Anatomy

1. Anatomy of pons
Dr. Mohammed Mahmoud Mosaed

2. Pons
• pons means “bridge”
• The pons lies anterior to the cerebellum
and connects the medulla oblongata to
the midbrain. It is about 1 inch (2.5 cm)
long
• Contains:
Sensory and motor nuclei for the middle
4 cranial nerves
• Trigeminal (5), Abducens (6), Facial (7),
and Vestibulochoclear (8)

3. Gross Appearance of the Pons
The pons has 2 surfaces:
• Anterior (ventral) and
• Posterior ( dorsal)

4. Anterior Surface
• The anterior surface is convex from
transverse pontine
side to side and shows
fibers
many
that
converge on each side to form the
middle cerebellar peduncle.
• There is a shallow
midline, the basilar
groove in the
groove, which
lodges the basilar artery.

6. Nerves emerging from Pons
Nerves emerging from Pons (4 nerves):
• Trigeminal (5th): from the middle of ventrolateral
aspect of pons, as 2 roots: a small medial motor
root and large lateral sensory root
• Abducent (6th), Facial (7th) and vestibulocochlear
(8th): In the groove between the pons and the
medulla oblongata, there emerge, from medial to
lateral, the abducent, facial, and vestibulocochlear
nerves

8. Posterior Surface
• The posterior surface of the pons is hidden from
view by the cerebellum. It forms the upper half of
the floor of the fourth ventricle and is triangular in
shape.
• The posterior surface is limited laterally by the
superior cerebellar peduncles and is divided into
symmetrical halves by a median sulcus. Lateral to
this sulcus is an elongated elevation, the medial
eminence, which is bounded laterally by a sulcus,
the sulcus limitans.

9. Posterior Surface
• The inferior end of the medial eminence is slightly
expanded to form the facial colliculus, which is
produced by the root of the facial nerve winding
around the nucleus of the abducent nerve. The
floor of the superior part of the sulcus limitans is
bluish-gray in color and is called the substantia
ferruginea; it owes its color to a group of deeply
pigmented nerve cells. Lateral to the sulcus
limitans is the vestibular area produced by the
underlying vestibular nuclei

11. Internal structures of the pons
• The pons is commonly divided by the transversely
running fibers of the trapezoid body into:
• Basilar part (the ventral part of the pons)
• Tegmentum (the dorsal part of the pons)
• To study the internal structures of the pons, transverse
sections are done in the pons at the following levels;
1. Level of facial colliculis; lower level (caudal level)
2. Level of trigeminal nuclei; upper level (cranial level)

12. Transverse Section Through the Lower Part
• The following structures are seen at this level
• Nuclei
• Cranial nerve nuclei: Spinal nucleus of trigeminal nerve, facial
nucleus, abducent nucleus, medial vestibular nucleus, posterior and
anterior cochlear nuclei
• pontine nuclei.
• Motor tracts
• Corticospinal and corticonuclear tracts,
• Transverse pontine fibers,
• Medial longitudinal fasciculus
• Sensory tracts
• Spinal tract of trigeminal nerve
• Lateral, spinal, and medial lemnisci

14. Transverse Section Through the Upper Part
sensations from the
• The following structures are seen at this level
• Nuclei
• Cranial nerve nuclei: Main sensory nucleus and motor nucleus of
trigeminal nerve,
• pontine nuclei,
• trapezoid nuclei
• Motor tracts
• Corticospinal and corticonuclear tracts, transverse pontine fibers,
medial longitudinal fasciculus
• Sensory tracts
• Medial lemniscus which carries proprioceptiveand, fine touch sensations
from the opposite side of the body
• Lateral lemniscus: which carries auditory sensations from the opposite and
partly from the same side.
• Spinal lemniscus: which carries pain, temperature and crude touch
sensations from the opposite side of the body
• Trigeminal lemniscus: which carries the general
opposite side of the head and face

