Brainstem and Cerebellum

Brainstem and cerebellum are the contents of the posterior fossa, delimited superiorly by the tentorium cerebelli.

The brainstem comprises the midbrain, pons and medulla oblongata. It is directely connected with the diencephalon, composed by thalamus, pineal body, posterior commissure, habenula, and stria medullaris. From anterior to posterior it can be divided into pes, tegmentum and tectum. The tectum of the brainstem is represented by the quadrigeminal plate, while the tectum of the pons and medulla is the cerebellum. The main pathways and nuclei of the brainstem are described in chapters 4 and 5.

I. Midbrain

The midbrain is about 2 cm in length. The posterior part is represented by the tectum, or quadrigeminal plate. This contains two pairs of tubercles (superior and inferior), also termed collides. They are separated from the tegmentum by a virtual plane passing through the cerebral aqueduct of Sylvius. In front of the tegmentum, the pes of the brainstem is constituted of the cerebral peduncles, also called crura, two thick bundles of fibers diverging upward from below, rostrally limited by the substantia nigra. Between them, the interpeduncular fossa is found. Anteriorly, it is limited by the two mammillary bodies (part of the hypothalamus), while in its depth is the posterior perforated substance through which perforating arteries pass.

II. Pons

The pons is the thicker portion of the brainstem of about 2530 mm in length. It bulges from midbrain and medulla and is separated from them by the superior and inferior pontine sulci. It is posteriorly covered by the cerebellum, united to it by means of the middle cerebellar peduncles (brachia pon-tis).

III. Medulla Oblongata

The medulla is the lowest part of the brainstem. It continues caudally with the spinal cord. The cord's sulci are present even in the medulla, where they are more prominent.

These are:

the median (anterior) sulcus, which disappears at the level of the pyramidal decussation, and the anterolateral (preoli-vary) sulcus, through which the hypoglossal nerve emerges. The eminence between the median and anterolateral sulcus is called the pyramid and hosts the motor fibers of the pyramidal tract. Lateral to the anterolateral sulcus is the olive. Posterior to the olive, the fossetta lateralis is found. This region is supplied by the lateral medullar arteries, most of which come from the postero-inferior cerebellar artery. Infarct of this area is responsible for the Wallenberg syndrome. Posterior to this area, the inferior cerebellar peduncle (corpus restiforme - restiform body) is found.

IV. Cerebellum

The cerebellum consists of two hemispheres and a median structure: the vermis. On the superior surface, there is no clear separation between the hemispheres. In the sagittal cuts, nine segments and two sulci are recognizable. The first segment, lingual, is adjacent to the superior medullary velum (Vieussen's valve), a membrane that delimites superiorly the fourth ventricle. The nodulus is against the inferior medullary velum, which delimites the ventricle inferiorly.

The cerebellar hemispheres are made up of a thick core of white matter (corpus medullaris) covered by the cerebellar cortex. This presents some fissures, of which the most important is the horizontal one that divides the cerebellar hemispheres into two lobi: anterior and posterior. Other fissures subdivide the lobi into lobuli, and these into folia.

Within the white matter of the cerebellum, four gray nuclei are recognized, around and close to the posterior aspect of the fourth ventricle: fastigial, dentate, globose and embo-liform nuclei.

The cerebellum is attached to the brainstem by three bridges: the superior, middle and inferior cerebellar peduncles. The superior cerebellar peduncle (brachium conjunc-tivum) is thin, oblique, vertically and mid-laterally oriented, starting immediately below the inferior colliculum. It follows the velum medullaris superior. The fibers of the superior cerebellar peduncles decussate in the midbrain, and most of them end at the opposite red nucleus. The middle cerebellar peduncle is the biggest cerebellar bridge, and most of the pathways from and to the cerebellum pass through it.

The inferior cerebellar peduncle (restiform body) contains fibers from the medulla and spinal cord. It is thin, originates from the dorsolateral aspect of the upper medulla oblongata, ascends and diverges before entering the cerebellum. Phylogenetically, the cerebellum is divided into three portions: archi, paleo, and neo-cerebellum. The archicerebel-lum, constituted by flocculus and nodulus, is mostly devoted to the maintenance of orthostatic equilibrium. The paleo-cerebellum (amygdala, pyramis, uvula and anterior lobe) governs proprioceptive stimuli coming from muscles and articulations. The neocerebellum includes most of the cerebellar hemisphere and the intermediate portion of the vermis; it seems mostly devoted to the finest regulation of motoricity in both voluntary and involuntary movements.

, Lobulus simplex

Superior semilunar lobule

Horizontal fissure

Inferior semilunar lobule

Quadrangular lobule ^

Flocculus

Biventral lobule -,

Lobulus simplex

Superior semilunar lobule

Horizontal fissure

Inferior semilunar lobule

Quadrangular lobule ^

Flocculus

Lobulus simplex

Superior semilunar lobule

Biventral lobule -,

Horizontal fissure

Inferior semilunar lobule

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Essentials of Human Physiology

Essentials of Human Physiology

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