Pons

PONS (Bridge,L)

Pons 

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The middle part of the brainstem. Situated in the posterior cranial fosa on the clivus (formed by union of basisphenoid and basiocciput). Connected to cerebellum by two middle cerebellar peduncles.

PONS in MRI

Expanding lesions of clivus can invade the pons

parts

Dorsal part is tegmentum

Relation to the IV ventricle  Forms

the upper half of the floor of the IV ventricle.  (the backs of pons and medulla form the floor of the IV ventricle)

Cisterna pontis  Situated

on the ventral aspect of pons.  Contains basilar artery.

External features 

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Convex ventral surface (facing the clivus) – basilar part of pons(basis pontis) This has a basilar groove (sulcus basilaris) for the artery of same name. Dorsal part (back) is called the TEGMENTUM forms the floor of the IV ventricle.

Basilar surface and trigeminal nerve  The

V nerve is attached to basilar pons by two roots and demarcates the pons from middle cerebellar peduncle.  A vertical line drawn just lateral to sensory root is the junction of the two.

BORDERS AND RELATED STRUCTURES

Basilar artery and pons   

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Formed at the lower border of pons. Terminates at the upper border of pons by dividing into two posterior cerebral arteries. Superior cerebellar arteries curve along the upper border and cerebral peduncles enter here. AICA curves around the lower border.

Arterial supply  Basilar

artery gives number of pontine branches.  Also by AICA and SCA (superior cerebellar artery)

Cranial nerve nuclei in the pons  V,VI,VII,VIII

nerves are attached to pons.  (remember!! except the first two cranial nerves (olfactory and optic) the rest are attached to brain stem in this fashion:  III and IV- midbrain  V to VIII- pons  IX to XII- medulla

Cerebello pontine angle  This

clinically important angle has the following structures:  VII and VIII nerves,flocculus of cerebellum, lateral apertures of IV ventricle with choroid plexus. Cerebello pontine angle

CP ANGLE

Acoustic neuroma and CP angle A thickening of Schwann cells of VIII nerve can lead to CP angle syndrome.

 Features

of this syndrome include:  1.Progressive deafness and vertigo (VIII nerve damage)  2.ipsilateral ataxia and staggering gait (cerebellar peduncle injury)

CP angle syndrome  3.ipsilateral

lower motor neuron type of

facial palsy.  4.ipsilateral loss of pain and temperatures sensation and loss of corneal reflex due to involvement of spinal tract and nucleus of V nerve.

The correct medical term for acoustic neuroma is vestibular schwannoma, as it rarely arises from the the acoustic division of VIII nerve!!

P

Internal structure  Grey

matter and white matter  The structure of basis pontis is similar throughout the pons.  Tegmentum on the other hand, differs in structure in the upper and lower halves.

Grey matter and white matter  Pontine

nuclei.  Nuclei of V,VI,VII and VIII cranial nerves.  Pontine respiratory center.  White matter contains ascending and descending tracts and transverse ponto cerebellar fibers.

Structure of basis pontis (white matter)  Longitudinal

fibers (corticopontine,corticonuclear and corticospinal fibers).  Corticopontine fibers relay in ipsilateral pontine nuclei.  Corticonuclear fibers terminate mainly in contralateral motor nuclei of cranial nerves.  Corticospinal fibers descend to form pyramids of medulla.

Transverse fibers  Arise

in the pontine nuclei and pass to the opposite cerebellar hemisphere and form ponto cerebellar fibers.  The pontine nuclei form an important part of the cortco-ponto-cerebellar pathway, connecting the cerebral cortex of one side to the cerebellar hemisphere of the opposite side.

IV

Internal structure of pons

Tegmentum (dorsal part)  Since

the structure differs in the upper and lower parts of pons, lets examine T.S of pons at these two levels.

T.S. through lower (caudal) pons  Grey

matter- nuclei of VI,VII,VIII and spinal nucleus of V nerves.  Abducent nucleus is looped by facial nerve fibers which forms an elevation called “facial colliculus”. This phenomenon of migration of nerve fibers is called “neurobiotaxis”.

V

Remember! Facial colliculus is not formed by facial nerve nucleus but by abducent nucleus!!

VI

Neurobiotaxis

Other nuclei in tegmentum   

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Superior and inferior salivatory nuclei. 4 vestibular and 2 cochlear nuclei. Spinal tract and its nucleus of V on their way to midbrain above. White matter at this level contains trapezoid body (fibers from cochlear nuclei), medial lemniscus (becomes transverse), MLF and tectospinal tracts.

Medial lemniscus In medulla

T.S. Through upper pons  Main

features are:  Sensory and motor nuclei of V  Lateral lemniscus along with other lemnisci

T.S THROUGH UPPER PONS

Main structural differences in tegmentum at two levels

Lower pons

Grey matter

Upper pons

Nuclei of VI,VII,VIII Motor and principal nerves, spinal sensory nucleus of nucleus of V. V. 2 lemnisci; medial and spinal.

4 lemnisci; medial,lateral,spinal and trigeminal lemnisci.

Trapezoid body is present.

Trapezoid body absent.

White matter

Trapezoid body and the lateral lemniscus are concerned with The auditory pathway

Clinical aspects of pons

Millard- Gubler syndrome  Results

from a vascular lesion in lower pons where pyramidal tracts, abducent and facial nerves are situated.  Features:  Contra lateral hemiplegia (involvement of corticospinal tracts)  Ipsilateral facial palsy  Ipsilateral medial squint (involvement of abducent) Alternating abducent hemiplegia?

The area of close relationship between the VI nerve, pyramids and basilar artery

Millard-Gubler syndrome

Pontine hemorrhage  Involvement

of basilar artery branches or AICA (bilateral and extensive) results in:  Pin-point pupil (sympathetic fibers)  Hyperpyrexia  Deep coma (reticular formation)  Bilateral paralysis of limbs and face

Astrocytoma of pons The most common tumour of the brainstem. Usually occurs in childhood.