16. Basilar part of the pons
• The basilar part (basis pontis) is the
ventral part of the pons
• Contents
• 1. Bundles of the pyramidal tract
• 2. Corticopontine fibers
• 3. Nuclei pontis
• 4. Transverse pontine fibers

17. Corticopontine fibers
• Corticopontine fibers are descending from cerebral cortex to the
pontine nuclei
pontine nuclei
• pontine nuclei are groups of small neurons scattered in the basilar
part of the pons, these neurons receive the terminations of the
corticopontine fibers on the same side.
• The axons of the cells of the pontine nuclei cross to the opposite
side forming the transverse pontine fibers which collect into the
middle cerebellar peduncle to reach the cerebellar cortex of the
opposite side. The corticopontocerebellar pathway is the main
pathway connecting the cerebral cortex and the cerebellar cortex
• The corticopontine fibers is the first neurone
• The pontocerebellar fibers form the second neurons
Bundles of pyramidal tract
• Bundles of pyramidal tract are descending from the crus cerebri of
the midbrain. The pyramidal tract is broken into bundles by
transverse pontine fibers.

19. Facial Motor Nucleus
• Most of the fibres of the facial nerve are SVE fibres.
• The large facial motor nucleus is located in the ventrolateral
tegmentum of the caudal pons.
• These innervate the muscle of the second branchial arch.. These
are the muscles of facial expression and stapedius.
• The facial motor nucleus is involved in the corneal blink reflex,
which is a reflex of considerable clinical importance.
• If either cornea is touched with a foreign object, both eyes
automatically blink.
• The sensory afferent is by way of the ophthalmic division of the
trigeminal nerve.
• The afferents enter the spinal trigeminal tract and synapse on
interneurons in the spinal trigeminal nucleus.
• They also synapse on the interneurons of the main sensory nucleus.
• These interneurons project bilaterally to the motor neurons of the
facial motor nucleus, which forms the efferent limb.

20. Abducens Nerve and the abducens nucleus.
• This supplies the lateral rectus muscle.
• The fibres originate from the ipsilateral abducens nuclei.
• This is located in the caudal pons, beneath the floor of the
4th ventricle.
• The facial fibres project dorsomedially and wrap around the
abducens nucleus
• They then turn back ventrally to exit from the brainstem.
• Here, the fibres are known as the internal genu of the facial
nerve. The abducens nuclei along with the internal genu
are responsible for the facial colliculus in the floor of the
4th ventricle.

22. Superior Salivatory Nucleus
• This nucleus is located near the internal genu of the
facial nerve.
• A small collection of GVE (parasympathetic) fibers
originate in this nucleus and travel in the facial
nerve.
• They innervate: The submandibular gland;
The sublingual gland; and the lacrimal gland.
Inferior Salivatory Nucleus
• A small collection of GVE fibers originate in this
nucleus and travel in the glossopharyngeal nerve.
• These fibers supply the parotid gland.

23. Cochlear nuclei
• There are 2 cochlear nuclei
• The dorsal cochlear nucleus; lies on dorsal aspects of the inferior
cerebellar peduncle
• The ventral cochlear nucleus lies on anterolateral aspects of the
inferior cerebellar peduncle.
• As the cochlear nerve enters the pons it divides into ascending
branch which terminate in the ventral nucleus and descending
fibers terminate in the dorsal nucleus
• The axons of the cells in the 2 nuclei on each mostly cross to the
opposite side forming a transverse bundle in the anterior part of
the tegmentum of the pons called trapizoid body
• As they reach to the opposite side the fibers of trapizoid body
change their direction and ascend forming the lateral lemniscus
• A smaller number of fibres from the cochlear nuclei do not cross
the midline. They instead join the ipsilateral lateral lemniscus.
Thus, each lateral lemniscus carries some information from both
ears. Nearly all fibres of the lateral lemniscus terminate in the
inferior colliculus

