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The pons is a special relay center for the cortico-ponto –cerebellar tract. It is responsible for highly coordinated repititive movements that require skill and extensive control.

The pons is divided into a ventral basis pontis and a dorsal pontine tegmentum.

Grossly it has two massive structures on the sides of the ventrally placed basis pontis. The groove or sulcus between the two lateral structures contain the basilar artery which provide pontine branches that reach the pons circumferentially.

On the lateral aspect of the pons we have two massive middle cerebellar peduncles which course forwards and downwards to reach the cerebellum.  They are carrying fibers which are second order neurons of the corticopontocerebellar pathway to the deep cerebellar nuclei.


1 Cerebellar peduncles cut away from cerebellum: 1superior cerebellar peduncle, 2 middle cerebellar peduncle 3 inferior cerebellar peduncle 4 pons 5 cerebellum

Posteriorly the pons is provided with the floor of the 4th ventricle. The roof of the ventricle is made up of superior medullar velum the fastigium of the cerebellum.

Behind the pons the 4th ventricle has the following prominent structures

  • Tuberculum acousticum which is part of the vestibular area

  • Striae medullaris

  • Median eminence which is continuous with that of  the medulla

  • Facial colliculus

 

 

 

 


  • Front of pons: 1 pyramid 2 olive 3 hypoglossal n 4 pons 5 tirgeminal n

 

 

 

 

 

Internal structure of the pons

Basis pontis

This is ventrally placed containing massive corticopontine fibers which terminate on the second order neurons in the basis pontis.

These fibers arise from all the lobes of the cortex hence we have

  • Temporopontine

  • Parietopontine

  • Occipitopontine

  • Frontopontine fibers

Frontopontine fibers occupy the anterior position in the pyramidal tract. The fibers terminate at different levels in the pons and synapse on the second order neurons which then run transversely to reach the middle cerebellar peduncle of the opposite side. These fibers are termed the transversely running pontine fibers. They will project through the middle cerebellar peduncle to enter the deep cerebellar nuclei which are as follows

  • Globose 

  • Emboliform

  • Fastigial

  • Dentate nuclei

Pontine tegmentum

This carries two important structures

1. Pontine reticular formation

2. The cranial nerve nuclei associated with the pons which are as follows V, VI, VII.

Pontine reticular formation

This is an extension of the medullary reticular formation and it consist of two parts

Central tegmental part

Lateral part which contains the following nuclear groups

  • Nucleus reticularis pontis pars oralis. This is the most cranial

  • Nucleus reticularis pontis pars caudalis. This is continuous with the reticular formation of the medulla

  • Nucleus reticularis pontis pars interpolaris. This is placed between the pas oralis and pars caudalis.

Pontine cranial nerve nuclei

Facial nerve

This nerve has its nucleus buried in the facial colliculus, a little ventral to the nucleus of the abducent nerve.


Facial nerve coloured yellow

 It then forms a genu  which sweeps across the dorsal surface of the abducent nucleus in order to reach the lateral aspect of the pontine tegmentum where it runs a ventral course to escape at the cerebellopontine angle, medial and inferior to the point of escape of the abducent nerve. It, immediately at its extrabulbar course, forms the external genu of the facial nerve before it expands to form the geniculate ganglion.  Its nervus intermedius part then gives off two branches

  • Chorda tympani

  • Greater petrosal nerve

The chorda tympani enters the lingual nerve and it carries secretomotor fibers which reach the submandibular ganglion and are distributed to oral glands and the submandibular and sublingual salivary glands. The greater petrosal nerve passes forwards to enter the pterygopalatine ganglion where it relays and its postganglionic fibers then enter nasal and palatine glands. The facial nerve itself proceeds through the stylomastoid canal and emerges at the stylomastoid foramen to enter the face and supply the muscles of facial expression.

The nucleus associated with the facial nerve is the motor nucleus of facial nerve. Other nuclei are the superior salivatory nucleus, and the nucleus of the tractus solitarius. Hence the facial nerve, together with its nervus intermedius component has the following functional components

  1. GSA fibers, which supply the auricle

  2. GVE fibers, which are secretomotor from the superior salivatory nucleus. They will supply the submandibular and sublingual glands including nasal and palatine glands and the lacrimal apparatus via the nerve which hitch hikes in the zygomaticotemporal nerves.

