Cerebellum Seminar

8/18/2019 Cerebellum Seminar

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General View

Cerebellum (Latin, little brain): only 10 % total volume of the brain but

more than half of all its neurons. arranged in a highly regular manner as repeating units but with input and

outputs from different parts similar computational operations

but on different inputs.

the cerebellum is provided with extensive information (40 times more

axons project into the cerebellum than exit from it) three sections of cerebellum: (i) gray matter: cerebellar cortex (ii) white

matter (iii) deep nucleus: fastigial, interposed, dentate.

the cerebellum is not necessary to basic elements of perception or

movement.

damage to the cerebellum disrupts the spatial accuracy and temporal

coordination of movement. !t impairs balance and reduces muscle tone

and motor learning and certain cognitive functions.



General View

Gross features of the cerebellum, including the

nuclei, cerebellar peduncles, lobes, folia, and

fissures. (Adapted from Nieuwenhuys et al. 1988)

A. Dorsal view. Part of the right hemisphere has

been ut out to show the underlying erebellar

pedunles.

!. "entral view of the erebellum detahed from

the brain stem.

#. $idsagittal setion through the brain stem and

erebellum% showing the branhing strutures ofthe erebellum.



The cerebellum is divided into anatomically distinct lobes. A. &he erebellum is unfolded to reveal the lobes normally hidden from view.

!. &he main body of erebellum has three funtional regions' the entral vermis and the lateral

and intermediate ones in eah hemisphere. t is divided by the primary fissure into anterior and

posterior lobes. &he posterolateral fissure separates the floulonodular lobe. *hallower fissures

divide the anterior and posterior lobes into nine lobules (anatomists onsider the floulonodular

lobe as the tenth lobule).

The Cerebellum Has Three Functionall !istinct "e#ions




The three functional regions of the cerebellum have different inputs and outputs.



"eurons in the Cerebellar Cortex #re $rgani%ed into

&hree Layers



&he 'urinje Cells eceive

*xcitatory !nput +rom &wo

#fferent +iber ystems and

#re !nhibited by &hreeLocal !nterneurons

Synaptic organization of the

basic cerebellar circuitmodule. $ossy and limbing

fibers onvey output from the

erebellum via a main

e+itatory loop through the

deep nulei. &his loop is

modulated by an inhibitory side

loop passing through the

erebellar orte+. &his figure

shows the e+itatory (/) and

inhibitory () onnetions

among the ell types.



-eometrical 'lan of 'arallel and Climbing fibers

The geometry of the mossy andparallel fiber system contrasts

with that of the climbing fiber

system. $ossy fibers e+ite granule

ells whose parallel fibers branh

transversely to e+ite hundreds of

Pur,in0e ells several millimeters

from the branh point% both mediallyand laterally. !y ontrast% limbing

fibers branh in the sagittal

dimension to e+ite 1 or so Pur,in0e

ells anterior and posterior to the

branh point. &he transverse

onnetions of the parallel fibers andthe sagittal onnetions of the

limbing fibers thus form an

orthogonal matri+.



ossy and Climbing +ibers *ncode 'eripheral and

/escending !nformation /ifferently

Simple and complex spikes

recorded intracellularly from

cerebellar Purkine cells.

#omple+ spi,es (right bra,et)

are evo,ed by limbing fiber

synapses% while simple spi,es

(left bra,et) are produed by

mossy fiber input. (2rom

$artine et al. 1931.)



*ynhroniation of omple+ spi,es in the Pur,in0e neurons

A. A rat performing trained

li,ing.

!. &he grid represents thespatial loations of 49

Pur,in0e ells from whih

omple+ spi,es were

reorded while the rat was

li,ing. &he ells in red fired

synhronously at one time5

those in blue firedsynhronously at another

time5 ells represented by

open irles were not

synhronied. *ynhronous

omple+ spi,es ourred in

neighboring Pur,in0e ells

even after the peripheral

nerves from the fae had

been setioned% suggesting

that the synhronied firing

was entral in origin. (from

6elsh et.al. 1997 Nature)



Cerebellum as a forward model$ Theoretical and neural or#aniation of forward models& a' &heoreticalorgani%ation of information processing streams that use forward models for motor control. otor commands directedto systems that control movement are also copied to forward models that mimic input5output relationships exhibited

by these systems (blue, direct route6 red, side0loop). b' #natomical correlates of this theoretical organi%ation. "otethat the anatomical model contains additional components that exert control over motor control systems (for example,

by modulating rubrospinal circuits) (", red nucleus). c' #nalogous anatomical model involving prefrontalinteractions. &he organi%ation is the same as that in panel b. !nformation arising in the prefrontal cortex is copied tothe cerebellum in the same way that motor commands are copied from the primary motor cortex to the spinal cord. !nthis scheme, cerebellar forward models mimic the input5output relationships of prefrontal targets (note that the target

of a prefrontal neuron can be neurons outside the prefrontal cortex, but can also be another prefrontal neuron).+orward models might therefore be able to mimic information processing that is intrinsic to the prefrontal cortex.



