Theoretical motor-to-somatosensory predictions (Popa, Hewitt, & Ebner, 2012). Relation

 

Theoretical model highlight the idea that the
cerebellum is a learning machine (Pope, Bracewell, & Miall, 2011). The
structural organization of the cerebellar internal circuitry collectively with
the content of its inputs restrains theoretical models of cerebellar function
(Ioffe, Chernikova, & Ustinova, 2007). A significant theoretical model of
cerebellum motor function is feedback and feedforward control system. The
feedback controller system involves continuous comparison of desired output
with the actual output, and adjustments are created during the execution of the
movement until the actual output matches the desired output. The myotatic
reflex function in posture maintenance is an example of a feedback controller,
and the cerebellum participates in coordinating these postural adjustments. In
its function as a feedforward controller, the cerebellum receives motor command
from the primary motor cortex through mossy fibre activation. The mossy fibres
may provide information regarding the desired output by delivering it to many
Purkinje cells through granule cells and parallel fibres. The cerebellum makes
a prediction regarding the sensory effects of such motor commands by evaluating
sensory information. It allows the musculoskeletal system to prepare to
correctly complete a movement. During movement, predicted output are compared
with the actual output. If there is a positive match, the pattern is maintained
for the next movement. The climbing fibres reveals information about movement
errors, which supplies a signal encouraging cerebellum to generate the right
movement the next time the output is requested. The feedforward controller is
more appropriate for fast movements and feedback control system is for slow
movements (e.g. postural adjustment).

Experimental evidence of Model:

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In a study by (Gilbert and thach,1977), the
climbing fibres error detecting function were identified while recording
Purkinje cell activity in monkeys during an arm movement task (Popa, Hewitt,
& Ebner, 2012). It has been suggested as the principal cerebellar form of
operation. Study on the internal feedforward controller revealed that the
cerebellum generates motor-to-somatosensory predictions (Popa, Hewitt, &
Ebner, 2012).

Relation of Model to Anatomy:

Marr (1969) and Albus (1971) anticipated that
cerebellar learning was an outcome of climbing fibres regulating Parallel
fibres -Purkinje cells plasticity (D’Angelo et al., 2011). This occurs through
phenomena known as Synaptic plasticity, which describes the ability of synapses
to change the strength of their connections. The “strength” is the response
that is generated in the excitatory post-synapse because of the presynaptic
activity. The two forms of synaptic plasticity are long-term potentiation (LTP)
and long-term depression (LTD).  LTP is the addition of new receptors and
enlargement of the synapse. Whereas, LTD is the weakening of synaptic contacts
through the removal of receptors and shrinkage of synapse.  The plasticity
responsible for learning was predicted to be Long-term potentiation (LTP) by
Marr and inverted into long-term depression (LTD) by Albus. Like LTP, LTD is
mediated by the activation of either metabotropic-glutamate receptor or
NMDA-glutamate receptor.

Learning is initiated through granule cells
mediated glutamate release, triggered by action potential generated at axon
hillock.  Repeated activation of this pre-synapse at a lower frequency,
between 1 and 5 Hz, for several minutes usually initiates a LTD. The glutamate
molecules bind to their receptors: type I metabotropic glutamate receptor
(mGluR1 and mGluR5), NMDAR and AMPAR on the postsynaptic Purkinje cell
dendritic spine. Co-activation of AMPAR is required to provide the
depolarisation needed to open NMDAR (ligand and voltage gated channel). The
activation of this receptors initiates multiple signalling cascade responsible
for the removal of AMPAR. The exact mechanism of this action is unknown but is
believed to be the fundamental mechanism for LTD.  This unclear mechanism
could occur pre-synapse (granule cell) reduced glutamate release or decrease
postsynaptic dendritic glutamate receptor expression. This change in strength
can last for hours to a lifetime, depending on how often the stimulation is
repeated.

