cerebellum is located in the posterior fossa of the skull,
dorsal to the pons and medulla from which it is separated by
the Aqueduct of Sylvius and the fourth ventricle.
Like the cerebrum,
the cerebellum is covered by cortex and consists of two
hemispheres, each of which is divided into lobes. The
hemispheres are separated from one another by a thin
structure called the vermis, or "worm."
of the Cerebellum
anterior lobe, or paleocerebellum, is the second oldest part
of the cerebellum. It receives proprioceptive input from the
spinal cord and controls the anti-gravity muscles of the
body, thus regulating posture.
lobe, or neocerebellum, is the newest part of the
cerebellum. It is involved in the coordination of muscle
movement via the inhibition of involuntary movement.
Inhibitory neurotransmitters, especially GABA, are found
here. This lobe plays an important role in fine motor
consists of the flocculi, the most ancient part of
the cerebellum, and the nodulus, the narrowest and most
inferior part of the vermis. This lobe is involved in the
maintenance of equilibrium.
nuclei are located deep within each cerebellar hemisphere;
the dentate nucleus, the emboliform
nucleus, the globose nucleus, and the
fastigal nucleus. These
nuclei have axons that project to the brain
stem, sending messages out to be conveyed to other parts of
the central nervous system.
The deep nuclei
are regulated by radish-shaped cells located in the
cerebellar cortex called Purkinje cells. The Purkinje cells
control the output of the cerebellum by inhibiting the
firing of the deep nuclei. The Purkinje cells located in the
lateral cortex of the cerebellum project to the dentate
nuclei, while those in the intermediate cortex synapse with
the emboliform and globose nuclei. The fastigial nuclei
receive input from Purkinje cells found in the cerebellar
cortical covering of the vermis.
cerebellum is involved in a feedback loop for muscle
movement. When the cortex sends a message for motor movement
to the lower motor neurons in the brain stem and spinal
cord it also sends a copy of this message to the
cerebellum.This is conveyed from pyramidal fibers in the
cortex on the corticopontinecerebeller tract to the
cerebellum. In addition, information gets to the cerebellum
from muscle spindles, joints and tendons. This information (proprioception and kinesthesia) lets the cerebellum know
about the movements that have been executed, so that it can
determine how well motor commands coming from the cortex are
being carried out. This has been called its comparator
plays a major role in the coordination of muscle activity
for the production of smooth movement through its
connections with the pyramidal and extrapyramidal systems
and the descending reticular formation .Due to its role in
the coordination of fine motor movements the cerebellum
makes important contributions to the control of rapid,
alternating muscle movements necessary for speech.
fiber bundles called peduncles connect the cerebellum to the
brain stem. On these tracts, information runs in both
directions, with all messages sent and received by the
cerebellum traveling on these fibers.
superior cerebellar peduncle also called
the superior brachium
conjunctivum connects the cerebellum to the
midbrain and contains efferent fibers from the dentate,
emboliform, and globose nuclei. These are the axons that
send feedback to the motor cortex in the frontal lobe via
the red nucleus in the midbrain and the thalamus. Afferent
fibers traveling in this peduncle take proprioceptive
information to the cerebellum from the lower body.This
information ascends along the spinal cord in the
tract, before entering the cerebellum.
cerebellar peduncle or the middle brachium
pontis is the largest of the peduncles and
links the cerebellum with the pons. Via this connection, the
cerebellum receives a copy of the information for muscle
movement that the pyramidal tract is carrying down to lower
peduncle or the restiform
bodies connects the cerebellum with the
medulla, but also to the vestibular nuclei located in the lower pons and medulla and
with cells of the reticular formation.
information from the upper body, traveling along the
dorsospinocerebellar tract enters the cerebellum on
the inferior cerebellar peduncle.
Sensory and motor input to the
The cerebellum receives both proprioceptive and kinesthetic
information from the periphery. It also gets information
about the strength and type of muscle movements occurring.
Proprioception, according to Love and
Webb, 1992, refers to sensory information about equilibrium, pressure,
movement, vibration, position, and pain
received from muscles, joints and tendons.
Kinesthesia is a more specific term
than proprioception. It refers to feedback that comes only
from muscle spindles. Kinesthesia is the "ability to detect
the range and direction of movements of the limbs"
(Bhatnager & Andy, 1995, pg. 341).
information ascends to the cerebellum along the spinal cord.
