Download Input to the Cerebellar Cortex

The nervous system
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Dr.Abdul-Aziz
MOTOR NEUROPHYSIOLOGY
Overview of the somatic motor system
The human skeleton is a system of levers that are moved by contraction
of skeletal muscles. The motor system is comprised of skeletal muscles
and the neurons that control them. Muscle contraction only occurs in
response to action potentials in alpha motor neurons, which originate in
the ventral gray matter of the spinal cord (and brainstem nuclei) and
constitute the final common path for motor control. The hierarchy of
motor control within the CNS is as follows:
1.The highest level of control resides in areas of association cortex and
the basal ganglia, which determine the goal of movements.
2.The primary motor cortex and cerebellum determine the correct
sequence of commands that will allow the goal to be achieved.
3.Neuronal circuits in the spinal cord are the lowest level of control and
function to implement descending commands.
Cerebellum
The cauliflower-like cerebellum “small brain”, accounts for about 11% of
total brain mass. It is located dorsal to the pons and medulla (and to the
intervening fourth ventricle). It protrudes under the occipital lobes of
the cerebral hemispheres, from which it is separated by the transverse
cerebral fissure, it has two lateral hemispheres and central vermis.
figure :Anatomy of cerebellum.
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cerebellar peduncles
The cerebellum connected to the brain stem by three bundles of axons
called the cerebellar Peduncles:
1. Superior cerebellar peduncle, connect the cerebellum with the
midbrain.
2. Middle cerebellar peduncle, connects the cerebellum with the pons.
3. Inferior cerebellar peduncle, connect the cerebellum with the medulla
oblongata and the spinal cord.
Anatomy
The cerebellum is bilaterally symmetrical; its two apple-sized cerebellar
hemispheres connected medially by the wormlike vermis. Its surface is
heavily convoluted, with fine, transversely oriented pleat like gyri known
as folia (“leaves”). Deep fissures subdivide each hemisphere into
anterior, posterior, and flocculonodular lobes. From midline out: It is
divided into the central part the vermis, and a cerebellar hemisphere
on each side. Each cerebellar hemisphere divided into intermediate, and
lateral zone.
figure: lobes of cerebellum
The cerebellum has a thin outer layer of gray matter, the cerebellar
cortex, and a thick, deeper layer of white matter. Located within the
white matter a collection of neurons form the deep cerebellar nuclei. the
dentate, interposed, and fastigial.
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Figure:layers of cerebellum.
Layers of the cerebellar cortex,The cerebellar cortex has three layers:
I.The granular layer is the innermost layer. It contains granule cells,
Golgi II cells, and glomeruli. The axons of mossy fibers synapse on
dendrites of granule and Golgi type II cells.
II.The Purkinje cell layer is the middle layer. It contains Purkinje cells,
and its output is always inhibitory.
III.The molecular layer is the outermost layer. It contains stellate cells,
basket cells, dendrites of Purkinje and Golgi II cells, and axons of granule
cells. The axons of granule cells form parallel fibers, which synapse on
the dendrites of Purkinje cells, basket cells, stellate cells, and Golgi type
II cells.
figure: Layers of the Cerebellar Cortex
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Functional division
Functionally the cerebellum divided into three subdivisions :
1) the vestibulocerebellum composed of the fluculonodular lobe, the Purkinje
cells from this lobe synapse on vestibular nuclei.
2) The spinocerebellum is composed of the vermis and the intermediate zone
of cerebellum hemisphere.. Purkinje cells, from the vermis, synapse on the
fastigial nuclei. Those from the intermediate cerebellum, synapse on the
interpositus nuclei.
3) The cerebrocerebellum, the newest part, projects to the motor and
premotor cortex via the dentate nuclei. this area joins with the cerebral cortex
in the overall planning of sequential motor movements. Without this lateral
zone, most discrete motor activities of the body lose their appropriate timing
and sequencing and therefore become incoordinate
.
figure: Functional division of cerebellum.
Input to the Cerebellar Cortex
Two systems provide excitatory input to the cerebellar cortex:
1.Climbing fibers originate in the inferior olive of the medulla
(Olivocerebellar fibers) and project directly onto Purkinje cells, each
Purkinje cell receives input from only one climbing fiber. It is believed
that climbing fibers "condition" the Purkinje cells and modulate their
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responses to mossy fiber input. Climbing fibers also may play a role in
cerebellar learning.
