Transcript 6-Cerebellum 2009

Cerebellum ( Latin : Little Brain )
Dr Taha Sadig Ahmed ,



MBBS , PhD ( England ) .
Consultant , Clinical
Neurophysiology .
Associate Professor , Physiology
Department , College of Medicine
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Physiologic Anatomy (1)
The cerebellum is the largest
part of the hindbrain
Relations :
It is s located in the posterior
cranial fossa , & has the
following relations :
Anteriorly: 4th ventricle,
pons, and medulla oblongata
Superiorly: it is covered by
tentorium cerebelliI
Inferiority: occipital bone
It consists of 2 cerebral
hemispheres which are
interconnected by the vermis
in the center.
Surface shows parallel
running folds known as Folia.
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4th Ventricle
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Physiologic Anatomy (2)
The cerebellum influences movement on the ipsilateral
side of the body.
Although it weighs only 10 % as much as the cerebral
cortex , its surface area is about 75 % of that of the
cerebral cortex .
It is connected to the brainstem on each side by the :
(1) Superior Peduncle  has main connections to the
Cerebrum .
(2) Middle peduncle  has main connections to the
Pons .
(3) Inferior Peduncle  has main connections to the
Medulla Oblongata .
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Cerebellar Peduncles : Carry afferents from where ?
Inputs to the Cereellum
from the Cerebrum
Middle Cerebellar
Peduncle
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Inputs to the Cerebellum
from from the Pons
Inputs to the Cerebellum
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from the Medulla Oblongata
Anatomical Divisions of the
Cerebellum
Corpus Cerebelli ( main body
of cerebellum ) is divided into
Anterior and Posterior lobe by
Primary fissure.
Floculonodular Lobe lies
behind the posterolateral
fissure
Two cerebellar hemisphere are
interconnected by the vermis.
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Anatomical Divisions of the , & Flocculonodular Lobe Cerebellum :
Anterior Lobe , Posterior Lobe
The Primary Fissure divides the Corpus Cerebelli ( main body of
cerebellum ) into Anterior and Posterior lobes.
The Floculonodular Lobe lies behind the posterolateral fissure
Thw two cerebellar hemisphere are interconnected by the vermis.
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Spinocerebellum ( medial
parts of hemispheres+
Vermis )
Neocerebellum
(Lateral parts
of hemispheres )
Hemisphere
Posterolateral Fissure
Flocculonodular Lobe
Physiologic ( Functional ) divisions of the
Cerebellum  Neocerebellum , Spinocerebellum
snd Vestibulocerebellum
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Functionally , the Cerebellum is divided into 3 parts :
Neocerebellum , Spinocerebellum , & Vestibulocerebellum
Neocerebellum
( Posterior lobe )
 Comprises the lateral portions of
cerebellar hemispheres.
 Is newest from a phylogenetic point of
view .
 It interacts with motor cortex in
planning & programming of
movements.
The Neocerebellum is involved , in
conjunction of the cerebral cortex , in
planning & execution of skilled
movements.
Anterior Lobe
It coordinate movements particularly of
the distal limb muscles ( e.g., hand )
which are employed in skilful
movement .
NB the vemis projects to the brainstem
& control the movement of axial and
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proximal
Spinocerebellum
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Vestibulocerebellum
Spinocerebellum
( Paleocerebellum)
 Consist of vermis & medial parts
of the cerebellar hemispheres .
 It receives 
(1) Proprioceptive inputs ( afferents )
from all the body : Hence  It is
concerned with regulation of
muscle tone .
and it also receives 
(2) a copy of the “ Motor Plan “”from
the motor cortex
Therefore , by comparing plan with
performance , it acts as a “
comparator “, and sends
impulses back to the cortex to
correct movement  thereby it
ccordinates & smoothes ongoing
body movements
The vermis projects to the brainstem areas
concerned with control of axial and proximal
limb movements .
NB : Whereas the Neocerebellum controls
particularly distal limb muscles that are neede
for skilled movements , the vemis controls
movement of axial and proximal limb muscle
which are mainly concerned with gross
postural adjustments .
Anterior Lobe
Spinocerebellum
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Vestibulocerebellum
Vestibulocerebellum
( Floculonodular Lobe):

Phylogenetically , it is the oldest
part of the cerebellum ( hence it
is also called Archicerebellum )
 It has connections to the
vestibular nuclei , consequently ,
it is concerned with balance &
equilibrium
 And can induce changes in the
VOR
Anterior Lobe
Spinocerebellum
( Vestibulocular Reflex )
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Vestibulocerebellum
Cerebellar Organization
The cerebellum has grey matter areas
comprising the cerebellar cortex and
the deep cerebellar nuclei ( DCN) .
