Internal structure of pons

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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 at the three levels is divided into 2
  main divisions
  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.
  -its contents vary in the 3
  levels of pons. -Superiorly, the tegmentum of
  pons is continous with tegmentum of midbrain.-

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Ventral (basilar) part of pons in all levels

• It is marked by numerous transverse
  pontocerebellar fibres which arise from pontine
  nuclei buried in its substance.
• These transverse Pontocerebellar fibres cross
  the midline to pass to contralateral cerebellar
  hemisphere forming middle cerebellar peduncle
  (brachium pontis), where trigeminal N. (V)
  pierce it.

.
Mid-pons at level of trigminal nerve.
Caudal pons at level of facial colliculus
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Ventral (Basilar)part of pons in all levels

• Corticospinal fibres corticobulbar (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 rostral pons at level of 4 lemnisci
T.S of mid-pons at level of trigeminal nerve
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Dorsal( tegmental) part of caudal pons at level
of facial colliculus

• 1-Trapezoid body consists of acoustic fibres
  responsible for hearing, arising from cochlear
  nuclei (dorsal ventral cochlear nuclei lying
  dorsal ventral to inferior crebellar peduncle
  in the most 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 caudal pons
T.S of rostral pons
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Dorsal( tegmental) part of caudal pons at level
of facial colliculus

• 2-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 fine touch sensation
  from opposite side of body, via Gracile Cuneate
  tracts, ending in P.L.V.N. of thalamus.

T.S of caudal pons
T.S of rostral pons
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Dorsal (tegmental)part of caudal pons at level of
facial colliculus

• 3-Medial longitudinal fasciculus or bundle
  
  -it is an association tract present close to
  midline , found throughout the brain stem,and
  descend into spinal cord.
  -It links vestibular
  nuclei with motor ocular nuclei, supplying
  extraocular muscles (oculomotor,trochlear
  abducent nuclei) to serve corrdination of head
  and eye movements.

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Dorsal (tegmental)part of caudal pons at level of
facial colliculus

• 4-Abducent nerve nucleus
  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 T.bundles
  to emerge anteriorly at junction between pons
  medulla, supplying lateral rectus muscle

• 5-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 medulla, (supplying ms. of facial
  expression)

T.S through caudal part of pons at the level of
facial colliculus.
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Dorsal (tegmental) part of caudal pons at level
of facial colliculus
6-Vestibular nuclei they are 4 nuclei lie
subjacent to vestibular area of floor of 4th
ventricle.
-they receive afferent fibres
from the vestibular nerve.
-they send efferent fibres
as vestibulo-ocular
fibres through medial longitudinal
fasciculus.

• 7-Spinal tract nucleus of trigeminal nerve lie
  on the anteromedial aspect of inferior cerebellar
  peduncle. -carrying
  pain temperature sensations from the face.
  
  -The axons of the cells of the nucleus
  cross to opposite side and ascend in trigeminal
  lemniscus to end in thalamus.
  
• 8-Inferior cerebellar peduncle.
  

T.S through caudal part of pons at the level of
facial colliculus.
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Dorsal (tegmental)part of mid-pons at level of
trigeminal nerve

• 1-Trigeminal motor nucleus medial in position.
  its axons form the motor root of trigeminal N.
  which passes along mandibular N. (supplying
  muscles of mustication).
• 2-Trigeminal sensory nucleus lateral in
  position. It receives afferent touch pressure
  sensation from face. -It sends
  efferent fibres which cross to join trigeminal
  tract or lemniscus.
• 3-Superior cerebellar peduncle lies
  posterolateral to motor nucleus of trigeminal
  nerve.
• 4-Medial longitudinal fasciculus, spinal
  lemniscus , medial lemniscus trapezoid body.

T.S of mid-pons a level of trigeminal N. .
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Dorsal (tegmental) part of Rostral Pons at level
of 4 lemnisci
1-Superior cerebellar peduncle lies in the
rostral part of pons, forming lateral walls of
4th ventricle. -They are connected together by
superior medullary velum which forms roof of 4th
ventricle.
T.S of rostral pons
T.S of mid-pons at level of trigeminal nerve.
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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 (Extrapyramidal nucleus) in
  midbrain.
  2-Dentato-thalamic tract from dentate
  nucleus of cerebellum to end in ventral lateral
  nucleus of thalamus.

T.S of rostral pons
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Dorsal (tegmental) part of Rostral Pons at level
of 4 lemnisci
2-Lateral lemniscus the most lateral
lemniscus. it is a band of ascending fibres
carrying hearing sensation from both ears
( mainly from opposite side), via acoustic Fs.
Of cochlear nuclei, Ending in auditory area in
temporal lobe. 3-Spinal lemniscus just medial
to lateral lemniscus. it is a band of ascending
Fs. Carrying pain, tempreture crude touch from
opposite side of body via spinothalamic tract
Ending in P.L.V.N.of thalamus.
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Dorsal (tegmental) part of Rostral Pons at level
of 4 lemnisci

• 4-Trigeminal lemniscus just medial to spinal
  lemniscus.it is a band of ascending Fs.carrying
  pain, temp., touch proprioception from opposite
  side of face scalp, via sensory Fs. Of
  trigeminal N. ending in P.M.V.N.of thalamus.
• 5-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
  fine touch sensation from opposite side of body,
  via Gracile Cuneate tracts, ending in P.L.V.N.
  of thalamus.