 

ANALYSIS OF CEREBELLAR tDCS EFFECT ON
MOTOR LEARNING

Cerebellar-tDCS has been used to modify posture
(Craig & Doumas, 2017; Inukai et al., 2016; Poortvliet et al., 2017),
visuomotor (Ehsani et al., 2016; Galea et al., 2011; Leow, Marinovic, Riek,
& Carroll, 2017; Shah, Nguyen, & Madhavan, 2013; Wessel et al., 2016) ,
grip force (John et al., 2017) and to compare with M1-tDCS (Ehsani et al.,
2016; Galea et al., 2011)

Figure 2. Feedforward control system

 

Theoretical model highlight the idea that the
cerebellum is a learning machine (Pope, Bracewell, & Miall, 2011). The
structural organization of the cerebellar internal circuitry collectively with
the content of its inputs restrains theoretical models of cerebellar function
(Ioffe, Chernikova, & Ustinova, 2007). A significant theoretical model of
cerebellum motor function is feedback and feedforward control system. The
feedback controller system involves continuous comparison of desired output
with the actual output, and adjustments are created during the execution of the
movement until the actual output matches the desired output. The myotatic
reflex function in posture maintenance is an example of a feedback controller,
and the cerebellum participates in coordinating these postural adjustments. In
its function as a feedforward controller, the cerebellum receives motor command
from the primary motor cortex through mossy fibre activation. The mossy fibres
may provide information regarding the desired output by delivering it to many
Purkinje cells through granule cells and parallel fibres. The cerebellum makes
a prediction regarding the sensory effects of such motor commands by evaluating
sensory information. It allows the musculoskeletal system to prepare to
correctly complete a movement. During movement, predicted output are compared
with the actual output. If there is a positive match, the pattern is maintained
for the next movement. The climbing fibres reveals information about movement
errors, which supplies a signal encouraging cerebellum to generate the right
movement the next time the output is requested. The feedforward controller is
more appropriate for fast movements and feedback control system is for slow
movements (e.g. postural adjustment).

Experimental evidence of Model:

In a study by (Gilbert and thach,1977), the
climbing fibres error detecting function were identified while recording
Purkinje cell activity in monkeys during an arm movement task (Popa, Hewitt,
& Ebner, 2012). It has been suggested as the principal cerebellar form of
operation. Study on the internal feedforward controller revealed that the
cerebellum generates motor-to-somatosensory predictions (Popa, Hewitt, &
Ebner, 2012).

Relation of Model to Anatomy:

Marr (1969) and Albus (1971) anticipated that
cerebellar learning was an outcome of climbing fibres regulating Parallel
fibres -Purkinje cells plasticity (D’Angelo et al., 2011). This occurs through
phenomena known as Synaptic plasticity, which describes the ability of synapses
to change the strength of their connections. The “strength” is the response
that is generated in the excitatory post-synapse because of the presynaptic
activity. The two forms of synaptic plasticity are long-term potentiation (LTP)
and long-term depression (LTD).  LTP is the addition of new receptors and
enlargement of the synapse. Whereas, LTD is the weakening of synaptic contacts
through the removal of receptors and shrinkage of synapse.  The plasticity
responsible for learning was predicted to be Long-term potentiation (LTP) by
Marr and inverted into long-term depression (LTD) by Albus. Like LTP, LTD is
mediated by the activation of either metabotropic-glutamate receptor or
NMDA-glutamate receptor.

Learning is initiated through granule cells
mediated glutamate release, triggered by action potential generated at axon
hillock.  Repeated activation of this pre-synapse at a lower frequency,
between 1 and 5 Hz, for several minutes usually initiates a LTD. The glutamate
molecules bind to their receptors: type I metabotropic glutamate receptor
(mGluR1 and mGluR5), NMDAR and AMPAR on the postsynaptic Purkinje cell
dendritic spine. Co-activation of AMPAR is required to provide the
depolarisation needed to open NMDAR (ligand and voltage gated channel). The
activation of this receptors initiates multiple signalling cascade responsible
for the removal of AMPAR. The exact mechanism of this action is unknown but is
believed to be the fundamental mechanism for LTD.  This unclear mechanism
could occur pre-synapse (granule cell) reduced glutamate release or decrease
postsynaptic dendritic glutamate receptor expression. This change in strength
can last for hours to a lifetime, depending on how often the stimulation is
repeated.

 

ANALYSIS OF CEREBELLAR tDCS EFFECT ON
MOTOR LEARNING

Cerebellar-tDCS has been used to modify posture
(Craig & Doumas, 2017; Inukai et al., 2016; Poortvliet et al., 2017),
visuomotor (Ehsani et al., 2016; Galea et al., 2011; Leow, Marinovic, Riek,
& Carroll, 2017; Shah, Nguyen, & Madhavan, 2013; Wessel et al., 2016) ,
grip force (John et al., 2017) and to compare with M1-tDCS (Ehsani et al.,
2016; Galea et al., 2011)