• Pons is divided into Basal (ventral, basilar) part Tegmental (dorsal) part

B

T

4th V

BT

Ventral (Basal) Pons• The basal part of

the pons consists of: Longitudinal fiber

bundles Transverse fiber

bundles Pontine nuclei,

collection of neurons that lie among the bundles.

LFTF

TF

LF

PN

• The longitudinal fibers are descending fibers that enter the pons from the midbrain.

• They are composed of:• corticospinal (pyramidal tract)• corticobulbar & • corticopontine fibres

LF

LF

Rostral

Pons

Caudal

Pons

• Corticospinal fibers originate in cerebral cortex and make synapses with motor

neurons of contralateral ventral horns of spinal cord at all levels,

are numerous, form small separate bundles, at rostral levels but many coalesce at caudal levels and reassemble as the pyramids of the medulla.

• Corticobulbar fibers originate in cerebral cortex and make synapses with cells of

motor nuclei of the cranial nerves (3,4,5,6,7,12) in brain stem.

• Corticopontine fibers originate in cerebral cortex and make synaptic contacts with

cells of the pontine nuclei of the same side (ipsilateral).

• The transverse fibers, composed of pontocerebellar fibers, originate in pontine nuclei, cross the midline, run laterally, converge to form the middle cerebellar peduncle (brachium pontis) and enter into the contralateral cerebellar hemispheres

TF

MCP

TF

• The pontine nuclei pontine nuclei are small groups of cells scattered among the longitudinal and transverse fasciculi.

• The axons of the neurons of the pontine nuclei cross the midline, form the transverse bundles of pontocerebellar fibers, and enter the cerebellum through the middle cerebellar peduncle.

PN

• The activities of the cerebral cortex are thus connected to the cerebellar cortex through relay in the pontine nuclei. (Corticopontine-pontocerebellar- cerebellodentate-dentatothalamic-thalamocortical)

• This circuit contributes to the precision and efficiency of voluntary movements.

Dorsal Pons (Tegmentum)• The pontine tegmentum is

structurally similar to the medulla & midbrain and contains:

• Ascending & descending fibersAscending & descending fibers: medial, spinal, trigeminal and lateral lemnisci, ventral spinocerebellar tract, medial longitudinal fasciculus, and superior & inferior cerebellar peduncles

• Cranial nerves nucleiCranial nerves nuclei: vestibulocochlear, facial, abducent and trigeminal nerve nuclei

MLTL

SL

LL

Lateral lemniscus: Lateral lemniscus: most laterally located, is a band of ascending fibres carrying hearing sensation from both ears (mainly from opposite side) to the auditory cortex in temporal lobe.

Spinal lemniscus:Spinal lemniscus: located just medial to lateral lemniscus, it is a band of ascending fibers carrying pain, tempreture & crude touch from opposite side of body to the thalamus.

Trigeminal lemniscus:Trigeminal lemniscus: located lateral to the medial lemniscus, a band of ascending fibers carrying pain, temperature, touch & proprioception from opposite side of face & scalp to the thalamus.

Medial lemniscus: Medial lemniscus: twists as it leaves the medulla, and lies horizontally in the ventral pontine tegmentum. The fibers from the cuneate nucleus are medial to those from the gracile nucleus. It carries proprioceptive & fine touch sensation from opposite side of body to the thalamus.

• Ventral spinocerebellar tract Ventral spinocerebellar tract traverses the most lateral part of the tegmentum and then curves dorsally and enters the cerebellum through the superior peduncle

• Medial longitudinal fasciculus: Medial longitudinal fasciculus: located near the midline in the pontine tegmentum

• Inferior cerebellar peduncles Inferior cerebellar peduncles enter the cerebellum from the caudal part of the pons. At this level, they lie medial to the middle cerebellar peduncles and form the lateral walls of the fourth ventricle

ICP

MLF

SCPMCP

ICP

4th V

• Superior cerebellar peduncles Superior cerebellar peduncles consist:

• mainly of dentatoruberal & dentatothalamic fibers that originate in cerebellar nuclei and enter the brain stem immediately caudal to the inferior colliculi of the midbrain. The fibers cross the midline at the level of the inferior colliculi in the decussation of the superior cerebellar peduncles

• also contains afferent fibers that enter the cerebellum: the ventral spinocerebellar tract and tectocerebellar fibers.

SCP

4th V

• Vestibulocochlear NerveVestibulocochlear Nerve• Fibers from the cdorsal and ventral

cochlear nuclei ascend in the pons• Most of the fibers cross in the midline.