The two main tracts that bring information from the
periphery to the cerebellum are the ventro-spino-cerebellar
and the dorso-spino-cerebellar
ventro-spino-cerebellar tract contains proprioceptive
fibers from the lower body. (The axons of this tract
decussate and travel upward on the contralateral side of the
spinal cord for some distance before crossing again and
continuing upward ipsilaterally.) The axons of this tract
enter the cerebellum on the superior cerebellar peduncle.
dorso-spino-cerebellar tract takes proprioceptive
information from the upper body that reaches the cerebullum
on the inferior cerebellar peduncle (restiform body).
reticulo-cerebellar tract carries messages received
by the reticular nuclei in various parts of the brain stem
from the cortex, spinal cord, vestibular system and red
vestibulo-cerebellar tract brings information from
the semi-circular canals of the inner ear to the cerebellum
via the vestibular nucleus located in the lower pons and
medulla. These fibers travel to the flocculi on the inferior
cortico-pontine-cerebellar tract brings motor information
to the cerebellum from the frontal lobe. It leaves the
precentral gyrus (motor strip) and descends in the internal capsule along
with pyramidal tract fibers. Its axons synapse with cells in
the pons. These pontine nuclei then send second order
neurons to the cerebellum on the middle cerebellar peduncle.
The axons of the corticopontine tract bring the cerebellum a
copy of the information that the corticobulbar and
corticospinal tracts are conveying
to the cranial and spinal nerves. Thus, the cerebellum
"knows" the details of the messages being sent to lower
motor neurons by the upper motor neurons. The messages sent by the corticopontinecerebellar
tract includes information about the nature of the motor
impulse being sent by the precentral gyrus: its destination,
its strength and its speed. This connection is
contralateral, meaning that information from the frontal
lobe of one hemisphere is sent to the posterior cerebellar lobe on the
opposite side of the brain.
acts on the motor messages carried by the
corticopontinecerebellar tract by
integrating the information and exerting control over the
message through the firing of Purkinje cells.
I consider the cortico-pontine-cerebellar tract to be
part of the extrapyramidal system, although it originates in
the premotor, precentral (motor strip) and postcentral (sensory strip) gyri it synapses with cells in the pons rather than with
spinal or cranial nerves. The portion of the tract from the
cortex to the pons consist of first order neurons, and from
the pons to the cerebellum, second order. The first order
neurons may be considered by some to be part of the
from the cerebellum
tracts discussed below may be considered part of the
extrapyramidal system since they carry involuntary signals and are indirect and multisynaptic. As
mentioned previously some sources (not current) consider the
extrapyramidal tract to consist of the basal ganglia only.
Output to the
nuclei, which receive inhibitory messages from the Purkinje
cells in the lateral cerebellar cortex, and the emboliform
and globose nuclei, which are controlled by the Purkinje
cells in the intermediate cortical areas all project to the
contralateral red nucleus in the midbrain via the
superior cerebellar peduncle.
Some of the
information sent on the superior cerebellar peduncle travels
from the red nucleus to the ventrolateral nucleus of the
thalamus. From there it passes on to the
precentral gyrus of the frontal lobe. Other ascending
projections travel to the cortex directly bypassing
the thalamus. These connections allow the cerebellum to
perform its comparator function by providing feedback to the
motor cortex regarding the messages that it has received
from cortico-pontine-cerebellar fibers as well as feedback
from the muscles innervated by lower motor neurons that
receive impulses from the pyramidal and extra pyramidal
Structure of the Feedback Loop for Motor Movement
Precentral gyrus --> pontine
nuclei -- > cerebellum -- > (after input from muscles
innervated by lower motor neurons, and comparisons made by
the cerebellum between output from the cortex and input from
muscles) --> red nuclei --> thalamus --> precentral
gyrus where adjustments are made.
Fibres from the
red nucleus also descend to synapse with cranial and spinal nerves,
forming the rubrospinal tractThe
fastigal nucleus, which receives inhibitory control from the
Purkinje cells of the vermis, sends messages to the
vestibular nuclei in the lower pons and medulla. From the
vestibular nuclei, information is sent to lower motor
neurons in the brain stem and spinal cord. This tract brings
information about the body's position in space to the
This tract carries information
related to the functioning of the autonomic nervous system
including circulation of the blood, dilation of blood
vessels, respiration and visceral activity. (Note that while
this tract influences autonomic activity, it is not part of
the autonomic nervous system.)
Ataxia, is an incoordination
of motor movement, resulting from cerebellar lesions. The term
ataxia is also used to describe the unsteady walk and usual
postures seen in patients who have suffered injury to the
cerebellum. People sometimes compensate for these problems
by walking with their feet far apart in a
broad-based gait seen on the tandem walking test (Webb, Adler and Love, 2008). It is also the heel to toe walk that police require during a driving under the influence stop.