2.Mossy fibers constitute the majority of the cerebellar input:
a. Cerebropontocerebellar fibers--arise from pyramidal cells in the
cerebral cortex, synapse on pontine nuclei which send their axons to the
contralateral cerebellar cortex via pontocerebellar fibers (form middle
peduncle). Alerts cerebellum regarding anticipated movements.
b. Vestibulocerebellar fibers--arise mainly from the vestibular nerve and
vestibular nuclei; project to flocculonodular lobe and fastigial nucleus
(coordinate head and eye movement).
c. Spinocerebellar Fibers- arise from spinal cord and go to rostral lobe;
makes cerebellum aware of ongoing movements via proprioceptive
input from muscle spindles and joint receptors.
The vestibulocerebellar, spinocerebellar, and cerebropontocerebellar
afferents project to granule cells, which are excitatory interneurons
located in collections of synapses called glomeruli. Axons from these
granule cells then ascend to the molecular layer, where they bifurcate
and give rise to parallel fibers. Parallel fibers from the granule cells
contact the dendrites of many Purkinje cells, producing a "beam" of
excitation along the row of Purkinje cells.
(Note:The climbing fiber system and the mossy fiber also sends
collateral branches directly to deep cerebellar nuclei, in addition to their
projections to the cerebellar cortex.
Major Cerebellar Cerebellar Output (efferents)
Outputs (arise from neurons in deep cerebellar nuclei):
1. Fastigial Nucleus Projections: to --> vestibular nuclei and reticular
formation--> vestibulospinal & reticulospinal tracts influence spinal
motor neurons--> effect extensor muscles related to maintaining
posture and balance.
2. Interpositus Nucleus Projections: -->go to red nucleus to influence
rubrospinal tract--> correct errors related to the gross movements .
3. Dentate Nucleus Projections: --> projects to thalamus to influence the
output from the motor cortex- ->makes delicate adjustments related to
fine, skilled movements.
Summery of cerebellar function
1.Cerebellum helps to sequence the motor activities and also monitors
and makes corrective adjustments in the body’s motor activities while
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Dr.Abdul-Aziz
they are being executed so that they will conform to the motor signals
directed by the cerebral motor cortex and other parts of the brain.
2.The cerebellum receives continuously updated information about the
desired sequence of muscle contractions from the brain motor control
areas; it also receives continuous sensory information from the
peripheral parts of the body, giving sequential changes in the status of
each part of the body—its position, rate of movement, forces acting on
it, and so forth. The cerebellum then compares the actual movements as
depicted by the peripheral sensory feedback information with the
movements intended by the motor system. If the two do not compare
favorably, then instantaneous subconscious corrective signals are
transmitted back into the motor system to increase or decrease the
levels of activation of specific muscles.
3.The cerebellum also aids the cerebral cortex in planning the next
sequential movement a fraction of a second in advance while the
current movement is still being executed, thus helping the person to
progress smoothly from one movement to the next.
4. the cerebellum damp movements and prevent overshoot of
movements
5. Another important function of the lateral zones of the cerebellar
hemispheres is to provide appropriate timing for each succeeding
movement. .
Cerebellar disorders usually result from:
lesion of cerebellum may be due to
1. Tumors (i.e., cerebellar cystic meningioma)
2. Viral Infections
3. Heavy metal poisoning
4. Genetic Disorders: cerebellar degeneration
The main abnormalities include
lesions of cerebellum usually lead to
1.dysmetria and ataxia, in the absence of the cerebellum, the
subconscious motor control system cannot predict how far movements
will go. Therefore, the movements ordinarily overshoot their intended
mark; then the conscious portion of the brain overcompensates in the
opposite direction for the succeeding compensatory movement.This
effect is called dysmetria, and it results in uncoordinated movements
that are called ataxia.
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2. Dysdiadochokinesia, when the motor control system fails to predict
where the different parts of the body will be at a given time, it “loses”
perception of the parts during rapid motor movements. As a result, the
succeeding movement may begin much too early or much too late, so
that no orderly “progression of movement” can occur.
3. Dysarthria, another example in which failure of progression occurs is
in talking because the formation of words depends on rapid and orderly
succession of individual muscle movements in the larynx, mouth, and
respiratory system
4. Intention tremor occur when a person performs a voluntary act.
5. Hypotonia. Loss of the deep cerebellar nuclei, particularly of the
dentate and interposed nuclei, causes decreased tone of the peripheral
body musculature on the side of the cerebellar lesion.
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Dr.Abdul-Aziz