Brainstem -Neuroanatomy-

Brainstem:

The Brainstem occupies the posterior cranial fossa of the skull and comprise of three parts:

1. Medulla (Myelincephalon)
2. The Pons (Metincephalon)
3. The Midbrain (Mesencephalon)

The pons and medulla together form Rhombincephalon. The brainstem connects the spinal cord inferiorly to the brain (diencephalon specifically) superiorly.  It has three functions:

1. it channels the ascending and descending tracts between the spinal cord and brain.
2. it houses the reflex centers e.g. those associated with respiratory, cardiovascular system and with control of consciousness
3. it contains the nuclei of all the cranial nerves except the first 2.

Medulla connects the pons superiorly with the spinal cord. On its anterior surface is a fissure called Anterior Median Fissure which continues with the fissure of spinal cord. Lateral to the fissure, on either side are large elevations called Pyramids that houses bundles of descending nerve fibers called Corticospinal Fibers. Lateral to pyramids are Olives which marks the location of underlying Olivary Nuclei.

 The groove between the pyramids and the olives bear the rootlets of Hypoglossal (12^th cranial) Nerve.  Posterior to olives, the inferior cerebellar peduncles connect the medulla to the cerebellum. The grove between the inferior cerebellar peduncle and the olives are rootlets for the glossopharyngeal (9^th), Vegas (10^th) and cranial roots of accessory nerve (11^th). Posterior Surface of superior half medulla forms the inferior part of the floor of the 4^th ventricle. The posterior surface possesses the posterior median sulcus, lateral to which are Gracile and Cuneate tubercle respectively.

Pons connects the Midbrain to the Medulla. Its anterior surface is convex and bears many transverse fibers the form the Middle Cerebellar Peduncle. There is a shallow grove in the midline, the basilar grove for the basilar artery. On the anterolateral surface of the pons, the sensory and motor roots of trigeminal nerve emerge. At the pontomedullary junction, from medial to lateral emerges the Abducent (6^th), Facial (7^th) and Vestibulocochlear (8^th) nerve. Posterior surface of the pons form the superior part of the floor of the 4^th ventricle. Posterior median sulcus divides the posterior surface into symmetric halves, limited by the superior cerebellar peduncles. Lateral to the sulcus are elevations called medial eminence that are bounded laterally by Sulcus limitans. Inferior end of median eminence is slightly expanded to form the Facial Colliculus.

Midbrain connects the pons and medulla to the cerebrum. It is traversed by narrow channel, the cerebral aqueduct which is filled with CSF. On the posterior surface are four Collicili (corpora quadrigemina). The superior colliculi are centers for visual reflex while the inferior colliculi are lower auditory centers. In the midline, below the inferior Colliculus, the trochlear (4^th) nerve emerges. On the anterior surface of midbrain there is a deep depression called the interpeduncular fossa which is bounded on either side by crus cerebri. The Occulomotor nerve emerges for from the groove on the medial side of crus cerebri and passes forward in the lateral wall of cavernous sinus.  

Histology Of Neuroanatomy.

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Base of the Skull

Base of the Skull:

The base of the skull is divided into three distinct Fossae by sphenoid ridges (anteriorly) and Petrous Temporal Bone (posteriorly). These are the Anterior, Middle and Posterior Cranial Fossae.

Anterior Cranial Fossa:

Anterior cranial fossa lies above the nasal cavity and the orbits and houses the frontal lobe of the two cerebral hemispheres. Bones that form the anterior cranial fossa are:

·         The Ethmoid bone (in the midline)

·         The orbital part of frontal bone (anterolateral to Ethmoid bone)

·         The Sphenoid Bone (posteriorly)

The Ethmoid bone found in the middle of the anterior cranial fossa has a small prominent bony ridge called the Cristal Galli. This along with the Frontal Crest on the anteriorly provide attachment site for Falx Cerebri –vertical extension of Dura matter, partially separating the two cerebral hemispheres-. On either side of the Crista Galli are cribriform plates –for the olfactory bundle- that form the narrow roof of the nasal cavity. Anteriorly between the Crista Galli and frontal crest is Foramen Cecum.

Lateral to the Ethmoid bone are the orbital part of the frontal bone, forming the floor of the anterior cranial fossa and roof of the orbits. Posterior to these two bones is the Ethmoid bone, the body of which extends posteriorly into the middle cranial fossa. Laterally projecting from the body are the lesser wings of the sphenoid bone that extends as far as the junction between the temporal and greater wing of sphenoid bone on the lateral wall of the skull. The lesser wing of sphenoid bone also forms boundary between the anterior and middle cranial fossa. Medially the lesser wing of sphenoid fossa form the Anterior Clinoid Process that provide attachment site for the Tentorium Cerebri –sheet of dura matter separating parts of cerebral hemisphere from cerebellum. Anterior to it is an opening –optic canal- for the ophthalmic artery and optic nerve.

Middle Cranial Fossa:

Middle cranial fossa is bounded anteriorly by the chiasmatic sulcus and the lesser wing of the sphenoid bone. While the posterior boundary is formed by the anterior surface of the superior part, of the petrous part of the temporal bone. The bones that form the floor of this fossa are parts of sphenoid and temporal bone.

The floor at the midline of the middle cranial fossa is formed by the body of the sphenoid bone. On either side of it are large depressions formed by the greater wing of the sphenoid and squamous part of temporal bone. These depressions contain the temporal lobe.

Sella Tursica is a complex bony structure formed by the sphenoid bone. The anterior wall of the Sella Tursica is formed called the Tuberculum Sellae. The posterior wall – the Dorsum Sellae- of Sella Tursica have prominent bony projection on the lateral edges which form the Posterior Clinoid Process, which like its anterior counterpart, provide attachment site for the Tentorium Cerebri. Between the two walls is the Hypophysial Fossa containing the pituitary gland.

On the petrous part of temporal bone, medially there is a slight depression called the trigeminal impression for the sensory ganglia of trigeminal nerve. Lateral to that is a round bony protuberance – the Arcuate eminence –produced by the underlying semicircular canal (internal ear). Anterolateral to that is Tegmen Tempani which is a slight depression that marks the roof of the middle ear cavity.

Foramen and fissures that are found in the middle cranial fossa are:

·         The Superior Orbital Fissure – Occulomotor, Trochlear, Opthalmic (nerve and vein) and Abducent  Nerve

·         Foramen Rotundum – maxillary branch of trigeminal nerve

·         Foramen Ovale – Mandibular nerve, Petrosal Nerve and the accessory middle meningeal artery

·         Foramen Spinosum – The Middle meningeal artery and vein.

Posterior Cranial Fossa:

It is formed mainly by the occipital bone and temporal bone. It is the largest and deepest cranial fossa and houses the brainstem (medulla, pons, and midbrain) and cerebellum. The anterior boundary of the posterior cranial fossa is the Dorsum Sellae and Clivus. The Clivus slopes from the foramen magnum and is partly formed by the sphenoid and partly by the occipital bone. It also forms the base of the occipital bone. Laterally the anterior boundary of the posterior cranial fossa is formed by part of the petrous part of temporal bone. Posteriorly it is bounded by the internal surface of the squamous part of the occipital bone.

There is a large foramen in the deepest part of the posterior cranial fossa, the Foramen magnum, allows the spinal cord to pass into the skull and connect to the brainstem.

Foramen:

·         Internal acoustic meatus –Facial nerve, Vestibulocochlear nerve

·         Jugular foramen –Glossopharyngeal, Vegas and Accessory nerve

·         Hypoglossal foramen –Hypoglossal nerve