Cerebellum
Cerebellar Function
The cerebellum is tasked with bringing finesse to the motor system. Although not
primarily involved in the mechanisms for production of muscle power, it is necessary for
normal control and regulation of muscle contraction.
The major function of the cerebellum, from a clinical point of view, is the coordination
of movement.
The cerebellum is the portion of the brain through which the cerebral motor cortex
achieves the coordination of individual muscle contractions required for normal voluntary
movements. Without it, movements are gross, uncoordinated, clumsy, and tremulous,
and precise movements become impossible. Lesions of the cerebellum do not cause
weakness, but rather loss of coordination and inability to gauge and regulate, as Gordon
Holmes said, the “rate, range, and force” of movement. Although motor strength and
power are preserved, active movements are severely compromised.
In order to perform any movement—especially a complex act involving many muscle
groups—contractions of the agonists, antagonists, synergists, and muscles of fixation
must be adequately coordinated.
To begin a movement, the agonists contract to execute the movement; the antagonists
relax or modify their tone to facilitate it; the synergists reinforce the movement; and the
fixating muscles prevent displacements and maintain the appropriate posture of the
limb. To terminate the movement, the antagonists contract and the agonists relax.
“The individual muscles that enter into the act must be controlled and coordinated, as
a conductor would direct an orchestra, precisely regulating the action of the individual
parts. The cerebellum is the conductor. It is essential to the synergy of muscle
contraction and is the center of coordination for voluntary movement. It does not provide
power and does not play an instrument, but without it the symphony of normal
movement degenerates into a mixture of disorganized muscle contractions”
A major manifestation of cerebellar lesions is ATAXIA (Gr. a “without,” taxis “order”); a
rough translation is “not orderly.”
1
, The essential feature in ataxia is that movements are not normally organized. Although
the term is a general one, indicating chaotic and disorganized movement, it is used
clinically primarily to refer to the motor control abnormalities—including incoordination,
tremor, and impaired rapid alternating movements (RAMs)—that occur with cerebellar
lesions.
Ataxia is not specific for cerebellar disease, and lesions in other parts of the nervous
system must be excluded before attributing ataxia to cerebellar disease. Impaired
proprioception may cause sensory ataxia, and lesions involving pathways that originate
in the frontal lobe may cause frontal lobe ataxia.
The cerebellum is involved in the following functions:
1-Maintenance of balance and posture: The cerebellum is important for making
postural adjustments in order to maintain balance. Through its input from vestibular
receptors and proprioceptors, it modulates commands to motor neurons to compensate
for shifts in body position or changes in load upon muscles.
2-Coordination of voluntary movements: Most movements are composed of a
number of different muscle groups acting together in a temporally coordinated fashion.
One major function of the cerebellum is to coordinate the timing and force of these
different muscle groups to produce fluid limb or body movements.
3-Motor learning: The cerebellum plays a major role in adapting and fine-tuning motor
programs to make accurate movements through a trial-and-error process (e.g., learning
to hit a baseball).
4-Adjustment of muscle tone
5-Cognitive functions: Although the cerebellum is most understood in terms of its
contributions to motor control, it is also involved in certain cognitive functions, such as
language. Thus, like the basal ganglia, the cerebellum is historically considered as part of
the motor system, but its functions extend beyond motor control in ways that are not yet
well understood.
2
Cerebellar Function
The cerebellum is tasked with bringing finesse to the motor system. Although not
primarily involved in the mechanisms for production of muscle power, it is necessary for
normal control and regulation of muscle contraction.
The major function of the cerebellum, from a clinical point of view, is the coordination
of movement.
The cerebellum is the portion of the brain through which the cerebral motor cortex
achieves the coordination of individual muscle contractions required for normal voluntary
movements. Without it, movements are gross, uncoordinated, clumsy, and tremulous,
and precise movements become impossible. Lesions of the cerebellum do not cause
weakness, but rather loss of coordination and inability to gauge and regulate, as Gordon
Holmes said, the “rate, range, and force” of movement. Although motor strength and
power are preserved, active movements are severely compromised.
In order to perform any movement—especially a complex act involving many muscle
groups—contractions of the agonists, antagonists, synergists, and muscles of fixation
must be adequately coordinated.
To begin a movement, the agonists contract to execute the movement; the antagonists
relax or modify their tone to facilitate it; the synergists reinforce the movement; and the
fixating muscles prevent displacements and maintain the appropriate posture of the
limb. To terminate the movement, the antagonists contract and the agonists relax.
“The individual muscles that enter into the act must be controlled and coordinated, as
a conductor would direct an orchestra, precisely regulating the action of the individual
parts. The cerebellum is the conductor. It is essential to the synergy of muscle
contraction and is the center of coordination for voluntary movement. It does not provide
power and does not play an instrument, but without it the symphony of normal
movement degenerates into a mixture of disorganized muscle contractions”
A major manifestation of cerebellar lesions is ATAXIA (Gr. a “without,” taxis “order”); a
rough translation is “not orderly.”
1
, The essential feature in ataxia is that movements are not normally organized. Although
the term is a general one, indicating chaotic and disorganized movement, it is used
clinically primarily to refer to the motor control abnormalities—including incoordination,
tremor, and impaired rapid alternating movements (RAMs)—that occur with cerebellar
lesions.
Ataxia is not specific for cerebellar disease, and lesions in other parts of the nervous
system must be excluded before attributing ataxia to cerebellar disease. Impaired
proprioception may cause sensory ataxia, and lesions involving pathways that originate
in the frontal lobe may cause frontal lobe ataxia.
The cerebellum is involved in the following functions:
1-Maintenance of balance and posture: The cerebellum is important for making
postural adjustments in order to maintain balance. Through its input from vestibular
receptors and proprioceptors, it modulates commands to motor neurons to compensate
for shifts in body position or changes in load upon muscles.
2-Coordination of voluntary movements: Most movements are composed of a
number of different muscle groups acting together in a temporally coordinated fashion.
One major function of the cerebellum is to coordinate the timing and force of these
different muscle groups to produce fluid limb or body movements.
3-Motor learning: The cerebellum plays a major role in adapting and fine-tuning motor
programs to make accurate movements through a trial-and-error process (e.g., learning
to hit a baseball).
4-Adjustment of muscle tone
5-Cognitive functions: Although the cerebellum is most understood in terms of its
contributions to motor control, it is also involved in certain cognitive functions, such as
language. Thus, like the basal ganglia, the cerebellum is historically considered as part of
the motor system, but its functions extend beyond motor control in ways that are not yet
well understood.
2