25. Vestibular nuclei
• These are four nuclei
• superior in the lower part of the pons
• Inferior in the open medulla medial to the inferior
cerebellar peduncle
• Medial in the lower part of the pons and the upper part of
the medulla
• Lateral in the lower part of the pons
• The vestibular nuclei receive afferent fibers from the
vestibular nerve
•The efferent fibers
Vestibulospinal fibers
Vestibulocerebellar fibers
Vestibuloocular
Vestibuloreticular fibers

26. Trigeminal nuclei
• 4 Trigeminal nuclei; one motor and 3 sensory
• Trigeminal Motor Nucleus special visceral
efferent (SVE) nucleus
• The nucleus is located in the midpons. It innervates
the muscles of the first branchial arch, which
tensor
consists of the muscles of mastication and
tympani and tensor palatini muscles
• Fibres of the trigeminal motor nucleus emerge as a
separate motor root. They are distributed
peripherally with the mandibular division.

27. Sensory Nuclei of the Trigeminal Nerve
• There are 3 sensory nuclei:
• The main (or principle) sensory nucleus;
• The spinal nucleus or nucleus of spinal
tract of trigeminal nerve
• The mesencephalic nucleus.

28. Main Sensory Nucleus
• The main sensory nucleus located lateral to the
trigeminal motor nucleus.
• It is concerned with discriminative touch and
proprioception. It receives large-diameter, heavily
myelinated tactile afferents.
Nucleus of Spinal Tract of Trigeminal Nerve
• The spinal nucleus of the trigeminal nerve and its tract
lie on the anteromedial aspect of the inferior cerebellar
peduncle and extend caudally to the 3rd cervical
segment of the spinal cord. The nucleus blends into the
posterior horn.
• The nucleus is known to be particularly important in
the processing of pain and temperature information in
the head

29. Mesencephalic Trigeminal Nucleus
The fibres to this nucleus carry proprioceptive fibres.
• These afferents from muscle spindles in the muscles of
mastication and some from mechanoreceptors of the gums,
teeth and hard palate don't have their cell bodies in the
trigeminal ganglion.
• Instead, they are located in the slender column of cells called
the mesencephalic trigeminal nucleus.
• This nucleus extends all the way rostrally to the posterior
commissure.
• The cells of this nucleus are pseudounipolar (analogous to
dorsal root ganglion cells) and their myelinated processes
collect in a bundle, called the mesencephalic trigeminal tract.
• This lies adjacent to the nucleus.
• The central processes of these neurons end in the motor and
main sensory nuclei and a few other brainstem sites.

30. Blood Supply
• Basilar artery through
pontine arteries
• Anterior inferior &
superior cerebellar
arteries

31. Applied anatomy
• Medial pontine lesion
• Causes mainly vascular due to obstruction of one or more of pontine
arteries
Manifestations
• Hemiplagia on the opposite side due to injury of the pyramidal tract
• Loss of proprioceptive sensation on the opposite side of the body due to
injury of the medial lemniscus
• Lower motor neurone paralysis of the muscle of mastication on the same
side due to injury of the motor nucleus of the trigeminal nerve.
• Lower motor neurone paralysis of the lateral rectus on the same side
• Lower motor neurone facial paralysis on the same side due to injury of the
facial nucleus and facial nerve
• Medial lesion of the lower part of the pons leading to middle or facial
alternating hemiplagia i.e. facial paralysis on the same side and hemiplagia
on the opposite side also called Millard-gubler syndrome
• Foville syndrome in which abducent and facial paralysis on the same side
and hemiplagia on the opposite side

32. Lateral pontine lesion
Manifestation
• Loss of pain, temperature and crude touch sensations on
the opposite side of the body due to injury of spinal
lemniscus
• Loss of pain, temperature and touch sensations on the
same side of the face due to injury of the main sensory
nucleus and nucleus of spinal tract of trigeminal nerve
• Cerebellar ataxia due to injury of cerebellar peduncles
• Vertigo and nystagmus on the same side due to injury of
the vestibular nuclei
• Complete loss of hearing on the same side due to injury of
the cochlear nerve and nucleus
• Horner’s syndrom