The decussating fibers constitute the trapezoid body which intersects the medial lemnisci and then turn rostrally in the lateral part of the tegmentum to form the lateral lemniscus

• Some fibers ascend ipsilaterally to join the ipsilateral lateral lemniscus.

• Lateral lemniscus lies lateral to the medial lemniscus in the first part of its course and then moves dorsally to end in the inferior colliculus of the midbrain

TB

Trapezoid body

• The superior vestibular nucleus, extends into the pons. Fibers from the vestibular

nuclei, some crossed and some uncrossed, ascend in the medial longitudinal fasciculus.

The fibers terminate mainly in the abducens, trochlear, and oculomotor nuclei, establishing connections that coordinate movements of the eyes with movements of the head.

• Facial NerveFacial Nerve• The facial motor nucleus lies in the

ventrolateral part of the tegmentum

• Axons arising from the nucleus course dorsomedially and then form a compact bundle, which loops over the abducens nucleus beneath the facial colliculus forming the internal genu .

• After leaving the genu, the fibers pass between the nucleus of origin and the spinal trigeminal nucleus, emerging as the motor root of the facial nerve at the junction of the pons and medulla.

VIV-Spinal nucleus

• The abducens nucleus The abducens nucleus is located beneath the facial colliculus.

• The efferent motor fibers of the nucleus run ventrally and leave the brain stem as the abducens nerve between the pons and the pyramid of the medulla

• The internuclear neurons have axons that travel in the contralateral medial longitudinal fasciculus to the division of the oculomotor nucleus that supplies the medial rectus muscle.

• This arrangement provides for simultaneous contraction of the lateral rectus and contralateral medial rectus when the eyes move in the horizontal plane.

Trigeminal Nerve Nuclei

• Trigeminal NerveTrigeminal Nerve• Sensory nuclei:Sensory nuclei:

• Spinal tract and nucleus:• Located in the lateral part

of the tegmentum of the caudal half of the pons lateral to the fibers of the facial nerve.

• Chief or Principal nucleus:• Located at the rostral end

of the spinal trigeminal nucleus

• Receives fibers for touch, especially discriminative touch.

V-SpinalTract & Nucleus

V-Chief Nucleus

• Mesencephalic nucleus: • A slender column of cells

located beneath the lateral edge of the rostral part of the fourth ventricle, extending into the midbrain

• Contains pseudo-unipolar cells (cell bodies of primary sensory neurons and the only such cells in the central nervous system).

• The peripheral fibers of the unipolar neurons is distributed through the mandibular division of the nerve to proprioceptive endings in the muscles of mastication

• Motor nucleus:Motor nucleus:• Located medial to the

chief sensory nucleus• Contains motor

neurons that supply the muscles of mastication, anterior belly of digastric and mylohyoid muscles

Motor nucleus

• Trigeminothalamic tracts:Trigeminothalamic tracts:• Fibers from the trigeminal sensory

nuclei project to the thalamus via the ventral & dorsal trigeminothalamic tracts collectively forming the trigeminal lemniscus.

• The ventral trigeminothalamic tract contains crossed fibers from both chief sensory and spinal tract nuclei

• The dorsal trigeminothalamic tract, contains crossed and uncrossed fibers, originating exclusively in the chief sensory

Caudal pons (Level of abducent & facial nerve nuclei)

MLF

Facial colliculus

Spinal trigeminal tract

& nucleusFacial nerve fibers

Trapezoid bodyMedial

lemniscus

Abducent nerve nucleus

Abducent nerve fibers

Facial nerve nucleus

Basilar Pons

4th ventricle MCP

Dentate nucleus

MCP

Middle pons (Level of trigeminal nerve nuclei)

MLF

Chief sensory nucleus

Trigeminal nerve fibers

Trapezoid bodyMedial

lemniscusBasilar Pons

4th ventricle

MCP

SCP

MCP

Motor nucleus

Rostral pons

V -Mesencephalic nucleus

(Level of lemnisci)

Basilar Pons

Sup. Medullary velum

Medial lemniscus

SCP

4th ventricle

Central tegmental tract

MLF

Lateral lemniscus

Spinal lemniscus

trigeminal lemniscus

Mainly by the paramedian and circumferential branches of the basilar arterybasilar artery. The anterior inferior cerebellar artery anterior inferior cerebellar artery and the superior superior cerebellar artery cerebellar artery contribute branches to the middle and superior cerebellar peduncles and to dorsal and lateral portions of the pontine tegmentum  

BBlloooodd

SSuupppplly y