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.