They are separated from each other
by white matter ( nerve fibers ).
The deep cerebellar nuclei are 4 in
number , & are called the:
(1) Dentate ,
(2) Globose ,
(3) Emboliform , &
(4) Fastigial nuclei .
Mossy Fibers , which are the primary
afferents to the cerebellum , send
collaterals to the deep nuclei and then
proceed ( pass on ) to the cortex
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Cerebellar Cortex Efferenrts to the Deep Cerebellar Nuclei
Neocerebellar Cortex :
Projects ( sends its efferents ) to the Dentate Nucleus 
& from there to the Ventrolateral Nucleus of the
Thalamus .
Spinocerebellar Cortex :
The Vermis projects to the Fastigial Nucleus  & from
there to the brainstem nuclei .
The hemispheric portions of the Spinocerebellum ( i.e.,
medial parts of the cerebellar hemispheres ) project to
the Emboliform and Globose nuclei  & from there to
the brainstem nuclei .
Vestibulocerebellar Cortex :
Its efferents pass directly to the brainstem ( & not via the
DCN)  to regulate balance , equilibrium & the VOR ).
Consequently , the Deep Cerebellar Nuclei provide the
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only output of the Neocerebellum and Spinocerebellum .13
Layers of the Cerebellar Cortex
The cerebellar
cortex is made of
layers 
(1) External
Molecular layer ,
( 2) Middle
Purkinje Cell
layer that is only
one cell thick , &
(3) Internal
Granular layer
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Cells of
the
Cerebellar
Cortex
The cerebellar cortex contains 5 types of neurons : Purkinje , Granule ,
Basket , Stellate & Golgi cells .
(1) Purkinje Cells :
Are amongst the biggest neurons in the body .
Have very extensive dendritic arbors that extend throughout the Molecular
Layer .
Their axons , which are the only output from the cerebellar cortex , pass to
the
deep
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2016 nuclei .
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(2) Granule Cells
( Origin of Parallel Fibers )
Their cell-bodies are situated
in the Granular layer .
They receive input from the
Mossy fibers and innervate
the Purkinje cells .
Each sends an axon to the
Molecular layer , where the
axon bifurcates to form a T .
Because the branches of this
“ T ” are straight and run for
long distances , they are
called Parallel Fibers .
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Granule
cells
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Granule Cells ( continued )
Because the
dendrites of
Purkinje cells are
oriented at right
angles to the
Parallel fibers
( which are ,
actually , axons of
Granule cells ) 
Each parallel fiber
makes synaptic
contacts with the
dendrites of many
Purkinje cells ,
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Granule cells
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And thus the
parallel fibers and
Purkinje cell
dendritic trees
form a grid of
remarkably
regular proportions
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The other 3 types of neurons in the cerebellar cortex are
inhibitory neurons :
(3) Basket cells ( inhibitory to Purkinje ):
Are located in the Molecular layer
They are excited by Parallel fibers of Granule cells , &
their output inhibits Purkinje cell discharge by a process
of Feed-Forward Inhibition .
Their axons form a basket around the cell-body and
axon hillock of each Purkinje cell they innervate .
(4) Stellate cells ( inhibitory to Purkinje ):
Similar to Basket cells  they are excited by Parallel
fibers of Granule cells , & their output inhibits Purkinje
cell discharge by a process of Feed-Forward Inhibition .
They differ from Basket cells only in being more
superficially located in the cortex than Basket cells .
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(5) Golgi cells
Golgi cells are located in the Granular
layer .
Their dendrites , which project into the
Molecular layer , receive inputs from
the Parallel fibers .
Their cell bodies receive input via
collateralsfrom the incoming Mossy
fibers and the Purkinje cells
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Their axons project to inhibit the
dendrites of the Granule cells .
They are excited by
(1) Mossy fibers
(2) Purkinje cells , &
(3) Parallel fibers ( of Granule cells ).
They inhibit the excitatory action
of Mossy fibers on Granule cells .
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The Main Inputs
(Afferents ) to the Cerebellar
Cortex (1)
There are 2 main inputs to the
cerebellar cortex : the Clombing
Fibers and Mossy Fibers , both of
which are excitatory .
