Brainstem - Gross Anatomy of Medulla Oblongata, Pons and Midbrain

BRAINSTEM: GENERAL CONSIDERATIONS

Components:

The brainstem consists of (from above downward):

Midbrain (continuous with the cerebral hemisphere above)

Pons

Medulla (continuous with the spinal cord below)

Location:

The brainstem is located in posterior cranial fossa

Relations:

Above, midbrain is continuous with the cerebral hemisphere.

Below, medulla is continuous with the spinal cord.

Posteriorly, the pons and medulla are separated from the cerebellum by the fourth ventricle.

The fourth ventricle is continuous,

below, with the central canal, which traverses the lower part of the medulla and is continuous with the central canal of spinal cord and

above, with the cerebral aqueduct of midbrain.

The midbrain, pons and medulla are connected to the cerebellum by superior, middle and inferior cerebellar peduncles, respectively.

Cranial nerves related to the brainstem

The third and fourth nerves emerge from the surface of the midbrain;

The fifth from the pons;

The sixth, seventh and eighth nerves emerge at the junction of the pons and medulla;

The ninth, tenth, eleventh and twelfth nerves emerge from the surface of the medulla.

Functions:

The brainstem has three broad functions:

1.     Serves as a conduit for the ascending tracts and descending tracts connecting the spinal cord to the different parts of the higher centers in the forebrain;

2.     Contains important reflex centers associated with the control of respiration and the cardiovascular system and with the control of consciousness; and

3.     Contains the important nuclei of cranial nerves III through XII.

GROSS ANATOMY OF MEDULLA OBLONGATA

The medulla oblongata is conical in shape. Its broad part joins the pons above and narrow part becomes continuous with the spinal cord. The junction between medulla and spinal cord coincides with the level of the upper border of Atlas (first cervical vertebra).

Its length is about 3 cm and its width is about 2cm at its upper end.

It is divided into

1.     A lower closed part with central canal and

2.   An upper open part posteriorly which is related to the lower part of the 4^th ventricle

Features on the anterior surface of Medulla Oblongata

Anterior median fissure, is an upward continuation of similar fissure present on the spinal cord

Anterolateral sulcus, on each side, is in line with the ventral roots of spinal cord

-         Gives attachment to the rootlets of the hypoglossal nerve

Pyramid is an elevation on each side of the midline between anterior median fissure and anterolateral sulcus.

-         Composed of bundles of nerve fibers of corticospinal tract that descends from the cerebral cortex to the spinal cord

-         Tapers inferiorly where the majority of fibers cross over to the opposite side, obliterating the medulla. These crossing fibers constitute the decussation of the pyramid.

Olive is a prominent, elongated oval swelling that lies in the upper part of medulla posterolateral to the pyramid separated by anterolateral sulcus.

The elevation is produced by the underlying inferior olivary nucleus.

Features on posterior surface of the medulla oblongata

Posterior median sulcus is upward continuation of the similar fissure on the spinal cord.

Posterolateral sulcus lies in line with the dorsal roots of spinal nerves.

-         Gives attachment to the rootlets of 9^th, 10^th and 11^th cranial nerves.

Between the posterior median sulcus and posterolateral sulcus, the medulla contains tracts (asccending) that enter it from the posterior funiculus of the spinal cord.

-         Fasciculus gracilis lies medially and fasciculus cuneatus lies laterally

-         Both fasciculi end in rounded elevations called gracile tubercle (nucleus gracilis) and cuneate tubercle (nucleus cuneatus) respectively.

Just above these tubercles, medulla is occupied by a triangular fossa which forms the lower part of the 4^th ventricle.

This fossa is bounded on each side by inferior cerebellar peduncle  which connect the medulla to cerebellum.

Features on the posterior part of the medulla that forms the floor of the 4^th ventricle:

Presents median sulcus, on each side of which there is a longitudinal elevation called the median eminence (continuous above in the pontine part of the floor of 4^th ventricle). The eminence is bounded laterally by sulcus limitans.

The sulcus limitans is marked by a depression called inferior fovea. The part of the medulla below fovea presents hypoglossal triangle medially and vagal triangle laterally.

Between the vagal triangle, above and gracile tubercle, below lies a small area called area postrema.

The lowest part of the floor is called the calamus scriptorius (for its resemblance to a nib).

The inferior angle where the lateral margins of the floor meet is called obex.

GROSS ANATOMY OF PONS

Pons has a convex anterior surface marked by transversely running fibers which laterally forms a bundle called middle cerebellar peduncle.

Main Features

-   The trigeminal nerve emerges from the anterior surface at its junction with middle cerebellar peduncle.

-       Presents a  basilar sulcus in the midline which lodges basilar artery

-    In the groove between Pons and the medulla oblongata, there emerge, from medial to lateral, abducent, facial and vestibulocochlear nerves.

Posterior surface of the pons is limited laterally by superior cerebellar peduncle and forms the upper part of the floor of the 4^th ventricle.

Main Features:

-        The floor is divided into symmetrical halves by a median sulcus.

-      Lateral to this sulcus is an elongated elevation, the medial eminence, which is bounded laterally by a sulcus limitans.

-     Inferior end of medial eminence is slightly expanded to form facial colliculus, which is produced by facial nerve

-     The upper end of sulcus limitans presents a bluish-gray coloration and the area is called substantia ferruginosa.

-         Area vestibule lies lateral to sulcus limitans.

Parts of the Pons

·        a posterior part, the tegmentum, and

·        an anterior basilar part

GROSS ANATOMY OF MID-BRAIN

Anteriorly, it presents two large bundles of fibers, one on each side of the midline, called crus cerebri.

-    The oculomotor nerve emerges from the medial aspect of the crus of the same side

-        The crura bounds from behind the interpeduncular fossa.

Posteriorly, the midbrain presents four rounded swellings called colliculi.

-         Superior and inferior colliculi one on each side

-         Each colliculus is laterally related to a ridge called brachium

o   Superior and inferior brachium from respective colliculi

o   Superior brachium connects the superior colliculus to lateral geniculate body

o   Inferior brachium connects the inferior colluculus to medial geniculate body

o   In the midline below the inferior colliculus, the trochlear nerve emerges which then winds round the side of the midbrain to reach its ventral aspect.

Parts of the midbrain

The midbrain comprises two lateral halves, called the cerebral peduncles; which is again divided into an anterior part, the crus cerebri, and a posterior part, tegmentum, by a pigmented band of gray matter, substantia nigra.

The narrow cavity is the cerebral aqueduct, which connects the 3^rd and 4^th  ventricles.

The tectum is the part of the midbrain posterior to the cerebral aqueduct; it has four small surface swellings referred to previously; these are two superior and two inferior colliculi.

REFERENCES:

Following resources are used while preparing this post (readers are strongly recommended to go through them for more details):

Snell’s Clinical Neuroanatomy 7th Edition      

Lange Clinical Neuroanatomy 25th Edition