Is the middle part of the brain stem (also a part of the hind brain) its pos. surface forms the upper 1/2 of the floor for the 4th ventricle, the pons lies in contact with the clivus (basilar part of the occipital bone, body of sphenoid & dorsum sellae) its ventral (basilar) surface bulges anteriorly and is continuous laterally with the middle cerebellar peduncle on each side.
The basilar part: -
Is grooved by a longitudinal median sulcus (basilar sulcus) which lodges the basilar artery, it is continuous on each side (just outside the attachment of trigeminal nerve) as middle cerebellar peduncle.
The pons is continuous above with the 2 crura of the mid brain & below with the 2 pyramids of the medulla oblongata.
The bulge of the basilar part is due to (produced by):
Pontocerebellar fibers which goes to cerebellum via the middle cerebellar peduncle.
The trigeminal nerve is the only cranial nerve attached to the surface; it is seen at the junction between the basilar part & the middle cerebellar peduncle.
The dorsal (tegmental) part: -
Forms the upper 1/2 of the floor of the 4th ventricle, is continuous above with the tegmentum of the mid brain & below with the pos. surface of the upper 1/2 of the medulla oblongata.
It contains the nuclei of the middle 4 cranial nerves (5th -8th) and also contains 4 sensory lemnisci (medial, lateral, spinal & trigeminal).
The posterior surface of the Pons shows the following features:-
Pos. median sulcus.
On each side of which there is a vertical band of swelling called medial eminence.
At the lower part of the medial eminence we can see the facial colliculus contains the nucleus of the abducent nerve.
Each medial eminence is limited laterally by sulcus known as sulcus limitance.
At the upper part of sulcus limitance there is a pigmented area known as substantia ferrigenea.
Lateral to the sulcus limitance there is vestibular area contains some vestibular nuclei.
Nuclei in the Pons: Nuclei of trigeminal nerve ( 4 nuclei ) : - Motor nucleus: - gives motor fibers that join the mandibular nerve to supply the muscles of mastication, mylohyoid & ant. Belly of digastric. in addition to the nerves to the tensor palati & tensor tympani muscles
Main sensory nucleus : it receive touch sensation form the trigeminal area (side of the face and scalp)
Mesencephalic nucleus: - receives proprioceptive sensation from the muscles of mastication, face & eye ball.
Spinal nucleus: receive fibers of the spinal tract of trigeminal nerve (pain & temperature from the same side of the face and scalp).
Nucleus of the abducent nerve: -at the bottom of the facial colliculus and it is surrounded by fibers of facial nerve as they arise from the facial nerve nucleus , they loop around the abducent nerve creating a swelling called the facial colliculus
Nuclei of facial nerve (3 in number): - One motor, one parasympathetic (superior salivary) both of these nuclei lie in the Pons, while the 3rd nucleus (I.e. sensory for taste, the solitarius) lies in the medulla oblongata. Superior salivary nucleus gives fibers distributed via chorda tympanic (sublingual & submandibular) & greater superficial petrosal (to lacrimal & nasal glands).
Nuclei of vestibulo-cochlear nerve: - Vestibular & cochlear nuclei.
Notice the last page of lec.-3- in 14/12/2008 about the midbrain and now we will detail in some additions: -
The cerebral peduncle is divided into crus cerebri and tegmentum by the substantia nigra.
The crus contains the following descending tracks: -
Cortico-spinal fibers: - occupy the middle 3/5th of the crus cerebri.
Cortico-nuclear fibers: - situated medially to the Cortico-spinal.
Cortico-pontine fibers: - occupy the medial 1/5th and the lateral 1/5th of the crus according to the site of origin of these fibers, the fibers coming from the frontal lobe occupy the medial 1/5th, while those coming from the occipital and temporal lobes occupy the lateral 1/5th of the crus (there will form Cortico-ponto-cerebellar pathway from cerebral cortex to cerebellar cortex).
Substantia nigra separates the crus cerebri from the tegmentum & is an important extrapyramidal centre.
Each crus cerebri has the following relations: -
Laterally : on each side
Trochlear nerve , optic tract (cross the crus from behind forward )
Some blood vessels (posterior cerebral , superior cerebellar arteries and basal vein)
Is continuous below with the tegmental part of the Pons, the part of the tegmentum at the level +of the superior colliculus contains red nucleus (an important extrapyramidal centre), While at the level of inferior colliculus the tegmentum receives the decussation of the 2 superior cerebellar peduncles.
The nuclei in the midbrain: The nucleus of the occulomotor nerve : at the level of superior colliculus , it is a motor nucleus supplies 5 of the extra-ocular muscles , and also contains edinger-westphal nucleus as a parasympathetic part whose fibers goes to ciliary ganglion to supply constrictor pupillae muscle and ciliary body .
Nucleus of the trochlear nerve: - in the lower part of midbrain at the level of inf. Colliculus.
Red nucleus: - in the tegmentum at the level of sup. Colliculus , it receives afferent from the frontal cortex , corpus striatus and cerebellum , while it sends efferent as: -
Fibers or tracts
It lies in the posterior cranial fossa, behind the Pons and medulla oblongata and encloses with them the 4th ventricle. It has two surfaces (sup. and inf.), two notches (the ant. and pos. – the anterior one receives the back of the brain stem, while the posterior receives the falx cerebelli). It consist of two hemispheres connected by narrow median vermis, the part of vermis seen from above is the superior vermis, while that seen from inferior surface is the inferior vermis.
The outer cortex grey mater is highly folded with numerous transversely running fissures, the part of cortex between the fissures are called the folia of cerebellum.
The superior surface shows fissure prima that separates anterior lobe from middle lobe. The inferior surface shows a depressed area called vallecula, at the bottom of this vallecula the inferior vermis lies (forms by nodule, uvula & pyramid). The inferior surface also shows the tonsil of cerebellum situated on each side of the inferior vermis.
Cerebellum has three fissures: -
Fissure prima: - intervenes between the anterior lobe and the middle lobe.
Postero-lateral fissures: - lies on the inferior surface between the flocculo-nodular lobe and middle lobe.
Horizontal fissure: - extends along the lat. & pos. border of the hemisphere between the superior and inferior Borders of the hemisphere.
Lobes of the cerebellum: - Anterior lobe: - in front of the fissure prima and its part that extending above the superior medullary velum is called the lingula.
Middle lobe: - extends from fissure prima and postero-lateral fissure, the tonsil is part of this lobe.