Climbing Fibers :
The climbing fibers come solely
from the Inferior Olivary Nucleues
They provide an indirect
proprioceptive input to the
cerebellar cortex  bringing to it
proprioceptive information from all
parts of the body via relays in the
Inferior Olive (which receives
proprioceptive inputs from all over
the body parts )
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Each climbing fiber projects to
the dendrites of Purkinje cells ,
around which it entwines like a
climbing plant .
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The Main Inputs
(Afferents ) to the Cerebellar Cortex (2)
Mossy Fibers :
(1) These , unlike Climbing Fibers
( which provide an indirect
proprioceptive input ) do provide a
direct proprioceptive pathway (
input ) to the cerebellar cortex ,
from all parts of the body , and , in
addition
(2) Provide inputs from the Motor
Area ( M1) & related areas of the
Cerebral Cortex ( indirectly , via
relays in the pontine nuclei ).
They end on the dendrites of
Granule cells in complex synaptic
groupings called Glomeruli .
The Glomeruli also contain the
inhibitory endings of the Golgi
cells.
Climbing Fibers
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Mossy Fibers
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Corollary ( summary ) of effects of different cells &
afferents on Purkinje cells
(A) Excitatory
The fundamental circuits of cerebellar cortex are thus relatively simple :
(1) climbing fiber inputs exert a strong excitatory effect on single Purkinje
cells , whereas
(2) Mossy fiber inputs exert a weak excitatory effect on many Purkinje cells
via the Granule cells .
(B) Inhibitory
(1) Basket cells
Both are excited by Parallel fibers of Granule
cells , & their output inhibits Purkinje cells
(2) Stellate cells
( Feed-Forward Inhibition ) .
(3) Golgi cells 
Golgi cells are excited by
(1) Mossy fibers
(2) Purkinje cells , &
(3) Parallel fibers .
They inhibit the action of Mossy fibers on Granule cells
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Q : What are the Neurotransmitters Secreted by in
the Cerebellar Cortex Neurons ?
Purkinje cells
Basket cells
Stellate cells
Golgi cells
Secrete GABA
Granule cells  Glutamate
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Functional Significance of Cerebellar Cortex
Circuitry
The DCN are excitatory to the Brainstem nuclei & Thalamus .
The circuitry of the Cerebellar Cortex seems to be solely concerned
with modulating the
(1) Timing
(2) strength
of the excitatory action of the DCN on the Brainstem &
Thalamus
Remember that : The DCN are excited by both Mossy & Climbing
fibers , but are inhibited by Purjkinje cells .
Hence activity in Mossy & Climbing fibers excite the DCN .
But these are also excitatory to Purkinje cells which inhibit the DCN .
Thus the effect of the afferent inputs seems to activate the DCN
initially , & then , after a latency of time ( of a few ms perhaps ) , to
switch them off via exciting the Purkinje cells ( remember that more
synapses  mean more latency ) .
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The Primary Afferents that Converge to Form the Mossy Fiber or Climbing Fiber
Inputs to the Cerebellum,
Afferent Tract
Transmits Information About :
Pontocerebellar
Intended movements from M1 and other parts of cerebral
cortex related to initiation & execution of voluntary
movement ( indirect path , via relays pontine nuclei )
Ventral and Dorsal
( Ventral & Dorsal )
Proprioceptive information from muscles , tendons &
joints in the trunk & limbs
( direct paths)
Cuneocerebellar
Proprioceptive information from muscles , tendons &
joints in the head and neck
( direct paths)
Olivocerebellar
Proprioceptive information from whole body ( indirect path
, via relay in inferior olive )
Tectocerebellar
Visual ( from retina ) & Auditory ( from Cochlea )
information ( via Superior & Inferior Colliculi )
Vestibulocerebellar
Body position in space from Inner Ear Vestibular
Apparatus ( directly from the labyrinth + Indirectly via
vestibular nuclei )
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Summary of Connections & Functions
Deep
Nuclei
Cortex
Inputs
Outputs
Function
Neocerebellum
Dentate
Lateral portions
of Cerebellar
Hemisphere
Corticoponti SCP to
ne/
VA/VL
pontocereb
ellar
Planning and
executive of
voluntary &
skilledhand
movements
Spinocerebellum
Interpos
ed;
Fastigial
Vermis & Medial
portions of
Cerebellar
hemispheres
Spinal and
brainstem
paths
SCP to
Red
Nucleus;
Fastigial
to RF
Muscle tone ,
posture &
coordination of
movements
Vestibulocerebellum
Fastigial
Flocculonodular
Vestibular
nuclei
Vestibular
nuclei; RF
Balance ,
equilibrium &
VOR
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Remember
Cerebellum hemispheres control the same (
ipsilateral ) side of the body .
