PONS PONS

(Bridge,L)(Bridge,L)

PonsPons

The middle part of the The middle part of the brainstem.brainstem.

Situated in the posterior Situated in the posterior cranial fosa on the clivus cranial fosa on the clivus (formed by union of basi-(formed by union of basi-sphenoid and basi-sphenoid and basi-occiput).occiput).

Connected to cerebellum Connected to cerebellum by two middle cerebellar by two middle cerebellar peduncles. peduncles.

PONS in MRI

Expanding lesions of clivus can invade the pons

parts

Dorsal part is tegmentum

Relation to the IV ventricleRelation to the IV ventricle

Forms the upper half Forms the upper half of the of the floor of the IV floor of the IV ventricle.ventricle.

(the backs of pons (the backs of pons and medulla form the and medulla form the floor of the IV floor of the IV ventricle)ventricle)

Cisterna pontisCisterna pontis

Situated on the Situated on the ventral aspect of ventral aspect of pons.pons.

Contains Contains basilar basilar artery.artery.

External featuresExternal features

Convex ventral surface Convex ventral surface (facing the clivus) –(facing the clivus) –basilar part of pons(basis basilar part of pons(basis pontis)pontis)

This has a basilar groove This has a basilar groove (sulcus basilaris) for the (sulcus basilaris) for the artery of same name.artery of same name.

Dorsal part (back) is Dorsal part (back) is called the TEGMENTUM called the TEGMENTUM forms the floor of the IV forms the floor of the IV ventricle.ventricle.

Basilar surface and trigeminal Basilar surface and trigeminal nervenerve

The V nerve is The V nerve is attached to basilar attached to basilar pons by two roots pons by two roots and demarcates the and demarcates the pons from middle pons from middle cerebellar peduncle.cerebellar peduncle.

A vertical line drawn A vertical line drawn just lateral to sensory just lateral to sensory root is the junction of root is the junction of the two. the two.

BORDERS AND RELATED STRUCTURES

Basilar artery and ponsBasilar artery and pons Formed at the lower Formed at the lower

border of pons.border of pons. Terminates at the upper Terminates at the upper

border of ponsborder of pons by dividing into two by dividing into two

posterior cerebral posterior cerebral arteries.arteries.

Superior cerebellar Superior cerebellar arteries curve along the arteries curve along the upper border and upper border and cerebral peduncles enter cerebral peduncles enter here.here.

AICA curves around the AICA curves around the lower border.lower border.

Arterial supplyArterial supply

Basilar artery gives Basilar artery gives number of pontine number of pontine branches.branches.

Also by AICA and Also by AICA and SCA (superior SCA (superior cerebellar artery)cerebellar artery)

Cranial nerve nuclei in the ponsCranial nerve nuclei in the pons

V,VI,VII,VIII nerves are attached to pons.V,VI,VII,VIII nerves are attached to pons. (remember!! except the first two cranial (remember!! except the first two cranial

nerves (olfactory and optic) the rest are nerves (olfactory and optic) the rest are attached to brain stem in this fashion:attached to brain stem in this fashion:

III and IV- midbrainIII and IV- midbrainV to VIII- ponsV to VIII- pons IX to XII- medullaIX to XII- medulla

Cerebello pontine angleCerebello pontine angle

This clinically This clinically important angle has important angle has the following the following structures:structures:

VII and VIII VII and VIII nerves,flocculus of nerves,flocculus of cerebellum, lateral cerebellum, lateral apertures of IV apertures of IV ventricle with choroid ventricle with choroid plexus.plexus.

Cerebello pontine angle

CP ANGLE

Acoustic neuromaAcoustic neuroma and CP angle and CP angle

Features of this syndrome include:Features of this syndrome include: 1.Progressive deafness and vertigo (VIII nerve 1.Progressive deafness and vertigo (VIII nerve

damage)damage) 2.ipsilateral ataxia and staggering gait 2.ipsilateral ataxia and staggering gait

(cerebellar peduncle injury)(cerebellar peduncle injury)

A thickening of Schwann cells of VIII nerveA thickening of Schwann cells of VIII nerve can lead to CP angle syndromecan lead to CP angle syndrome..

CP angle syndromeCP angle syndrome

3.ipsilateral lower motor neuron type of 3.ipsilateral lower motor neuron type of facial palsy.facial palsy.

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

P

The correct medical term for acoustic neuroma is vestibular schwannoma,

as it rarely arises from the the acoustic division of VIII nerve!!

Internal structureInternal structure

Grey matter and white matterGrey matter and white matterThe structure of basis pontis is similar The structure of basis pontis is similar

throughout the pons.throughout the pons.Tegmentum on the other hand, differs in Tegmentum on the other hand, differs in

structure in the upper and lower halves.structure in the upper and lower halves.

