The cerebellum, principally a motor organ, is responsible for
the coordination of movements, especially skilled voluntary
ones, the control of posture and gait, and the regulation of muscular
tone. In the last decade it has come to be appreciated that the
cerebellum may play a role in the modulation of the emotional state
and some aspects of cognition. The mechanisms by which
these functions are accomplished have been the subject of intense investigation
by anatomists and physiologists. Their studies have yielded a mass
of data, testimony to the complexity of the organization of the
cerebellum and its afferent and efferent connections. A coherent
picture of cerebellar function is now emerging, although it is not
yet possible, with a few notable exceptions, to relate each of the
symptoms of cerebellar disease to a derangement of a discrete anatomic
or functional unit of the cerebellum.
Knowledge of cerebellar function has been derived mainly from
the study of natural and experimental ablative lesions and to a
lesser extent from stimulation of the cerebellum, which actually produces
little in the way of movement or alterations of induced movement.
Furthermore, none of the motor activities of the cerebellum reaches
conscious kinesthetic perception; its main role, a critical one,
is to assist in the initiation and modulation of willed movements
that are generated in the cerebral hemispheres. The following discussion
of cerebellar structure and function has, of necessity, been simplified;
a fuller account can be found in the writings of Jansen and Brodal,
of Gilman, and of Thach and colleagues.
The classic studies of the comparative anatomy and fiber connections
of the cerebellum led to its subdivision into three parts (Fig. 5-1): (1) The flocculonodular
lobe, located inferiorly, which is phylogenetically the
oldest portion of the cerebellum and is much the same in all animals
(hence archicerebellum). It is separated from the
main mass of the cerebellum, or corpus cerebelli, by the posterolateral fissure.
(2) The anterior lobe, or paleocerebellum,
which is the portion of the corpus cerebelli rostral to the primary
fissure. In lower animals, the anterior lobe constitutes most of
the cerebellum, but in humans it is relatively small, consisting
of the anterosuperior vermis and the contiguous paravermian cortex.
(3) The posterior lobe, or neocerebellum,
consisting of the middle divisions of the vermis and their large
lateral extensions. The major portions of the human cerebellar hemispheres
fall into this, the largest, subdivision.
Diagram of the cerebellum, illustrating the major fissures,
lobes, and lobules and the major phylogenetic divisions (left labels).
This anatomic subdivision corresponds roughly with the distribution
of cerebellar function based on the arrangement of its afferent
fiber connections. The flocculonodular lobe receives special proprioceptive
impulses from the vestibular nuclei and is therefore also referred
to as the vestibulocerebellum; it is concerned