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Cerebellum • Overview and structure of cerebellum • Microcircuitry of cerebellum • Motor learning

Cerebellum Overview and structure of cerebellum Microcircuitry of cerebellum Motor learning

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Page 1: Cerebellum Overview and structure of cerebellum Microcircuitry of cerebellum Motor learning

Cerebellum

• Overview and structure of cerebellum

• Microcircuitry of cerebellum

• Motor learning

Page 2: Cerebellum Overview and structure of cerebellum Microcircuitry of cerebellum Motor learning

Overview

• “small brain”

• Weight: 10% of entire brain

• Number of neurons: 50% (due to large number of small granule cells)

• Cerebellar cortex: highly regular structure

• Function: coordination, balance, motor learning, etc.

Page 3: Cerebellum Overview and structure of cerebellum Microcircuitry of cerebellum Motor learning

Dorsal view Sagital view

fastigial nuclei

The cerebellum consists of the cerebellar cortex and a set of deep nuclei

interposed nuclei

dentate nuclei

Page 4: Cerebellum Overview and structure of cerebellum Microcircuitry of cerebellum Motor learning

• Vermis fastigial nuclei, control proximal muscles

• Intermediate zone interposed nuclei, control distal muscles

• Lateral zone dentate

• Flocculonodular

Descending system to brain stem, motor execution

Connect to motor & premotor cortices, planning and initiation of movement

Connect to vestibular nuclei, control balance and eye movement

Different regions of cerebellar cortex are connected to different deep nuclei

Page 5: Cerebellum Overview and structure of cerebellum Microcircuitry of cerebellum Motor learning
Page 6: Cerebellum Overview and structure of cerebellum Microcircuitry of cerebellum Motor learning

5 cell types

StellateBasket(molecular layer)

Purkinje (Purkinje layer)

GolgiGranular(Granular layer)

Microcircuitry of cerebellum

Page 7: Cerebellum Overview and structure of cerebellum Microcircuitry of cerebellum Motor learning

Inputs:

• Climbing fiber (“+”, excitatory)

• Mossy fiber (+)

Output:

• Purkinje cell axon (“-”, inhibitory)

Page 8: Cerebellum Overview and structure of cerebellum Microcircuitry of cerebellum Motor learning

Purkinje cell2-D dendritic tree perpendicular to the long

convolutions of the cortex

Page 9: Cerebellum Overview and structure of cerebellum Microcircuitry of cerebellum Motor learning

First direct pathway:

• climbing fiber (+) Purkinje cells (-) deep nuclei

• each climbing fiber projects to a single Purkinje cell

• each Purkinje cell receives input from a single climbing fiber

• Powerful excitatory connection, each climbing fiber spike cause a burst of spikes in Purkinje cell (called a “complex spike”)

Page 10: Cerebellum Overview and structure of cerebellum Microcircuitry of cerebellum Motor learning

Second direct pathway:

• Mossy fiber (+) granule cells (axon: parallel fibers, +) Purkinje cells (-) deep nuclei

• each parallel fiber projects to thousands of Purkinje cells (high divergence)

• each Purkinje cell receives input from ~200,000 parallel fibers (high convergence)

• Weak excitatory connection, spatiotemporal summation of inputs from many parallel fibers causes a single spike in Purkinje cell (called a “simple spike”)

glomeruli

Page 11: Cerebellum Overview and structure of cerebellum Microcircuitry of cerebellum Motor learning

Lateral inhibition:

• granule cells (axon: parallel fibers, +) stellate and basket cells (-) Purkinje cells in a different row

Negative feedback:

• granule cells (axon: parallel fibers, +) Golgi cells (-) granule cells

Page 12: Cerebellum Overview and structure of cerebellum Microcircuitry of cerebellum Motor learning

Everything together

Page 13: Cerebellum Overview and structure of cerebellum Microcircuitry of cerebellum Motor learning

Cerebellum and motor learning

• During learning of a new motor task, subject makes mistakes, but the error reduces with practice

• The standard notion is that the “error signal” causes changes in brain circuits involved in motor control (e.g., cerebellum), thus improving motor performance

Patient H.M., who had bilateral removal of medial temporal lobe and lack of ability to form episodic memory, but motor learning is intact, indicating it is mediated by different brain structure

Page 14: Cerebellum Overview and structure of cerebellum Microcircuitry of cerebellum Motor learning

Cerebellum and motor learning

• In a well-known model of cerebellum motor learning, climbing fiber activity represents error signals (difference between expected and actual sensory inputs, e.g., the template and the actual drawing).

• Experimentally, simultaneous activation of climbing fibers and parallel fibers converging onto the same Purkinje cell can cause long-term depression of parallel fiber Purkinje synapse, which may underlie motor learning.