
compacta, which consists of pigmented,
melanin-containing neurones that
synthesise dopamine as their transmitter.
-It projects to caudate nucleus + putamen
of basal ganglia in the forebrain –
’’Nigrostriatal dopaminergic neurons’’.
-It has extra-pyramidal motor function,
concerned with coordination of movements.
-lesion of pars compacta leads to
parkinson’s disease. It is due to absence
of dopamine into basal ganglia, this is
manifested by a mask face, resting tremors,
rigidity of muscles and a shuffling gait.
Caudal midbrain at level of Inferior
colliculus
Crus Cerebri : lies ventral to
substantia nigra. It consists entirely of
descending cortical efferent Fs.
-50% of crus consists of pyramidal tract
consists of cortico-bulbar Fibres (end in
motor cranial nerve nuclei of brain stem) +
cortico-spinal fibres which traverse pons,
down to medullary pyramid and spinal cord.
T.S.of caudal midbrain at level of inferior colliculus.
-on either side of corticobulbar &
corticospinal fibres, crus cerebri
contains cortico-pontine,
temporo-pontine + fronto-pontine fibres
-These Fs. arise from cerebral cortex
and ends in pontine nuclei of ventral
pons to pass via M.C.P into cerebellum,
(cortico-ponto-cerebellar pathway ) to
involve in coordination of movement.
Rostral midbrain at the level of Superior
Superior colliculus :
Colliculus:
-lies in upper part of tectum of midbrain.
-it is a centre of visual reflexes.
-Its main afferent Fs. are :
Cortico-tectal Fs. arise from
-visual cortex of occipital lobe.
-frontal eye field of frontal lobe.
-function : control movements of eyes +
accomodation reflex.
T.S. of rostral midbrain at level of
superior colliculus.
Pretectal nucleus : It lies close to
superio- lateral part of superior colliculus.
-The visual Fs. running in optic tract just
rostral to superior colliculus end in
pretectal N.
-it has connections with parasymp.
nucleus of oculomotor N. of both sides
(Edinger-Westphal nucleus) to control
smooth ms. of eye (sphincter pupillae)
and to mediate pupillary light reflex.
Rostral midbrain at the level of Superior
Peri-aqueductal (central) grey.
Colliculus :
Oculomotor nucleus : lies ventral to periaqueductal grey at level of superior colliculus
of midbrain.
-efferent Fs. emerge from the medial
surface of crus cerebri. as oculomotor
nerve to supply extraocular ms. of eye
(except S.O + L.R.).
T.S. of rostral midbrain at level of
superior colliculus.
Red nucleus :
-it is a large mass of grey matter lies in
tegmentum of rostral midbrain.
-it has extrapyramidal function, motor function
-It receives afferents from motor cortex of frontal
lobe - cortico-rubral Fs.
-it sends efferents to 1-spinal cord as rubrospinal tract, which cross in ventral tegmental
decussation. / and to 2-inferior olivary nucleus of
medulla, via central tegmental tract as
rubroolivary Fs.
Reticular formation :
It is a complex matrix of neurones that
extends throughout the length of brain stem,
made up of deeply placed nerve cells & fibres.
It has widespread afferent & efferent
connections with other parts of CNS.
It has important functions for vital centers
as respiratory & cardiovascular centres, through
its connection to autonomic neurones of
hypothalamus & limbic system.
It has descending reticulospinal tracts
arise from medullary + pontine reticular
formation ,that influence muscle tone &
posture ( spinal motor function ).
It has ascending fibres, the reticular
activating system, for activation of thalamus,
hypothalamus & cerebral cortex, and arousal
(alertness) occurs. Damage to RAS can
produce coma.
Reticular formation:
Raphe nuclei : are a group of midline nuclei
that extend throughout the length of brain stem.
-they are serotonergic nuclei (their transmitter
is serotonine).
-Their ascending fibres to forebrain are involved
in neural mechanisms regulating sleep.
-Descending fibres to the spinal cord are
involved in pain control pathway.
Raphe nuclei
Locus coeruleus
Locus coeruleus : is a group of pigmented
neurones that lies in brain stem tegmentum of caudal
midbrain & rostral pons.
-it is noradrenergic cell group.
-it has ascending fibres to cerebellum, thalamus,
hypothalamus, limbic system and cerebral cortex.
-its descending fibres project to brain stem & spinal
cord.
-function : 1-neural mechanisms regulating
sleep, particularly REM (rapid eye movement)
sleep.
2-acts as an attention center,
(functional deficiency resulting in depression).
Brain stem lesions :
 A unilateral brain stem lesion : caused
by stroke,tumour or multiple sclerosis
causes :
1-epsilateral cranial nerve dysfunction +
contralateral spastic hemiparesis.
2-hyperreflexia & an extensor plantar
response (upper motor neurone lesion).
3-contalateral hemisensory loss.
4-ipsilateral incoordination (ataxia).
5-it can affect eye movements through
demyelination of medial longitudinal
fasciculus, producing internuclear
ophthalmoplegia which interferes with
conjugate ocular deviation (abducting eye
moves normally, but adducting eye fails to
follow), adduction is preserved on
convergence.
A bilateral lesion : destroys the ‘vital
centers’ for respiration & circulation, leading to
coma & death.