#'- model of

the Cerebellum:

7 'roposed by 8ou and 9arto in

1; (#djustable pattern generator)

7 'roduces motor commands with

adjustable duration and intensity

7 completely agree with physiological

aspects of the cerebellum

7 'ositive feedbac pathway between

cerebral cortex and cerebellar nuclei

7 'C receives input from '+,C+ and

baset cells



1. 'rogramming : balance between '+ and 9 determines the situation

of 'C

2. !nitiation : spread of positive feedbac in premotor networs

. &he intensity of command (positive feedbac) is determined by the

situation of 'C and this produces different elemental movements

3 . 'C becomes refractory to further inputs until near end of movement

< . &he movement is terminated by firing a large number of 'Cs which

were turned off in programming phase due to '+ inputs

= . C+ modifies synaptic weights in programming phase and in

termination phase due to '+ patterns

#'- model: 'hases of movement :



!sometric force fields



+orce fields evoed from microstimulation of the

interneuronal regions of the frog spinal cord. a, &he anle

of spinali%ed frogs was attached to a force transducer and

fixed at different locations in the worspace of the leg,

indicated by the filled circles in the figure. &he same site in

the spinal cord was electrically stimulated with the anle in

each location, and the resulting isometric force was

measured. b, shows an example of a force field resulting

from the stimulation of one such site in the spinal cord.

eproduced from 9i%%i and others (11).

otor

primitives inthe pinal

Cord



Linear

ummation'rinciple

>ector summation of force vectors when costimulating ites at

different activation levels. &he top panel shows the ndividual

fields obtained when site # was stimulated at the ower pulse

duration ('/) and site 9 at the higher one, and the actual (site

# and 9 activated simultaneously) and predicted (from linear

summation of the forces at each position) fields obtained from

costimulation of the two sites. &he bottom panel shows the

results when the levels of activation were switched. imilarity

between fields is indicated by a blac arrow indicating no

difference in average angular deviation across the measured

vectors, and a white arrow indicating that the force magnitude

ratio across positions is not different than 1. &he fields

produced by the two combinations of activation levels (top

and bottom panel) were different from each other, showing the

possibility of creating a variety of fields by modulating the

contribution of each original site to the summated response.

eproduced from Lemay and others (2??1).



7 esponses generated by combinations of muscle synergies may show lowcorrelation between the activities of muscle pairs. # muscle synergy isdefined here as the recruitment of a group of muscles with a specific balanceof activation. !f only one synergy is activated at a time (a), a set of responsesobtained by changing the level of activation of a synergy is obtained byscaling a single vector in the muscle activity space (b). !n this case, all pairsof muscles have highly correlated activations (c). !f instead two synergies arecoactivated (d ), the set of responses are generated by combining two vectorsand the activities of the muscles lie on a plane (e). &he correlation between

two of the three muscles is now much lower ( f ).



τ(t) -1((t)) Σ i ϕi((t))

*pinal ord solving

the inverse problem

!rain

:inear interation



#''- model of the Cerebellum



+igure illustrates the scheme of our proposed model for cerebellar

learning based on #''- modules. 'rimitive encoder represents the

-ranule cells, which provide cerebellum with sparse expansive

encoding of the coefficients of primitives (ci ) from the spinal cord

and motor cortex. # map transforms a low dimensional variable @(t)

into a multi0dimensional control signal6 input of this transformation

is the proprioceptive information of the motor apparatus and the

output represents the mossy fiber. &his process is performed in Astate

mappingB bloc shown in +ig. &he Cn represents the current

coefficient of motor primitive that corresponds to the efferent copy

of motor information coming from spinal cord to the cerebellum.

Cn41 represents the information from motor cortex to the cerebellum(e@uivalent to next motor coefficient). @,@DEn01 represents the

proprioceptive information from the limbs (the previous state of the

limb).

#''- model:

Cerebellum Seminar

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