Anatomy and Physiology of Children and Adolescents - M. R. Sapin 2007

The Nervous System (general structural and developmental plan of the nervous system)
Central Nervous System
Brain - Cerebellum

The Cerebellum plays a vital role in maintaining body balance and coordinating movements. It consists of two hemispheres and an unpaired median phylogenetically ancient Structure known as the vermis.

The surfaces of the hemispheres and vermis are marked by transverse and parallel sulci, which define narrow, elongated leaflets (folia) of the cerebellum. As a result, the total surface area of the cerebellar hemispheres in an adult averages about 850 cm². The cerebellum is divided by deep fissures into anterior, posterior, and flocculonodular lobes, which in turn form cerebellar lobules. Because the cerebellar sulci are continuous, extending uninterrupted from the vermis across to the hemispheres, each lobule of the vermis is directly connected to the corresponding right and left lobules of the hemispheres. The flocculus is the most isolated and phylogenetically ancient lobule of the hemispheres, lying adjacent to the ventral surface of the middle cerebellar peduncle on each side.

The cerebellum is composed of gray and White matter. On a sagittal section, the white matter (corpus medullare) resembles the branching pattern of a tree, commonly referred to as the arbor vitae (tree of life) of the cerebellum. Enclosing the white matter, the cerebellar cortex is a layer of Gray matter 1 to 2.5 mm thick.

Each cerebellar leaf (folium) is a thin lamina of white matter covered by the cortex (gray matter). The cortex is organized into three layers: the outer molecular layer, the middle Purkinje Cell (ganglionic) layer, and the inner granular layer. The molecular and granular layers consist primarily of small interneurons. Large Purkinje Neurons—reaching up to 40 µm in size and arranged in a single row within the middle layer—serve as the principal efferent neurons of the cerebellar cortex. Their axons originate from the Base of the cell body and form the initial segment of the efferent pathways, transmitting nerve impulses to the deep cerebellar nuclei and the thalamus, while their dendrites arborize extensively within the superficial molecular layer. The remaining cortical neurons function as interneurons or association Cells that Relay nerve impulses to the Purkinje cells; thus, all incoming signals reaching the cerebellar cortex ultimately converge on the Purkinje cells. Afferent impulses arrive at the cerebellar cortex via three distinct fiber systems. One group of fibers originates from the inferior olivary Nucleus of the Medulla Oblongata and winds around the dendrites of Purkinje cells to form synapses. A second group of afferent fibers projects from the pontine nuclei to the granule Cells of the cerebellum. A third group originates from the locus coeruleus in the tegmentum of the Pons and is believed to exert a neuromodulatory function, altering the excitability of cerebellar cortical neurons. Because Purkinje cells provide the sole output pathway from the cortex, their axons inhibit The activity of the deep cerebellar nuclei. Consequently, the primary functional output of the cerebellar cortex is inhibitory over the cerebellar nuclei.

Deep within the WHITE MATTER OF the cerebellum lie embedded masses of gray matter known as the deep cerebellar nuclei. The largest and phylogenetically newest is the dentate nucleus, located within the cerebellar hemisphere. Medial to the dentate nucleus lie the interposed nuclei—specifically the emboliform nucleus, followed medially by the globose nucleus—and most medially, the fastigial nucleus. Axons of neurons in the fastigial nucleus project to Deiters' vestibular nucleus and the reticular Formation of the medulla and pons. Axons from the emboliform and globose nuclei project to the red nucleus of the Midbrain and the thalamus, while axons from the large dentate nucleus target the thalamus.

Afferent and efferent fibers connecting the cerebellum with other PARTS OF THE Central Nervous system form three pairs of cerebellar peduncles. The inferior cerebellar peduncles link the cerebellum with the medulla oblongata, the middle peduncles connect it with the pons, and the superior peduncles connect it with the midbrain (tectum).

Functions of the cerebellum. Equipped with extensive afferent and efferent connections, the cerebellum is a key component of the motor control system and performs the following functions: 1) Regulation of Muscle tone and posture; 2) coordination of voluntary, goal-directed movements; and 3) coordination of rapid, targeted movements executed upon command from the Cerebral Cortex. The cerebellum is also involved in The regulation of visceral functions. Stimulation of the cerebellum elicits various autonomic Reflexes, such as pupil dilation, increased Blood pressure, and others.