Anatomy of the Brain Stem

External features

Anterior view

The anterior surface of medulla oblongata is grooved by an anteromedian fissure,on either side of which are the swellings due to the pyramidal tracts. These

pyramids, in turn, are separated from the olivary eminences by the anterolateral sulcus along which the rootlets of the 12th cranial nerve emerge.

Between the olive and the inferior cerebellar peduncle there is yet another groove corresponding to the posterolateral sulcus of the spinal cord; emerging from this groove are the rootlets of cranial nerves IX, X and XI

The pons lies between the medulla and the midbrain and is connected to the cerebellum by the middle cerebellar peduncles.

Its ventral surface presents a shallow median groove and numerous transverse ridges, which are continuous laterally with the middle cerebellar peduncle.

Its junction with the medulla is marked close to the ventral midline by the emergence of the 6th

7th and 8th cranial nerves nerves.

5th cranial nerves leave the lateral part of the pons near its upper border.

The anterior surface of the midbrain is formed by the cerebral peduncles (descending cerebral pathways).

The fibres of the 3rd nerves emerge between the two cerebral peduncles in the interpeduncular fossa.

Posterior view

The posteromedian sulcus of the spinal cord is continued half-way up the medulla, where it widens out to form the posterior part of the 4th ventricle.

On either side of the fissure the posterior columns of the spinal cord expand to form two distinct tubercles, the gracile and cuneate nuclei

The dorsal surface of the pons forms the upper part of the floor of the 4th ventricle

The dorsal surface of the midbrain presents corpora quadrigeminia,including two inferior,and two superior colliculi .

The corpora quadrigemina are reflex centers involving vision and hearing

Internal structure

Cranial nerve nuclei

Non-cranial nerve nuclei

Ascending and descending pathways

Reticular formation

Cranial nerve nuclei are referred to those nuclei located in the brain stem, where the cranial nerves originate or terminate except the olfactory and optic nerve

Non-cranial nerve nuclei are referred to those nuclei which are not linked directly to any cranial nerves functionally or structurally

Ascending pathways include 1) pathways originate in the spinal cord and pass

the brain stem to higher region of the brain; 2) pathways originate in the brain

stem to higher region of the brain

Descending pathways originate in the same motor area of the cortex, but terminate in 1) spinal cord and 2) brain stem

Reticular formation is recognized as an extensive field of intermingled grey and white matter outside the fiber bundles and nuclei of the brain stem

Long ascending pathways

Passing fibrous bundles which originate in the spinal cord:

1) Medial lemniscus: fibrous bundle formed by relayed fibers from fasciculi gracilis,

cuneatus. The fibers relay in gracile and cuneate nuclei, and decussate with fibers of opposite side, finally end in the ventral posteriomedial nucleus of thalamus

2) Spinothalamic lemniscus： composed of anterior and posterior spinothalamic tracts, and terminate in the ventral posterolateral nucleus of thalamus

Long ascending pathways

Fibrous bundles originated in the brain stem:

1) Trigeminal lemniscus: formed by fibers from contralateral spinal and pontine nuclei of trigeminal n. It conducts the tactile, pressure, pain and thermal impulses to the ventral posterolateral nucleus of thalamus

2) Lateral lemniscus: formed by the fibers from the ipsilateral and controlateral cochlear nuclei; some of fibers end in the inferior colliculus, others traverse the brachium of inferior colliculus to the nucleus of medial geniculate body

Long descending pathways

Pyramidal tract : descending fiber bundles originated in motor cortex to lower motor neurons in brain stem and spinal cord, It includes 1) corticospinal tract and 2) corticonuclear tract

The cerebellum

The cerebellum is the integrative organ for the coordination and fine-tuning of movement and for the regulation of muscle tone.

it is situated in the post cr fossa behind the pons & medulla.

EXTERNAL FEATURES

The cerebellum consists of two cerebellar hemispheres that are united to each other by the median vermis.

it has 2 surfaces sup & inf.

The sup surface is convex. The two hemispheres are continued each other on this surface.

The inf surface shows a deep median notch called the vallecula which separates the rt & lft hemispheres.

Internal structure

It consists of a cortex of grey matter (in which all the afferent fibres terminate) covering a mass of white matter, in which deep nuclei of grey matter are buried.

The cerebellar cortex contains several maps of the skeletal muscles in the body

The topographic arrangement of these maps indicates that the vermis controls the axial and proximal musculature of the limbs, the intermediate part of the hemisphere controls distal musculature, and the lateral part of the hemisphere is involved in motor planning.

THE DIENCEPHALON

The diencephalon is a midline structure

which is largely embedded in the cerebrum, and therefore hidden from the surface view.

Its cavity is the 3rd ventricle the hypothalamic sulcus extending from the interventricular foramen to the cerebral aqueduct, divides each half of the diencephalon in to dorsal & ventral parts.

Further sub divisions are follows

A . Dorsal part of the diencephalon

Thalamus ( dorsal thalamus )

Metathalamus, including the med & lat geniculate bodies and

Epithalamus, including the pinial body & habenula

B . Ventral part of the diencephalon

Hypothalamus,

Subthalamus ( ventral thalamus )