The Pons Internal Features

2. The Pons Internal Features Dr. ZeenatZaidi

3. 4th V T T B B • Pons is divided into • Basal (ventral, basilar) part • Tegmental (dorsal) part

4. Ventral (Basal) Pons LF TF • The basal part of the pons consists of: • Longitudinal fiber bundles • Transverse fiber bundles • Pontine nuclei, collection of neurons that lie among the bundles. TF PN LF

5. LF Rostral Pons Caudal Pons • The longitudinal fibersare descending fibers that enter the pons from the midbrain. • They are composed of: • corticospinal (pyramidal tract) • corticobulbar& • corticopontinefibres LF

6. 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).

7. TF MCP TF Thetransverse 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

8. PN The 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.

9. 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.

10. Dorsal Pons (Tegmentum) • The pontinetegmentum is structurally similar to the medulla & midbrain and contains: • Ascending & descending fibers: medial, spinal, trigeminal and lateral lemnisci, ventral spinocerebellar tract, medial longitudinal fasciculus, and superior & inferior cerebellar peduncles • Cranial nerves nuclei: vestibulocochlear, facial, abducent and trigeminal nerve nuclei

11. Medial lemniscus: twists as it leaves the medulla, and lies horizontally in the ventral pontinetegmentum. 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. LL SL TL 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. ML 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: locatedjustmedial to lateral lemniscus, it is a band of ascending fibers carrying pain, tempreture & crude touch from opposite side of body to the thalamus.

12. MLF SCP MCP 4th V ICP ICP 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: located near the midline in the pontinetegmentum 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

13. 4th V SCP 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.

14. TB Trapezoid body Vestibulocochlear 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 bodywhich 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

15. 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.

16. VI V-Spinal nucleus Facial 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.

17. 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.

18. Trigeminal Nerve Nuclei

19. V-Spinal Tract & Nucleus V-Chief Nucleus • Trigeminal Nerve • 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.

20. 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

21. 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

22. 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

23. Caudal pons (Level of abducent & facial nerve nuclei) Dentate nucleus Facial colliculus MLF 4th ventricle MCP MCP Abducent nerve nucleus Spinal trigeminal tract & nucleus Facial nerve fibers Facial nerve nucleus Abducent nerve fibers Medial lemniscus Trapezoid body Basilar Pons

24. Middle pons (Level of trigeminal nerve nuclei) SCP Chief sensory nucleus MLF 4th ventricle Motor nucleus MCP Trigeminal nerve fibers MCP Medial lemniscus Basilar Pons Trapezoid body

25. Rostral pons (Level of lemnisci) Sup. Medullary velum 4th ventricle V -Mesencephalic nucleus SCP Central tegmental tract Lateral lemniscus Spinal lemniscus MLF trigeminal lemniscus Basilar Pons Medial lemniscus

26. Supply Blood Mainly by the paramedian and circumferential branches of the basilar artery. The anterior inferior cerebellar artery and the superior cerebellar artery contribute branches to the middle and superior cerebellar peduncles and to dorsal and lateral portions of the pontinetegmentum

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