Brainstem 2

PONS

External features of Pons

Pons• Literally means

“bridge”• Wedged between the

midbrain & medulla.• Pons shows a convex

anterior surface with prominent transversely running fibres:Transverse pontine (pontocerebellar) fibers

PONS – VENTRAL SURFACE Basilar sulcus:• Divides the pons into 2

halves, occupied by basilar artery.

Transverse pontine (pontocerebellar) fibers:

• Originate from pontine nuclei, cross the midline & pass through the contralateral middle cerebellar peduncle to enter the opposite cerebellar hemisphere.

Nerves emerging from Pons (4 nerves):

• Trigeminal (5th): from the middle of ventrolateral aspect of pons, as 2 roots: a small medial motor root & a large lateral sensory root.

• Abducent (6th): from sulcus between pons & pyramid.

• Facial (7th) & vestibulocochlear (8th): at the cerebellopontine angle (junction between medulla, pons & cerebellum).

• Both nerves emerge as 2 roots:

• from medial to lateral: motor root of 7th, sensory root of 7th vestibular part of 8th & cochlear part of 8th

The dorsal part of the pons may be regarded as continuation of the part of the medulla behind the pyramids.

Superiorly continuous with the tegmentum of the midbrain.

Occupied predominately by the reticular formation

Posterior surface help to form floor of fourth ventricle

The dorsal part is bounded laterally by inferior cerebellar peduncle in the lower part of the pons and superior cerebellar peduncle in upper part.

DORSAL PART

Midpons

Upper pons

PONS – DORSAL SURFACE

The posterior surface shows:• Upper half of the floor of 4th

ventricle• Superior cerebellar peduncle• Median sulcus• Sulcus limitans• Medial eminence• Vestibular area• Facial colliculus

PONS – DORSAL SURFACE

PONS – DORSAL SURFACE Separated from the medulla

by an imaginary line passing between the caudal margins of middle cerebellar peduncle.

• On either side, the median sulcus divides the area into 2 parts (from medial to lateral):

Medial eminence & facial colliculus: overlies abducent nucleus.

Vestibular area: overlies vestibular nuclei.

Internal structures

CAUDAL PART OF THE PONS• Divided into an anterior part

(Basis Pontis) & a posterior part (Tegmentum) by the Trapezoid Body (consists of acoustic fibres from cochlear nuclei to ascend into midbrain as lateral lemniscus and terminate in inferior colliculus).

Lateral lemniscus• Auditory pathway that conducts most

contralateral cochlear inputTrapezoid body• – Formed by decussating fibers of the ventral

cochlear nuclei

• The ventral portion is marked by numerous transversely oriented fascicles of pontocerebellar fibres that originate from scattered cell groups, the pontine nuclei, and that pass to the contralateral side of the cerebellum through the massive middle cerebellar peduncle.

The ascending fibres of the medial lemniscus become separated from the pyramid and displaced dorsally.

The Medial Lemniscus rotates 90 degrees and lies almost horizontally.

It contains spinal nucleus & tract of Trigeminal.

Deep origin of cranial nerve nuclei:Abducent nucleusFacial motor nucleus

CAUDAL PART OF THE PONS

AT THE LEVEL OF THE TRIGEMINAL NERVE Motor nucleus of the

trigeminal nerve: Lies in the lateral part of the floor of the 4th ventricle. Main sensory nucleus of

the trigeminal nerve:Reaches its maximum extent in the pons and it lies lateral to the motor nucleus. Superior cerebellar

peduncles form the lateral boundary of the 4th ventricle

ROSTRAL PONS• Superior Medullary

Velum: – Passes between the

two peduncles & forms the roof of the 4th ventricle.

• Medial longitudinal fasciculus: – Lie close to the

midline beneath the floor of the 4th ventricle.

• Contains:– Sensory and motor nuclei

for 4 cranial nerves • Trigeminal (5), Abducent

(6), Facial (7), and Auditory/Vestibular (8)

– Respiratory nuclei:Apneustic & pneumotaxic centers work w/ the medulla to maintain respiratory rhythm– Nuclei & tracts that process

and relay info to/from the cerebellum

– Ascending, descending, and transverse tracts that interconnect other portions of the CNS

The End