Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 16Control of Posture & Movement 257

fiber inputs exert a weak excitatory effect on many Purkinje
cells via the granule cells. The basket and stellate cells are also
excited by granule cells via the parallel fibers, and their output
inhibits Purkinje cell discharge (feed-forward inhibition). Golgi
cells are excited by the mossy fiber collaterals, Purkinje cell col-
laterals, and parallel fibers, and they inhibit transmission from
mossy fibers to granule cells. The transmitter secreted by the
stellate, basket, Golgi, and Purkinje cells is GABA, whereas the
granule cells secrete glutamate. GABA acts via GABAA recep-
tors, but the combinations of subunits in these receptors vary
from one cell type to the next. The granule cell is unique in that
it appears to be the only type of neuron in the CNS that has a
GABAA receptor containing the α6 subunit.
The output of the Purkinje cells is in turn inhibitory to the
deep cerebellar nuclei. As noted above, these nuclei also receive
excitatory inputs via collaterals from the mossy and climbing
fibers. It is interesting, in view of their inhibitory Purkinje cell
input, that the output of the deep cerebellar nuclei to the brain
stem and thalamus is always excitatory. Thus, almost all the cer-
ebellar circuitry seems to be concerned solely with modulating
or timing the excitatory output of the deep cerebellar nuclei to
the brain stem and thalamus. The primary afferent systems that
converge to form the mossy fiber or climbing fiber input to the
cerebellum are summarized in Table 16–2.

FUNCTIONAL DIVISIONS

From a functional point of view, the cerebellum is divided into
three parts (Figure 16–16). The nodulus in the vermis and the

flanking flocculus in the hemisphere on each side form the ves-

tibulocerebellum (or flocculonodular lobe). This lobe, which is

phylogenetically the oldest part of the cerebellum, has vestibular

connections and is concerned with equilibrium and eye move-

ments. The rest of the vermis and the adjacent medial portions

of the hemispheres form the spinocerebellum, the region that

receives proprioceptive input from the body as well as a copy of

the “motor plan” from the motor cortex. By comparing plan

with performance, it smoothes and coordinates movements that

are ongoing. The vermis projects to the brain stem area con-

cerned with control of axial and proximal limb muscles (medial

brain stem pathways), whereas the hemispheres project the brain

stem areas concerned with control of distal limb muscles (lateral

brain stem pathways). The lateral portions of the cerebellar

hemispheres are called the cerebrocerebellum. They are the

newest from a phylogenetic point of view, reaching their greatest

development in humans. They interact with the motor cortex in

planning and programming movements.

Most of the vestibulocerebellar output passes directly to the

brain stem, but the rest of the cerebellar cortex projects to the

deep nuclei, which in turn project to the brain stem. The deep

nuclei provide the only output for the spinocerebellum and

the cerebrocerebellum. The medial portion of the spinocer-

ebellum projects to the fastigial nuclei and from there to the

brain stem. The adjacent hemispheric portions of the spino-

cerebellum project to the emboliform and globose nuclei and

from there to the brain stem. The cerebrocerebellum projects

to the dentate nucleus and from there either directly or indi-

rectly to the ventrolateral nucleus of the thalamus.

MECHANISMS

Although the functions of the flocculonodular lobe, spinocer-

ebellum, and cerebrocerebellum are relatively clear and the cere-

bellar circuits are simple, the exact ways their different parts

carry out their functions are still unknown. The relation of the

TABLE 16–2 Function of principal
afferent systems to the cerebellum.a

Afferent Tracts Transmits

Vestibulocerebellar Vestibular impulses from labyrinths, di-

rect and via vestibular nuclei

Dorsal spinocerebellar Proprioceptive and exteroceptive impuls-

es from body

Ventral spinocerebellar Proprioceptive and exteroceptive impuls-

es from body

Cuneocerebellar Proprioceptive impulses, especially from

head and neck

Tectocerebellar Auditory and visual impulses via inferior

and superior colliculi

Pontocerebellar Impulses from motor and other parts of

cerebral cortex via pontine nuclei

Olivocerebellar Proprioceptive input from whole body via

relay in inferior olive

aThe olivocerebellar pathway projects to the cerebellar cortex via climbing fibers; the
rest of the listed paths project via mossy fibers. Several other pathways transmit
impulses from nuclei in the brain stem to the cerebellar cortex and to the deep
nuclei, including a serotonergic input from the raphé nuclei to the granular and
molecular layers and a noradrenergic input from the locus ceruleus to all three layers.

FIGURE 16–16 Functional divisions of the cerebellum.

(Modified from Kandel ER, Schwartz JH, Jessell TM [editors]: Principles of Neural

Science, 4th ed. McGraw-Hill, 2000.)

Spinocerebellum To medial

descending

systems

To lateral

descending

systems

To motor

and

premotor

cortices

To

vestibular

nuclei

Motor

execution

Motor

planning

Balance

and eye

movements

Cerebrocerebellum

Vestibulocerebellum