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Examples for Questions of Pons 1. All of
these tracts are found in the pons EXCEPT
a.Corticospinal tract.
b.Corticobulbar tract. c.Medial
lemniscus. d.Medial
longitudinal fasiculus. e.Gracile tract.
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Pons 2. Which nucleus is not lying in the
tegmentum of pons ? a. Facial motor
nucleus. b. Abducent motor nucleus.
c. Inferior olivary nucleus. d.
Cochlear nucleus. e. Vestibular nucleus. 3.
The acoustic fibres ascend as a. Medial
lemniscus. b. Spinal lemniscus. c. Lateral
lemniscus. d. Trigeminal lemniscus.
e. Medial longitudinal tract.
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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 lower
  centers of vision.
  2-2
  inferior colliculi lower 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, decussation, nuclei, reticular formation.

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Caudal midbrain at level of Inferior colliculus
1-The inferior colliculus is a centre of hearing
reflex 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.
2-The cerebral aqueduct runs ventral to
colliculi, and surrounding by area of grey
matter, the peri-aqueductal
(or central grey ). 3-Trochlear nucleus lies
ventral to peri-aqueductal grey, its efferent Fs.
cross to opposite side to emerge from back of
midbrain, then turn forwards to reach base of
brain to supply extraocular ms.(sup.oblique).
4-Mesencephalic nucleus of Trigeminal
-lies lateral to aqueduct of
midbrain, at level of inferiorsuperior
colliculi.
Sensory nucleus receives proprioceptive sensation
from ms.of mastication.
s.
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Auditory pathway inferior colliculus of
midbrain (reflex center of hearing)

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Caudal midbrain at level of Inferior colliculus
5-Medial longitudinal fasciculus is a well
defined bundle of association fibres lies on each
side of median plane in midbrain tegmentum.

-It extends throughout the brain stem, and
descends into spinal cord.
-It lies close to oculomotor, trochlear
abducent nuclei.
-it receives
fibres from vestibular nuclei.
-it
sends efferents to ocular motor nuclei

-Its function coordination of eye,and
head neck movements.
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Medial longitudinal fasiculus in midbrain
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Caudal midbrain at level of Inferior colliculus
6-Decussation of superior cerebellar peduncles
(brachium conjunctivum) the fibres of each
peduncle cross to opposite side, forming
decussation in the central part of
tegmentum. 7-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.
.
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Caudal midbrain at level of Inferior colliculus
8-Substantia Nigra It is a large extrapyramidal
motor nucleus, lies at midbrain tegmentum.

-It contains subdivision part, the pars
compacta, which consists of pigmented,
melanin-containing neurones that synthesize
dopamine as their transmitter.
-It project to
caudate nucleusputamen of basal ganglia in the
forebrain.
--It has extra-pyramidal motor
function, concerned with movements.
-lesion of pars compacta leads to
parkinsons disease. It is due to absence of
dopamine into basal ganglia, this is manifested
by a mask face, resting
tremors, rigidity of muscles
(more in flexors giving flexor attitude)
and a shuffling gait.
T.S.of caudal midbrain at level of inferior
colliculus.
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Caudal midbrain at level of
Inferior colliculus
9-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 down to medullary pyramid and spinal
cord.
-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.
T.S.of caudal midbrain at level of inferior
colliculus.
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Rostral midbrain at the level of Superior
Colliculus

• Superior colliculus
  -lies in upper part of tectum of
  midbrain.
  -it is a centre of visual reflex.
  -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.
• Pretectal nucleus It lies above the superior
  colliculus.
  -It receives the visual Fs. running in optic
  tract just rostral to superior colliculus.
  -it has connections with parasymp.
  nucleus of oculomotor N.
  (Edinger-Westphal nucleus) to control smooth ms.
  of eye (sphincter pupillae) and to mediate
  pupillary light reflex.

.
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Rostral midbrain at the level of Superior
Colliculus

• Peri-aqueductal (central) grey.
• Oculomotor nucleus lies ventral to
  peri-aqueductal 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.).
• Red nucleus
  -it is a large mass of grey
  matter lies in tegmentum of rostral midbrain.
  -it has spinal
  extrapyramidal motor function.
  -It receives afferents from motor cortex
  cerebellum (cortico-rubral dentato-rubral F.).
  
  -it sends efferents to
  spinal cord as rubro-spinal tract.

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Reticular formation

• It is a gray matter extending throughout the
  length of brain stem, made up of , deeply placed
  nerve cells fibres.
• It has important functions for vital centers as
  respiratory cardiovascular centres.
• It has descending fibres, reticulospinal tracts
  that influence muscle tone posture.
• It has ascending fibres, the reticular activating
  system (RAS), plays a role in consciousness and
  awake / sleep cycle.

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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 modulation of nociceptive mechanisms.
• 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.
  
  -involved in neural mechanisms regulating sleep.

Raphe nuclei
Locus coeruleus
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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.
  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.

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Examples of Questions of Midbrain 1. Which
nucleus is not lying in the tegmentum of the
midbrain ? a. Oculomotor nucleus.
b. Trochlear nucleus.
c. Mesenchephalic nucleus of
trigeminal d. Red nucleus e. Abducent
nucleus. 2.Substantia nigra is concerned with
a.Hearing sensation. b.Visual
sensation. c.Motor function.
d.Pain and temperature sensation.
e.Neural
mechanisms regulating sleep.
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3. The extrapyramidal nucleus lying in tegmentum
of the midbrain is a. Oculomotor
nucleus. b. Trochlear nycleus. c.
Substantia nigra. d. Mesenchephalic
nucleus of trigeminal. e. Facial
nucleus. 4.Parkinson's disease results from
degeneration of a.Red nucleus.
b.Medial lemniscus. c.Pyramid.
d.Substantia nigra. e.Inferior olivary
nucleus. 5. Which is wrong regarding the contents
of the crus cerebri of midbrain
a.Corticospinal fibres. b.Corticobulbar
fibres. c.Frontopontine fibres.
d.Temporopontine fibres. e.Corticotectal
fibres.