Grey matter and white matterGrey matter and white matter

Pontine nuclei.Pontine nuclei.Nuclei of V,VI,VII and VIII cranial nerves.Nuclei of V,VI,VII and VIII cranial nerves.Pontine respiratory center. Pontine respiratory center. White matter contains ascending and White matter contains ascending and

descending tracts and transverse ponto descending tracts and transverse ponto cerebellar fibers.cerebellar fibers.

Structure of basis pontisStructure of basis pontis(white matter)(white matter)

Longitudinal fibers (corticopontine,corticonuclear Longitudinal fibers (corticopontine,corticonuclear and corticospinal fibers).and corticospinal fibers).

Corticopontine fibers relay in ipsilateral pontine Corticopontine fibers relay in ipsilateral pontine nuclei.nuclei.

Corticonuclear fibers terminate mainly in Corticonuclear fibers terminate mainly in contralateral motor nuclei of cranial nerves.contralateral motor nuclei of cranial nerves.

Corticospinal fibers descend to form pyramids of Corticospinal fibers descend to form pyramids of medulla.medulla.

Transverse fibersTransverse fibers

Arise in the pontine nuclei and pass to the Arise in the pontine nuclei and pass to the opposite cerebellar hemisphere and form opposite cerebellar hemisphere and form ponto cerebellar fibers.ponto cerebellar fibers.

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

IV

Internal structure of pons

Tegmentum (dorsal part)Tegmentum (dorsal part)

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

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

Grey matter- nuclei of VI,VII,VIII and spinal Grey matter- nuclei of VI,VII,VIII and spinal nucleus of V nerves.nucleus of V nerves.

Abducent nucleus is looped by facial nerve Abducent nucleus is looped by facial nerve fibers which forms an elevation called fibers which forms an elevation called “facial colliculus”. This phenomenon of “facial colliculus”. This phenomenon of migration of nerve fibers is called migration of nerve fibers is called “neurobiotaxis”.“neurobiotaxis”.

V

Neurobiotaxis

VI

Remember!Facial colliculus is not formed by facial nerve nucleus

but by abducent nucleus!!

Other nuclei in tegmentumOther nuclei in tegmentum Superior and inferior Superior and inferior

salivatory nuclei.salivatory nuclei. 4 vestibular and 2 cochlear 4 vestibular and 2 cochlear

nuclei.nuclei. Spinal tract and its nucleus Spinal tract and its nucleus

of V on their way to of V on their way to midbrain above.midbrain above.

White matter at this level White matter at this level contains trapezoid body contains trapezoid body (fibers from cochlear (fibers from cochlear nuclei), medial lemniscus nuclei), medial lemniscus (becomes transverse), (becomes transverse), MLF and tectospinal tracts. MLF and tectospinal tracts.

Medial lemniscusIn medulla

T.S. Through upper ponsT.S. Through upper pons

Main features are:Main features are:Sensory and motor nuclei of VSensory and motor nuclei of VLateral lemniscus along with other Lateral lemniscus along with other

lemniscilemnisci

T.S THROUGH UPPER PONS

Lower ponsLower pons Upper ponsUpper pons

Nuclei of VI,VII,VIII Nuclei of VI,VII,VIII nerves, spinal nerves, spinal nucleus of V. nucleus of V.

Motor and principal Motor and principal sensory nucleus of sensory nucleus of V.V.

2 lemnisci; medial 2 lemnisci; medial and spinal.and spinal.

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

Trapezoid body is Trapezoid body is present.present.

Trapezoid body Trapezoid body absent.absent.

Grey matter

White matter

Main structural differences in tegmentum at two levels

Trapezoid body and the lateral lemniscus are concerned with

The auditory pathway

Clinical aspects of ponsClinical aspects of pons

Millard- Gubler syndromeMillard- Gubler syndrome

Results from a vascular lesion in lower Results from a vascular lesion in lower pons where pyramidal tracts, abducent pons where pyramidal tracts, abducent and facial nerves are situated.and facial nerves are situated.

Features:Features:Contra lateral hemiplegia (involvement of Contra lateral hemiplegia (involvement of

corticospinal tracts)corticospinal tracts) Ipsilateral facial palsyIpsilateral facial palsy Ipsilateral medial squint (involvement of Ipsilateral medial squint (involvement of

abducent)abducent)

Alternating abducent hemiplegia?

The area of close relationship between

the VI nerve, pyramids and basilar artery

Millard-Gubler syndrome

Pontine hemorrhagePontine hemorrhage

Involvement of basilar artery branches or Involvement of basilar artery branches or AICA (bilateral and extensive) results in:AICA (bilateral and extensive) results in:

Pin-point pupil (sympathetic fibers)Pin-point pupil (sympathetic fibers)HyperpyrexiaHyperpyrexiaDeep coma (reticular formation)Deep coma (reticular formation)Bilateral paralysis of limbs and face Bilateral paralysis of limbs and face

Astrocytoma of ponsAstrocytoma of pons

The most common tumour of the brainstem.

Usually occurs in childhood.