Purkinje cells are the main output neurons of the
cerebellar cortex & project to the deep nuclei of
the cerebellum.
They are inhibitory to the DCN .
The deep cerebellar nuclei ( DCN ) project out
to brainstem and thalamic targets via the
superior cerebellar peduncles. They are
excitatory , but in turn , are themselves inhibited
( switched off ) by Purkinje cells .
Flocculonodular lobe is important for regulation
of balance , equilibrium & the VOR .
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Cerebellum Lesions
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Cerebellar Hemispheric
Lesions
Cerebellar lesions cause no paralysis or sensory
deficit .
When not moving , there are no externally
obvious signs .
However , upon physical examination , signs
such as hypotonia and pendular reflexes can be
elicited .
Once the patients attempts movement , ataxia
appears .
What is ataxia ? Ataxia is incoordination of due to
errors in the rate , range , force and direction of
movement .
With circumscribed lesions , the ataxia may be
confined/localized to only one part of the body . 31
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The Difference Between Lesions of the
Cerebellar Cortex & Lesions of DCN
If only the cortex of the cerebellum is involved ,
the movement abnormalities gradually disappear
as “ compensation ” occurs .
However , lesions of the DCN produce more
generalized defects , and abnormalities are
permanent .
For this reason , care should be taken to avoid
damaging the DCN when surgery is undertaken
to remove a tumor involving part of the
cerebellar cortex .
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A/ Hemispheric Lesions
I/Ataxia (lack of coordination of muscle
movement ) , which is manifested by 
(1) Wide-based , unsteady “ drunken , or
staggering “ gait .
(2) Scanning speech
(3) Dysmetria ( also called Past-Pointing ) :
attempting to touch an object with a finger
results in overstretching to one side or the
other this promptly initiates a gross correction
action ( corrective action ) , but the correction
overshoots to the other side  Consequently ,
the finger oscillates back and forth .
This oscillation is the (4) “ Intention Tremor ” ,
which is characteristic of cerebellar disease .
This cerebellar tremor , unlike that of
Parkinson’s
disease , is absent at rest .
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Hemispheric Lesions ( Contd )
II/ Inability to “ put on the brakes ”  i.e., inability to stop
movement promptly . Normally , for example , flexion of
the forearm against resistance is quickly checked when
the resistance force is suddenly broken off . The patient
with cerebellar disease can not break the movement of
the limb , and the forearm flies back in a wide arc . This
abnormal response is known as the “ Rebound
Phenomenon ”.
III/ Adiadochkinesia ( Dysdiadochkinesia ) : Inability to
perform rapidly alternating opposite movements such as
repeated pronation and supination of the hands .
IV/ Difficulty in performing actions that involve
simultaneous motions at more than one joint . The
patient dissects such movements and carries them out
one joint at a time , a phenomenon known as “
Decomposition of Movement ” .
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B/ Flocculonodular Lobe Lesions
Midline cerebellar tumors in children , arising
from the “ Nodule ” , early in their course
(& before affecting the rest of the cerebellum) ,
damage first the Flocculonodular lobe .
Such a child is afraid ( & reluctant ) to stand
erect and move without support .
This is because if he tries to walk , he does so in
a staggering fashion on a broad base , & tends
to fall .
Moreover , selective Flocculonodular lobe
lesions may cause vertigo
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Role of the Cerebellum in Learning (1)
The cerebellum is concerned with learned
adjustments that make coordination easier when
a given task is performed over & over
As a motor task is learned , activity in the brain
shifts from the Prefrontal ( cerebral ) Cortex to
the (1) Parietal Cortex , (2) M1 , & (3)
Cerebellum .
The basis of learning in the cerebellum is the
input via the Olivary Nucleus.
It is worth noting , in this regard , that each
Purkinje cell receives inputs from 250,000 to
1,000,000 Mossy fibers.
By contrast , each Purkinje cell receives only a
single ( only one ) Climbing fiber from the inferior
olive , and this fiber makes 200-3000 synapses
on the Purkinje cell .
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Finished
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