  3. SVE fibers supply the muscles of facial expression, including platysma, buccinator, posterior belly  digastric  and the stylohyoid. They are from the motor nucleus of facial nerve.

  4. SVA for taste in the anterior 2/3 of tongue via the chorda tympani.

Abducent nerve

This nerve has its nucleus placed within the facial colliculus. It is responsible for the formation of the colliculus at the floor of the 4th ventricle. The fibers of the internal genu of facial nerve sweep across its dorsal aspect and later lateral to it. It escapes from the brain stem above the pyramids at the pontomedullary junction.

The abducent nucleus is surrounded by the parabducent nucleus. This nucleus is thought to contain fibers which are part of the extrapyramidal system that control lateral gaze movement. Abducent nerve supplies the lateral rectus.  Damage to this nerve will cause paralysis of the lateral rectus. Damage to the parabducent nucleus will lead to paralysis of lateral gaze movements.

The rostral part of the pons begins to change its configuration so that of the isthmus rhombencephali. Hence certain nuclear groups belonging to the isthmus begin to show themselves. They include raphe nuclei and locus ceruleus.

 Trigeminal nerve 

The trigeminal nerve has the following nuclear groups in the pontine tegmentum

Sensory nuclear group which carry GSA fibers. They are 

  • Mesencephalic nucleus of V

  • Principal sensory nucleus of V (pontine nucleus)

  • Spinal nucleus of V (this is an extension from the upper cervical level through the medulla.

  • Motor nucleus of V which carry SVE fibers to structures derived from the pharyngeal arch 1- muscles of mastication and other.

Sensory nuclear group is quite massive. It extends from cervical level C4 through the medulla to the mesencephalon. It may therefore be divided into two

Superior group. This is found in the pons and midbrain. It is the principal sensory nucleus and the mesencephalic nucleus of V. The pontine nucleus or principal sensory nucleus is placed lateral to the motor nucleus in the pontine tegmentum. They both lie in the dorsal aspect of the tegmentum, close to the floor of the 4th ventricle. The mesencephalic nucleus is slightly displaced more dorsally, medial to the superior cerebellar peduncle and extending upwards into the midbrain.

The mesencephalic nucleus contains first order neurons for proprioception of the face region supplied by the fibers of V. Principal sensory nucleus is responsible for fine sensation of the face region while the spinal nucleus is responsible for coarse sensation in the face region. the sensory neurons are psedounipolar with their cell bodies in the trigeminal ganglion placed at the trigeminal cave of the middle cranial fossa. Their peripheral processes pass through the corresponding nerve divisions as follows

  • Ophthalmic 

  • Maxillary 

  • Mandibular

The sensory nuclear group is arranged in a lamina fashion. Ophthalmic fibers are ventral, maxillary intermediate and mandibular dorsal

Motor nucleus of V passes through the portio minor  being smaller than the sensory division, which passes through portio major. The fibers remain separate from the sensory division, but join the division distally by entering the mandibular nerve which carries them through e foramen ovale into the infratemporal fossa. The motor divions of V will supply the muscles of mastication, medial pterygoid, lateral pterygoid, masseter and temporalis, and then also the other muscles derived from the first pharyngeal arch, which are mylohyoid, tensor tympani, tensor veli palatini, and anterior belly of digastric muscle with SVE fibers.

Isthmus rhombencephali

The junction between the pons and the midbrain contains the isthmus rhombencephali. This means the isthmus of the the hindbrain. This is because it has a small cavity which joins the cerebral aqueduct superiorly in the midbrain. The isthmus has certain characteristics. For example, the raphe nucleus and locus ceruleus are found in its tegmentum. Its ventral portion is similar inconfiguration to the upper part of the pons. This contains prominent transversely running fibers which are equally maintained at rostral levels in the lower part of the midbrain.  It has a diagnostic feature which is the presence of the decussation of the trochlear which takes place dorsally, and dorsal to the superior edge of the 4th ventricle at the superior medullary velum which forms the roof of the 4th ventricle at the back of the pons. This is usually used to determine the presence of the isthmus in cross sections of the brain stem.

The superior cerebellar peduncle also begins its decussation at this level. This decussation will continue to lower levels of the midbrain. The lateral lemniscus is now placed lateral to the medial lemniscus beginning at rostral levels of the pons. The superior olive is placed between them. Gradually the lateral lemniscus begins to escape dorsally as it enters the tectum, initially to the inferior colliculus before it reaches the superior colliculus.

 

 

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