Download The Cerebellum

The Cerebellum
– The cerebellar cortex is folded into
numerous, small gyri, making it easy to
distinguish from the cerebral
hemispheres.
– The cerebellum (CBL) receives its inputs
from three major sources: spinocerebellar
tracts, pontocerebellar fibers, and
olivocerebellar fibers.
– The great majority of CBL output is via its
own subcortical nuclei; namely the
dentate, interposed, and fastigial nuclei,
and the emboliform.
– Phylogenetically, the CBL first appeared in the
lowest vertebrates. It enlarged in cartilaginous and
bony fish and increased further in Amphibia and
Reptilia.
– Birds show the greatest relative development in
submammalian species.
– Mammals exhibit the greatest relative enlargement
and degree of complexity.
– These phylogenetic differences correlate with the
appearance of the three functional subdivisions of
the CBL.
– The vestibulocerebellum is oldest, the
spinocerebellum is intermediate in evolutionary
age, and the cerebrocerebellum is most recent.
Functional Subdivisions of the
Cerebellum
–
The cerebellum is routinely divided into three,
somewhat arbitrary, functional divisions,
1. the vestibulocerebellum (archicerebellum),
2. the spino-cerebellum (paleocerebellum), and
3. the cerebrocerebellum (neocerebellum).
– Each division, as its name implies, receives its
predominant afferent input from a different source.
– Each division also has somewhat different outputs.
The Vestibulocerebellum
– The vestibulocerebellum (archicerebellum) is the
oldest part, first appearing in fish.
– It consists of the flocculonodular lobe and, according
to some authors, parts of the vermis.
– It receives input mainly from the vestibular system.
– It is richly interconnected with UMNs of the medial
brainstem motor pathway, especially those in the
vestibular nuclei.
– Some efferent fibers from the vestibulocerebellum
also end on reticulospinal neurons.
– The pathways to the vestibular nuclei are via direct
projections of Purkinje cell axons from the
flocculonodular lobe and via efferent fibers from the
fastigial nucleus.
– The vestibulocerebellum provides for coordination of
movements involving whole body equilibrium and
posture.
The Spinocerebellum
– The spinocerebellum (paleocerebellum) is of intermediate
phylogenetic age, first appearing in amphibians.
– Its inputs are mainly from proprioceptors and exteroceptors
of the limbs via spinocerebellar pathways.
– It is richly interconnected, via efferent fibers from the
globose and emboliform nuclei (together called the
interposed nucleus), with UMNs of the lateral brainstem
motor pathway (the red nucleus).
– It also connects with reticulospinal neurons and sends
information rostrally to the motor cortex via the
ventrolateral (VL) nucleus of the thalamus.
– The spinocerebellum provides for coordination of limb
movements for posture, progression, and other purposes.
It also is important for "updating evolving movements," a
role it shares to some extent with the cerebrocerebellum.
The Cerebrocerebellum
– The cerebrocerebellum (neocerebellum), the
youngest part, first appears in mammals.
– Its inputs arrive mainly from cerebral cortex via
the pontine nuclei.
– Because of this fact it is also sometimes called
the pontocerebellum.
– It is richly interconnected, mostly via the
ventrolateral (VL) nucleus of the thalamus, with
UMNs of the corticospinal motor pathway.
– The cerebrocerebellum provides for coordination
of independent limb movement and skilled
movement, especially in terms of
preprogramming movements.
Most Important Neuronal Connections
The following connections are most important:
1. Mossy fibers are excitatory to granule cells.
2.
Climbing fibers are excitatory to Purkinje cells.
3.
Both types of afferent fibers send collaterals to the CBL subcortical
nuclei.
4.
Granule cells, via their axons, the parallel fibers, are excitatory to
Purkinje, basket, stellate, and Golgi cells. Glutamate is the
neurotransmitter.
5.
Each parallel fibers run longitudinally for several millimeters along a
cerebellar folium, synapsing on a long strip of Purkinje cells.
6.
Basket and stellate cells are inhibitory to Purkinje cells.
7.
Axons of basket cells are perpendicular to the parallel fibers in the
plane of the CBL cortex.
