1. Brainstem 2 PONS

2. External features of Pons

3. Pons Literally means “bridge” Wedged between the midbrain & medulla. Pons shows a convex anterior surface with prominent transversely running fibres:Transverse pontine (pontocerebellar) fibers

4. PONS – VENTRAL SURFACE  Basilar sulcus: occupied by basilar artery. Divides the pons into 2 halves, occupied by basilar artery.  Transverse pontine (pontocerebellar) fibers:, middle cerebellar peduncle cerebellar hemisphere Originate from pontine nuclei, cross the midline & pass through the contralateral middle cerebellar peduncle to enter the opposite cerebellar hemisphere.

6.  Nerves emerging from Pons (4 nerves): Trigeminal (5 th ): from the middle of ventrolateral aspect of pons, as 2 roots: a small medial motor root & a large lateral sensory root. Abducent (6 th ): from from sulcus between pons & pyramid.

7. Facial (7 th ) & vestibulocochlear (8 th ): at the cerebellopontine angle (junction between medulla, pons & cerebellum). Both nerves emerge as 2 roots: from medial to lateral: motor root of 7 th, sensory root of 7 th vestibular part of 8 th & cochlear part of 8 th

8.  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

10. The posterior surface shows: Upper half of the floor of 4 th ventricle Superior cerebellar peduncle Median sulcus Sulcus limitans Medial eminence Vestibular area Facial colliculus PONS – DORSAL SURFACE

11.  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: abducent nucleus.  Medial eminence & facial colliculus: overlies abducent nucleus.  Vestibular area: vestibular nuclei.  Vestibular area: overlies vestibular nuclei.

12. Internal structures

13. 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).

14.  Lateral lemniscus Auditory pathway that conducts most contralateral cochlear input  Trapezoid body – Formed by decussating fibers of the ventral cochlear nuclei

15. 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.

16. medial lemniscus  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 nucleus  Facial motor nucleus CAUDAL PART OF THE PONS

18. AT THE LEVEL OF THE TRIGEMINAL NERVE  Motor nucleus of the trigeminal nerve: Lies in the lateral part of the floor of the 4 th 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 4 th ventricle

19. ROSTRAL PONS Superior Medullary Velum: – Passes between the two peduncles & forms the roof of the 4 th ventricle. Medial longitudinal fasciculus: – Lie close to the midline beneath the floor of the 4 th ventricle.

20. 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