Brain

The brain is the largest organs of the body, consisting of about 100 billion neurons and 10-50 trillion neuroglia with a mass of about 1300g.

The four major parts are the brain stem, diencephalon, cerebrum and cerebellum.

BRAIN STEM

The brain stem is the region of the brain that connect the cerebrum with  the spinal cord. It consists of the medulla oblongata,,midbrain and the pons.

1. Medulla Oblongata

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 9th, 10th and 11th 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.

 2. 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.

 

3. Midbrain

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 3rd and 4th  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.

DIENCEPHALON

The diencephalon, along with the telencephalon (cerebrum) comprise the two major divisions of prosencephalon (forebrain). Main structures of the diencephalon include the hypothalamus, thalamus, epithalamus (including the pineal gland), and subthalamus. The diencephalon relays sensory information between brain regions and controls many autonomic functions of the peripheral nervous system. It also connects structures of the endocrine system with the nervous system and works in conjunction with limbic system structures to generate and manage emotions and memories.

CEREBELLUM

In Latin, the word cerebellum means little brain. The cerebellum is the area of the hindbrain that controls motor movement coordination, balance, equilibrium and muscle tone. Like the cerebral cortex, the cerebellum is comprised of white matter and a thin, outer layer of densely folded gray matter. The folded outer layer of the cerebellum (cerebellar cortex) has smaller and more compact folds than those of the cerebral cortex. The cerebellum contains hundreds of millions of neurons for processing data. It relays information between body muscles and areas of the cerebral cortex that are involved in motor control.

CEREBRUM

The cerebrum, also known as the telencephalon, is the largest and most highly developed part of the human brain. It encompasses about two-thirds of the brain mass and lies over and around most of the structures of the brain. The outer portion (1.5mm to 5mm) of the cerebrum is covered by a thin layer of gray tissue called the cerebral cortex. The cerebrum is divided into right and left hemispheres that are connected by the corpus callosum. Each hemisphere is in turn divided into four lobes. The cerebrum or telencephalon, along with the diencephalon comprise the two major divisions of prosencephalon (forebrain).