8.
Purkinje cells are inhibitory to nuclear cells.
9.
Nuclear cells are excitatory to their target cells (UMNs or thalamic
neurons).
The Canonical Circuit
– This is the basic circuit,
the canonical circuit, that
exists in all parts of the
cerebellar cortex.
– All the redish colored
cells, the Golgi cell, the
Purkinje cell, the stellate
cell, and the basket cell,
are inhibitory.
– They release GABA.
– Only the granule cell and
the nuclear cells are
excitatory.
Mossy Fibers Excite a Strip of Purkinje Cells
Climbing Fibers
Climbing fibers originate from cells in the inferior olivary
nucleus and terminate with numerous, strong excitatory
synapses on the cell body and dendrites of Purkinje
cells.
Although the exact role of the climbing fibers is not
understood, they are believed to play a role in motor
learning. The frequency of firing of climbing fibers (as
evidenced by an increased frequency of firing of complex
spikes) increases when a subject encounters an
unexpected load during a movement. Once the subject
adapts to the load, or learns the new movement, the
frequency of complex spikes returns to baseline level.
Function of the Cerebellum
– All movements, postural or voluntary, of limbs, eyes, or
speech, require an appropriate pattern of activation of
agonist and antagonist muscles or muscle groups. The
relative timing of the bursts of activity of the agonists
and antagonists determines the smoothness and
coordination of the movement.
– The function of the cerebellum is to provide for
smooth, coordinated, synegistic movement by
adjusting the timing of the bursts of activity of agonist
and antagonist muscles.
– The notion of the CBL as a regulator of timing is entirely
compatible with the major clinical signs of CBL
dysfunction, incoordination and loss of muscle synergy.
These signs are quite different from the weakness and
spasticity that occur with UMN problems.
Function of the Cerebellum
– The neocerebellum adjusts (sets) this timing signal prior to a
voluntary movement and is said to be involved in
"preprogramming" of movement parameters
– The spinocerebellum adjusts this signal as a consequence of
information it receives about the movement via spinocerebellar
pathways. Since the spinocerebellum also receives information
("efference copy") about the intended movement via the pontine
nuclei, it is often considered to be a comparator.
– According to this formulation, the spinocerebellum compares
the intended movement (information from cerebral cortex via
the pontine nuclei) with the actual movement (information from
the spinal cord via spinocerebellar tracts) and adjusts its clock
signal to update the evolving movement to make it smooth
and accurate.
movement. The following common
signs of CBL dysfunction are due to
inappropriate timing signals:
1.Delay in movement initiation.
2.Dysmetria: alterations in the
rate and force of a movement.
3.Asynergia: decomposition of
movement.
4.Past Pointing.
5.Intention Tremor.
6.Dysarthria.
7.Dysdiadochokinesis.
Another important sign of CBL
dysfunction is hypotonia. This is due
to a decreased firing frequency of the
gamma motor neurons innervating the
muscle spindles of the affected limb.
Depending on the distribution of CBL
Clinical Signs of Cerebellar Dysfunction
•
•
•
With one exception, all cerebellar signs reflect inaccurate,
inappropriate, or absent clock signals necessary for smooth,
coordinated, synergistic movement.
The following common signs of CBL dysfunction are due to
inappropriate timing signals:
•
Delay in movement initiation.
•
Dysmetria: alterations in the rate and force of a movement.
•
Asynergia: decomposition of movement.
•
Past Pointing.
•
Intention Tremor.
•
Dysarthria.
•
Dysdiadochokinesis.
Another important sign of CBL dysfunction is hypotonia. This is
due to a decreased firing frequency of the gamma motor neurons
innervating the muscle spindles of the affected limb.
Depending on the distribution of CBL signs, the
following two syndromes are often recognized:
Archicerebellar Syndrome: Incoordination of the
muscles of equilibrium; dysequilibrium with widebased gate; disturbances of stance and gait. Heel-totoe walking is impaired.
Neocerebellar Syndrome: Incoordination of muscles
for voluntary movements. Heel-to-shin movements
and rapid alternating movements (diadochokinesis)
are impaired.